Lower Respiratory Tract Diseases

Amelia R. Woolums

Clinical Classification of Pneumonia

Amelia R. Woolums

In an effort to simplify the differential diagnosis of the array of lower respiratory diseases of cattle, a classification system based on pathophysiology and clinical signs can be used:

1.

2. The interstitial pneumonias are a very diverse group of (usually) noninfectious diseases. Although it is difficult to make gener­alizations, these diseases are characterized pathophysiologically by an interstitial reaction that usually results from ingestion or inhalation of toxins or allergens. Clinically affected animals tend not to be as depressed and septic, although dyspnea may be severe and disease can be rapidly fatal. Abnormal lung sounds and lesions are diffusely distributed, and there is little or no response to antibiotic therapy.

3. Metastatic pneumonia is characterized pathophysiologically by septic embolization of the lungs from other foci in the body, classically liver abscesses and postcaval thrombi. Clinically, cases of metastatic pneumonia exhibit signs of sepsis as with bronchopneumonia, but with widespread pulmonary lesions and abnormal lung sounds, and the eventual development of hemoptysis (see the Vena Caval Thrombosis and Metastatic Pneumonia section later).

The Bronchopneumonias (Respiratory Disease Complex of Cattle, Sheep, and Goats)

Amelia R. Woolums

The respiratory disease complex of ruminants consists of the single clinical entity of bronchopneumonia or pleuropneumonia but is caused by numerous combinations of infectious agents, compromised host defenses, and environmental conditions. Bronchopneumonia causes greater economic losses than any other disease of feedlot cattle or lambs and is one of the most common causes of mortality in other ruminant populations.

In dairy calves, bronchopneumonia, sometimes called enzootic pneumonia, is most common in group-housed calves. It is called shipping fever in beef calves because the greatest incidence of bronchopneumonia occurs after shipment to stocker operations or feedlots; the term bovine respiratory disease (BRD) or bovine respiratory disease complex (BRDC) is also used to describe bronchopneumonia in cattle. Bronchopneumonia also occurs in nursing beef calves, in mature beef or dairy cows, and in young or mature sheep and goats, sometimes in outbreaks that can have significant morbidity and mortality. The infectious agents and risk factors of bronchopneumonia of sheep and goats are similar to those of calves.

Bronchopneumonia of ruminants is a disease of multifactorial causation that occurs when a certain combination of host, environment, and infectious agent characteristics (risk factors) is active. The numerous infectious agents (Boxes 31.3 and 31.4) associated with bronchopneumonia are ubiquitous in ruminant populations, and the bacteria most often associated with pneumonic lesions are part of the normal resident flora of the ruminant nasopharynx. In addition to recognition of the microbial agents that contribute to the development of broncho­pneumonia in ruminants, an understanding of the management practices and host factors that also play a role is necessary for developing successful programs of prevention. Management practices and other risk factors for bronchopneumonia are discussed in the Host and Environmental Risk Factors for Undifferentiated Ruminant Respiratory Disease section later.

Infectious Agents Associated With the Respiratory Disease Complex of Cattle, Sheep, and Goats

■ Etiology Numerous infectious agents have been isolated from cases of bronchopneumonia in ruminants. Although infec­tious bronchopneumonia of ruminants is usually caused by two or more infectious agents acting together, some agents can also cause significant disease alone. Therefore the clinical and epidemiologic characteristics, means of diagnosis, and treatment and prevention of the infectious agents will be first considered individually. A description of epidemiology, diagnosis, and treatment of “undifferentiated” bronchopneumonia (broncho­pneumonia with no specific causative diagnosis attempted) will then follow. The clinical signs, gross pathologic lesions, and recommended methods of diagnosis for the major infectious causes of ruminant respiratory disease are listed in Table 31.14.

Viral Agents

Many different viruses have been identified in nasal secretions or respiratory tissues of cattle, sheep, or goats with or without respiratory disease. Recent development of next-generation sequencing approaches that allow identication of all viral genomes present in a sample (the “virome” or “viral metage­nome”) has revealed even more, sometimes newly recognized respiratory viruses.¹ Historically, five viruses have been con­sidered the most important contributors to the bovine respira­tory disease complex: bovine herpesvirus type 1 (BHV-1), bovine respiratory syncytial virus (BRSV), bovine viral diarrhea virus types 1 and 2 (BVDV1 and BVDV2), and parainfluenza virus

■ BOX 31.3

■ BOX 31.4

Viral Agents Associated With Respiratory Tract Diseases in Ruminants

Bacteria, Mycoplasma, Ureaplasma, and Chlamydophila Species Associated With Bronchopneumonia of Cattle, Sheep, and Goats

Bacteria of Major Importance (Commonly Isolated or Generally Accepted to Be Important Contributors to Ruminant Respiratory Disease)

Mannheimia haemolytica

Pasteurella multocida

Histophilus somni Mycoplasma bovis

Bibersteinia trehalosi

Mycoplasma dispar Ureaplasma spp.

Mycoplasma ovipneumoniae (sheep and goats only)

Mycoplasma mycoides subsp. mycoides, large colony variant (goats)

Trueperella pyogenes (formerly Arcanobacterium pyogenes) (secondary opportunistic pathogen)

Bacteria of Minor Importance (Uncommonly Isolated or of Uncertain Importance in Ruminant Respiratory Disease)

Pseudomonas aeruginosa

Escherichia coli

Streptococcus spp. Staphylococcus spp.

Moraxella spp.

Salmonella spp.

Bacteroides spp. (anaerobe)

Peptococcus indolicus (anaerobe) Fusobacterium spp. (anaerobe) Chlamydophila spp. type 3 (PIV-3). In sheep and goats, BRSV and PIV-3 have been considered important. However, in some recent surveys of cattle or cattle populations with respiratory disease, other viruses have been more prevalent than these five viruses. Also, anecdotal reports suggest that, at least in North America, BHV-1 and PIV-3 are less commonly found in cattle with respiratory disease than they were in the second half of the twentieth century. This may be related to broad population immunity induced by widespread use of vaccines containing these viruses. Thus the relative importance ascribed to the viruses described below may be changing.

Bovine Herpesvirus Type 1 (Infectious Bovine Rhinotracheitis Virus)

■ Definition and Etiology Bovine herpesvirus type 1 (BHV-1) is an enveloped DNA virus of the family Herpesviridae, subfamily Alpbaherpesvirinae, genus Varicellovirus. It is associ­ated with multiple, distinct disease syndromes of cattle that include infectious bovine rhinotracheitis (IBR), conjunctivitis (see Chapter 39), infectious pustular vulvovaginitis (IPV), balanoposthitis, abortion (see Chapter 43), encephalomyelitis (see Chapter 35), and mastitis. Two subtypes have been identi­fied on the basis of restriction endonuclease cleavage patterns. These subtypes are referred to as BHV-1.1 and BHV-1.2; BHV-1.2 can be further divided into BHV-1.2a and BHV-1.2b. Either subtype can cause respiratory or genital disease; only the respiratory manifestations of infection are discussed here.

■ Clinical Signs Clinical signs from infection with BHV-1 alone can range from inapparent to severe. Clinical signs can include any combination of pyrexia, anorexia, sharp drop in milk production in dairy cattle, increased respiratory rate, a slight degree of hyperexcitability, ptyalism, coughing, and nasal discharge that progresses from serous to mucopurulent. Clinical signs of lower respiratory tract infection may be caused by secondary bacterial pneumonia, which is a common sequela

■ Table 31.14

Clinical and Gross Pathologic Characteristics of Common Infectious Agents That Cause Respiratory Disease in Ruminants

Continued

■ Table 31.14

Clinical and Gross Pathologic Characteristics of Common Infectious Agents That Cause Respiratory Disease in Ruminants—cont'd

^aClinical signs not related to the respiratory system but possibly also prevalent are listed in parentheses. Note that respiratory disease is often caused by two or more agents, so clinical signs and gross pathology in an individual or herd may be any combination of those listed for individual agents.

^bAgent does not survive transport well or can be difficult to isolate; requires special handling or culture techniques to maximize likelihood of isolation. ^cUsually need to specifically request tests to isolate mycoplasmas and to specifically identify the species M. bovis.

BHV-1, Bovine herpesvirus type 1; BRSV, bovine respiratory syncytial virus; BVDV, bovine viral diarrhea virus; ELISA, enzyme-linked immunosorbent assay; IBR, infectious bovine rhinotracheitis; IFA, immunofluorescent assay; IHC, immunohistochemistry, most commonly used on formalin-fixed lung tissue collected at necropsy; PCR, polymerase chain reaction; PIV-3, parainfluenza type 3; PS, paired serology; RT-PCR, reverse transcriptase PCR; TTA, transtracheal aspirate; VI, virus isolation.

to BHV-1 infection. Severe hyperemia and reddening of the muzzle can occur, which is the reason for the common name of “red nose” to describe BHV-1 infection. Pustules may develop on the nasal mucosa and later form diphtheritic plaques. Conjunctivitis with excessive ocular discharge may be present. Conjunctivitis with corneal opacity can also occur as the principal manifestation of BHV-1 infection and may be mis­diagnosed as infectious bovine keratoconjunctivitis (pink eye). Abortions may occur concurrently with respiratory disease, but they can also occur well after infection. Abortions may also occur in cattle that escape serious respiratory disease.

On rare occasions, neonatal calves may suffer from an acute respiratory and/or systemic form of BHV-1 infection character­ized by rhinitis, marked lacrimation, inflammation and necrosis of the soft palate, laryngotracheitis, and ulceration of the GI tract.^2,3 Vaccination of calves within 3 days of age with modified live viral vaccines has also been associated with severe, fatal systemic BHV-1 infection.⁴ The severity of disease in vaccinated calves was suspected to be caused by lack of maternal antibody and resultant widespread multiplication of the vaccine virus. ■ Pathogenesis The route of infection is by direct contact with infected cattle or aerosol; apparent transmission by aerosol has been recognized among calves separated by as little as 4 m.⁵ At least three of the surface glycoproteins of BHV-1, gC, gD, and gB, mediate attachment to host cells and entry through interaction with heparan sulfate proteoglycans and other host cell proteins. Epithelial cells of the respiratory tract are the initial target of infection after respiratory exposure, and after initial infection the virus can spread intracellularly to neighboring cells. Lymphocytes and monocytes are also susceptible to infection; although infection with these cells produces little virus, they appear to be a means by which the virus reaches extrarespiratory sites after respiratory infection. In severe field cases of BHV-1 infection the virus can be found in multiple organs, including the esophagus, spleen, and liver.⁶

Respiratory disease caused by BHV-1 is mediated by two major mechanisms: (1) direct injury and destruction of infected epithelial cells in the upper respiratory tract and bronchi, with resultant inflammation and increased susceptibility to infection with secondary opportunistic pathogens; and (2) immunosuppression resulting from dysfunction of neutrophils, lymphocytes, and macrophages. Immunosuppressive actions of BHV-1 include decreased neutrophil chemotaxis and mitogen- induced blastogenesis of lymphocytes, decreased expression of MHC class I molecules, and induction of apoptotic death of lymphocytes and monocytes.

Another important mechanism of pathogenesis of BHV-1 is the ability to establish latent infection. After acute infection, BHV-1 can be found in a latent state in the trigeminal ganglia; the virus may also persist latently in the tonsil. During latent BHV-1 infection, virus is not actively produced in infected cells; thus latently infected cattle do not shed virus. However, an RNA molecule called the latency related transcript (LRT) is abundantly produced in latently infected cells, which prevents apoptosis in cells receiving signals that should trigger apoptosis.⁸ One or more proteins encoded by the LRT appear to be required for the virus to reactivate from latency. Stress or administration of glucocorticoids causes reactivation of the latent virus, leading to shedding of virus and the possibility of infection of in-contact susceptible animals. Latency appears to occur in effectively all cattle that are infected with BHV-1, thus ensuring that the virus can be spread during times of stress by animals that have been free of clinical disease from BHV-1 infection for months to years. Modified live BHV-1 vaccines are also capable of causing latent infections. Calves that are exposed to BHV-1 when they have moderate levels of circulating maternal antibody can develop latent infection while never showing signs of clinical disease. Latency of BHV-1 is thus an important mechanism by which the virus can persist and spread in a group of cattle.

■ Epidemiology Studies of antibody prevalence to BHV-1 indicate that infection is widely distributed in the cattle popula­tion. Latently infected adult cattle are thought to be the principal reservoirs of infection. Infections of the respiratory tract by BHV-1 are prevalent when large concentrations of beef or dairy cattle are assembled, although BHV-1 does not appear to have an important role in enzootic pneumonia of calves. Decreaed milk production has been linked to subclinical BHV-1 infection in dairy cows. In feedlots, “late breaks” of BHV-1 infection and disease can be seen to occur in cattle previously vaccinated against BHV-1. Although BHV-1 is a relatively genetically stable virus, there is evidence that mutations are occurring in BHV-1 isolates currently circulating in cattle populations, which may help the virus escape host immunity.¹³ Although commercially available vaccines can still prevent disease resulting from experimental challenge with modern BHV-1 isolates, viral shedding is increased relative to cattle challenged with older BHV-1 isolates that are more similar to the strains included in vaccines.^13,14 Repeat vaccination later in the feeding period has been recommended to decrease risk of late BHV-1 breaks.

■ Necropsy Findings IBR is rarely fatal in mature cattle unless it is complicated by secondary bacterial infection of the lung. Lesions include rhinitis, laryngitis, and tracheobronchitis. The mucosa of the nasal passages and trachea can be congested or hemorrhagic. Pustular lesions (sometimes referred to as “plaques”) may be observed, and these lesions may coalesce to form adherent necrotic material on respiratory (Color Plate 31.2), ocular, and reproductive mucosa. Usually the inflammatory lesions induced by BHV-1 do not extend into airways contained within the lung, but secondary bacterial bronchopneumonia with the expected pathology is common. Conjunctivitis and, less commonly, keratitis may be present. Esophageal erosions have been identified in severe natural outbreaks. Perinatal infection of calves can lead to fatal systemic disease, with necrotic foci found in the liver, adrenal glands, kidneys, and other organs.^3,4 Although intranuclear inclusion bodies are a feature of herpesvirus infections, they are not a common histologic feature of BHV-1.

■ Diagnosis BHV-1 can be diagnosed by PCR, virus isola­tion, immunofluorescent antibody (IFA) testing, or immuno­histochemistry (IHC) to identify virus or viral nucleic acid in nasal swabs or conjunctival scrapings antemortem or in tissues collected postmortem. Infection can also be diagnosed by paired serology, with tests for serum neutralizing (SN) antibodies most commonly used.

■ Treatment and Prevention Cattle infected with BHV-1 should receive supportive care. Good-quality feed and water should be made easily available, and additional stressors such as transport, castration or dehorning, or introducing new cattle to the group should be avoided or postponed. NSAIDs such as flunixin meglumine (1.1 to 2.2 mg/kg IV daily or divided twice daily) may help severely affected individuals maintain water and feed consumption. Administration of antimicrobials to treat or prevent secondary bacterial pneumonia may be appropriate (Table 31.15), although these are not invariably required. Steroids should not be administered. Because protec­tive immunity develops rapidly after either intranasal or IM vaccination,^16,17 BHV-1 vaccination in the face of an outbreak may help limit disease, although the efficacy of this practice has not been tested in a controlled study.

Efforts to prevent BHV-1 infection should include practices that optimize host immunity and avoiding management practices that put cattle at risk, such as mixing newly introduced cattle with established populations. Many brands of inactivated and attenuated BHV-1 vaccines for SC or IM administration are commercially available; products that contain BHV-1 alone or in combination with other pathogens are available. Modified live vaccines are also available for intranasal administration; intranasal vaccines also include PIV-3 with or without BRSV. Vaccination to prevent disease due to BHV-1 infection is discussed in detail in Chapter 48. It is important to remember that vaccination can complicate efforts to identify viruses that may be causing outbreaks of bovine respiratory disease. It appears that live virus is more likely to be shed from cattle vaccinated with modified live vaccines given intranasally, as compared to vaccines given by injection. However, viral DNA can be identified by PCR of nasopharngeal swabs or transtracheal aspirates for 28 days after cattle are vaccinated either intranasally or subcutaneously with modified live virus vaccine. This makes it difficult to interpret the results of PCR testing of respiratory samples when trying to diagnose BHV-1 infection in cattle that have been recently given modified live vaccines.

Other Herpesviruses

Antimicrobials Approved by the U.S. Food and Drug Administration for Treatment of Bovine Respiratory Disease

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Note: Many of the drugs listed in this table are not approved for use in veal calves; some are not approved for use in lactating dairy cattle.

^aIn cattle, use of an extralabel dose level, frequency, duration, or route prohibited by law. ^bExtralabel use in food animals prohibited by law.

^cVaries with preparation.

^dOxytetracycline is labeled for administration to sheep in feed; not a recommended route for treatment of respiratory disease.

^eDose that has been shown to be more effective than label-approved dose. Higher dose can be given by the same route of administration and with the same treatment interval, but use of extralabel dosages must comply with the Animal Medicinal Drug Use Clarification Act (AMDUCA) and withdrawal times must be extended.

BRD, Bovine respiratory disease; IM, intramuscular; IV, intravenous; PO, oral; SC, subcutaneous.

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to infected exotic hoofstock. MCF is discussed in detail in Chapter 32.

Bovine Respiratory Syncytial Virus

■ Definition and Etiology Bovine respiratory syncytial virus (BRSV) is an enveloped RNA virus recently moved from the family Paramyxoviridae to the new family Pneumoviridae, genus Orthopneumovirus.²⁰ This virus is named for the character­istic cytopathic effect it produces in vitro and in vivo, which is the formation of syncytial cells. BRSV shares many similarities in its biology and epidemiology with the human respiratory syncytial virus (HRSV), a common respiratory pathogen of humans with particular importance in disease of young children and the elderly. Although BRSV and HRSV are closely related, they are distinct viruses. BRSV has also been isolated from sheep and goats with respiratory disease. Research suggests that ovine respiratory syncytial virus (also known as ovine pneumovirus) may be distinct from HRSV and BRSV, whereas caprine respira­tory syncytial virus (also known as caprine pneumovirus) may be more closely related to BRSV²¹ A European serologic survey undertaken to determine whether nonbovine species were likely to harbor BRSV infection found that only goats, in addition to cattle, were commonly seropositive.²² These results indicate either that goats are commonly infected with BRSV or that antigenic similarities between caprine respiratory syncytial virus and BRSV led to production of cross-reactive antibodies in many goats. In spite of minimal to moderate differences among BRSV isolates at the genetic level, isolates can differ notably in their reactivity, with monoclonal antibodies directed against individual BRSV proteins; this indicates that apparently minor genetic differences could lead to changes that allow the virus to evade host immunity. Genetic variability is evident in BRSV isolates collected over several decades, and BRSV isolates commonly included in commercial vaccines can differ a relatively large amount from isolates collected from recent natural outbreaks.^23,24 Studies have also shown that genetic differences in BRSV isolates can be grouped by geographic origin, with some European isolates differing to a notable degree from isolates from the United States or Japan.²³ These changes appear to be driven by vaccination; thus commercial vaccines may need to be reformulated periodically to include BRSV similar to isolates currently circulating.

■ Clinical Signs Clinical signs of BRSV infection alone can vary from inapparent to severe in an infected group of cattle. Signs of disease are limited to the respiratory system. Infected animals may display elevated rectal temperature, depression, decreased feed intake, elevated respiratory rates, ptyalism, cough, and nasal and lacrimal discharges. Signs of disease may progress rapidly, and early signs may be missed. Thoracic auscultation may reveal increased bronchial and bronchovesicular sounds; fine crackles or wheezes may be heard, particularly in the middle or dorsocaudal lung fields. Absence of bronchovesicular sounds may be noticed in the dorsal or dorsocaudal lung fields if an emphysematous bulla is present, or if a bulla has ruptured, leading to pneumothorax. In later stages of the disease, dyspnea can become pronounced and is characterized by increased expiratory effort and mouth breath­ing. Subcutaneous emphysema and intermandibular edema are sometimes noted. Dramatic reduction in milk production has been reported in dairy cattle. Duration of disease is variable (1 to 2 weeks). A biphasic clinical course has been described but does not appear to be a consistent finding. Infection can also cause clinical signs of respiratory disease in sheep, goats, and deer; however, the severe clinical signs, including dyspnea and subcutaneous emphysema, which are sometimes seen in cattle and suggest AIP, do not seem to be seen in small ruminants or cervids.

■ Pathogenesis The means of transmission appears to be contact with infected cattle and aerosols. Fomites such as bottles and buckets are also a likely means of transmission, as HRSV is spread by fomites. The incubation period is 3 to 5 days. Infection with BRSV can cause bronchitis, bronchiolitis, alveolitis, and interstitial pneumonia, leading to clinical signs referable to small airway and alveolar disease, including expira­tory dyspnea, auscultable wheezing, and hypoxemia. The virus causes epithelial cells to fuse, resulting in the characteristic multinucleated cells, or syncytia, which are seen in airways and alveoli. Epithelial cells, which may undergo apoptosis after BRSV infection, slough into the lumen of airways and are phagocytized by neutrophils or alveolar macrophages. Infection of alveolar macrophages and circulating peripheral blood mononuclear cells occasionally occurs,^25,26 and BRSV infection of macrophages decreases important functions of these cells, including Fc receptor expression, phagocytosis, phagosome-lysosome fusion, and production of factors that ₂7₂₈

induce neutrophil chemotaxis.,

The severity of disease after BRSV infection is related to host immunity. Both humoral and cell-mediated immunity contribute to protection. Clinical signs of infection are less severe in animals with moderate to high levels of serum antibody against BRSV,^29,30 and more extensive pathology is seen in calves depleted of CD8+ T cells before infection.³¹ Protection after intranasal vaccination is related to rapid onset of BRSV- specific IgA production in nasal secretions,³² indicating that mucosal immunity is likely also important in protection against naturally occurring disease. Host immunity after exposure to BRSV can minimize disease, and clinical signs are usually the most noticeable in calves younger than 6 months of age. However, very young calves are not necessarily most likely to have the most severe disease; clinical signs after experimental infection were less severe in 1-day-old calves than in 6-week-old calves, apparently due to a less robust inflammatory response 33

in younger calves.³³ Ruminants appear to be susceptible to reinfection with BRSV throughout their lives, and adult cattle can develop severe disease after BRSV infection.^34,35 Because genetic variability among BRSV isolates is not great, reinfection does not seem to be largely due to viral strain variation. It appears that natural infection does not confer lifelong immunity, and some cattle may be reinfected annually,³⁶ although clinical signs are not always apparent.

Although host immunity can protect ruminants from severe disease after BRSV infection, the host immune response can also contribute to disease. Cattle with severe disease after natural infection have a higher proportion of degranulated mast cells in their lungs than less severely affected cattle,³⁷ indicating that mast cell mediators contribute to physiologic and pathologic changes seen in severe cases. In experimentally challenged cattle, BRSV-specific IgE has been associated with disease severity.³⁸ Exposure to the fungus Saccharopolyspora rectivirgula (formerly M. faeni), the spores of which induce pulmonary hypersensitivity in cattle and other species, enhanced BRSV- specific IgE production in calves,³⁸ indicating that environmental allergens can affect the immune response to BRSV infection. Calves infected with BRSV before infection with Histophilus somni developed higher levels of H. somni-specific serum IgE than did calves infected with H. somni alone.³⁹ Thus BRSV infection can lead to production of virus-specific IgE, which may be enhanced by coexposure to allergens, and BRSV infection may also increase IgE production in response to infection with other agents. Crosslinking of IgE bound to mast cells by binding of BRSV or other specific antigen is expected to lead to mast cell degranulation and subsequent broncho­constriction, pulmonary edema, and other sequelae of mast cell mediator release and immediate hypersensitivity.

Other evidence that BRSV can trigger immunopathogenesis is the fact that certain BRSV vaccines have on rare occasion been shown to cause enhanced disease when vaccinated animals are subsequently infected with BRSV. Vaccine-enhanced disease has been seen after both natural^40,41 and experimental infec­tion.^42,43 It appears that the dose of BRSV protein, the adjuvant included in the vaccine, and probably also other factors related to the formulation of the vaccine and the genetics of vaccinated cattle determine whether enhanced disease is likely after infec­tion of cattle given a particular BRSV vaccine. Apparent vaccine-enhanced disease was reported in cattle that received a modified live vaccine in the early stage of a natural BRSV outbreak, which suggests that BRSV vaccination in the face of a BRSV outbreak could be harmful.⁴⁰ It is important to note that BRSV vaccines are commonly used, and vaccine- enhanced disease is rarely reported; thus vaccination is still recommended as part of any plan to control disease caused by BRSV. Many gaps exist in our understanding of what aspects of the immune response contribute to disease severity after BRSV infection or vaccination. Most research completed to date has focused on enhanced disease in calves that received formalin-inactivated BRSV. The data suggest that when enhanced disease occurs in calves vaccinated with formalin- inactivated BRSV, it is related to a relatively strong response by T-helper type 2 (Th2) cells, as evidenced by decreased production of the Th1 prototype cytokine interferon gamma,⁴⁴ increased production of BRSV-specific IgE,⁴⁵ and pulmonary eosinophil infiltration.⁴² The association of BRSV-specific IgE production with disease severity in some experimental studies indicates that at least some individual ruminants will produce IgE after BRSV vaccination or infection, which may contribute to severe disease in these individuals.

The presence of co-infection with other pathogens such as M. haemolytica or bovine viral diarrhea virus (BVDV) can also increase the severity of disease after BRSV infection. Although genetic variation among BRSV isolates is not extensive, experimental challenge with certain isolates of BRSV can lead to serious disease, whereas challenge with other isolates leads to only minimal disease. This indicates that, despite limited differences among viral isolates at the genetic level, variation among BRSV isolates may still contribute to variations in disease severity.

■ Epidemiology Prevalence of antibodies to BRSV in the cattle population of the United States ranges from approximately 60% to 80%. Seropositive sheep, goats, and deer are commonly identified in domestic or wild populations, confirming that infection occurs in these species. Cattle of any age are susceptible to disease following BRSV infection. BRSV was demonstrated to be involved in 14% to 71% of respiratory disease outbreaks in North American and European studies of calf pneumonia outbreaks involving several farms. In a recent survey of more than 1300 dairy herds in Norway, herd size, geographic location, and closer proximity to neighboring herds were related to increased risk for a herd to have evidence of BRSV exposure, based on antibodies in bulk tank milk.⁴⁶ Herds that were positive for BRSV antibodies in bulk tank milk were more likely also to be positive for antibodies to bovine coronavirus. Seroconver­sion to BRSV has been significantly associated with treatment for respiratory disease in feedlot cattle,⁴⁷ and cattle with low antibody titers to BRSV at feedlot entry have increased risk of developing disease.⁴⁸ Although feedlot cattle are commonly infected with BRSV after arrival, a survey of necropsy findings at 72 Canadian feedlots found BRSV at postmortem of only 11% of the cattle that died or were euthanized because of pneumonia.⁴⁹ The low rate of isolation at necropsy suggests that if the virus contributes to disease in feedlot cattle, it is often no longer present by the time the animal dies or is euthanized. In general, morbidity rate tends to be high in outbreaks of BRSV, whereas case fatality rate is variable, ranging from 0% to as high as 20%.⁵⁰

Cattle are most likely the principal reservoirs of infection, although a European serologic survey of different species found that goats were also often seropositive,²² indicating that goats may be a source of BRSV infection for cattle, and vice versa. The mechanism by which BRSV persists in the cattle population is not known. Possibly a similar epidemiologic pattern to that described for HRSV also exists for BRSV HRSV is capable of reinfecting the host throughout his or her life, with the most severe disease occurring in the very young or elderly. Subsequent exposure results in mild upper respiratory tract disease. Similarly, adult cattle may periodically undergo subclini- cal to mild infections and serve as a source of infection for susceptible young stock. However, one study concluded that transmission among seropositive cattle was not a plausible mechanism of BRSV persistence in a dairy herd.⁵¹ These authors suggested that persistent BRSV infection in individuals is a more plausible explanation of population persistence of BRSV BRSV has been identified in B lymphocytes in tracheobronchial and mediastinal lymph nodes of calves 71 days after experimental infection,⁵² so it may be that individual cattle harbor the virus long term and periodically begin shedding it, allowing it to periodically reappear in herds even if they are closed to new introductions.

■ Necropsy Findings Grossly, in cattle that die or are euthanized after BRSV infection, individual cranioventral lobules may appear dark and collapsed because of atelectasis or consolidation. Dorsocaudal lung often fails collapse and may have a rubbery texture suggesting AIP. In severe cases, interstitial or bullous emphysema is often present in the dorsocaudal lung (Color Plate 31.3). Pathologic emphysema must be differentiated from that sometimes caused by agonal breathing of cattle. Histologic lesions depend on the stage of infection. Neutrophilic and later mononuclear bronchitis, bronchiolitis, and alveolitis are present in infected animals. Syncytial cells may be seen in the airways or alveoli; intracy- toplasmic inclusion bodies are rarely present. BRSV infection can cause lesions consistent with AIP; therefore differential diagnoses for the typical gross lesions include other causes of AIP (see the Feedlot Acute Interstitial Pneumonia section later in this chapter). Histologic changes consistent with AIP include alveolar epithelial hyperplasia, hyaline membrane formation, and interstitial inflammatory cell infiltrate, hemorrhage, and edema. Later, evidence of chronic bronchitis and bronchiolitis obliterans can be found.

■ Diagnosis Infection is diagnosed by identification of the virus in nasal secretions, tracheal aspirates, or lung lavage fluid from live calves or in lung tissue collected postmortem. BRSV is difficult to isolate, as it does not survive transport well; thus methods of identification that do not require the virus to be alive (such as IFA, IHC, and RT-PCR) are preferable to virus isolation for the identification of BRSV IHC of formalin-fixed tissue is convenient because the virus can be identified in tissue processed for histopathologic evaluation. Although virus is readily identifiable in the first days after infection, virus is less likely to be found by 10 to 15 days postinfection, even when lung tissue collected postmortem is tested by IHC.

Seroconversion as evidenced by paired serology also supports a diagnosis; virus neutralization or ELISA assays are most commonly used to identify BRSV-specific antibody. In live cattle, the arterial partial pressure of oxygen (PaO₂) is highly negatively correlated with the extent of lung pathology.⁵³

■ Treatment and Prevention Treatment of BRSV is supportive and aimed at limiting the inflammatory response in bronchioles and alveoli and preventing secondary bacterial infection. Antiinflammatory therapy with NSAIDs (such as flunixin meglumine at 1.1 to 2.2 mg/kg IV daily or divided twice daily) is considered appropriate. Administration of one or two doses of steroid therapy (dexamethasone, 0.05 to 0.2 mg/ kg IV or IM once or twice) is appropriate in selected individuals with severe respiratory distress or evidence of AIP. Antimicrobi­als to treat or prevent secondary bacterial infection may be indicated, although these are not invariably required. For rare individuals with severe signs suggesting AIP, insufflation with intranasal oxygen, if available, and diuretic therapy with furosemide (0.5 to 1 mg/kg IM or IV once or twice daily) may be helpful. Although the prognosis for animals with uncom­plicated BRSV infection is good, the prognosis for animals with AIP is guarded. Animals with severe respiratory distress should be handled with care, as even careful manipulation to administer treatment can lead to rapid respiratory decompensa­tion and death.

Both modified live and inactivated BRSV vaccines for intramuscular or subcutaneous administration are commercially available, as are modified live multivalent vaccines for intranasal administration. More information on the use of vaccines to prevent disease due to BRSV is presented in Chapter 48. There is no respiratory syncytial virus vaccine licensed for use in small ruminants, although BRSV vaccines ares sometimes administered to sheep or goats in an extralabel manner. Controlled studies of the effects of BRSV vaccines licensed for cattle on respiratory disease in sheep or goats have not been published.

Bovine Viral Diarrhea Virus

In addition to contributing directly to bovine respiratory disease by infecting the respiratory tract and enhancing the pathogenicity of co-infecting bacteria or viruses, BVDV may also contribute to respiratory disease by impairing the ability of cattle to respond properly to vaccination against other respiratory pathogens. Calves infected with BVDV before BHV-1 vaccination shed BHV-1 longer after subsequent BHV-1 infection than calves that were not co-infected with BVDV In another study, calves persistently infected with BVDV failed to develop a serologic response to a M. haemolytica vaccine, in contrast to control animals not infected with BVDV

In summary, BVDV infection is significantly associated with the development of bovine respiratory disease in some cases. Although experimental challenge studies indicate that BVDV may cause mild respiratory disease when acting alone, the virus appears to contribute most importantly by impairing the host's ability to resist infection and limit disease caused by other pathogens.

Bovine Parainfluenza Virus 3

■ Definition and Etiology Bovine parainfluenza virus 3 (PIV-3) (also known as bovine respirovirus 3) is an enveloped RNA virus classified in the family Paramyxoviridae, genus Respirovirus. The virus has been associated with respiratory tract disease in cattle, sheep, and goats. It hemagglutinates and hemadsorbs red blood cells of certain species, which means that serum antibodies can be identified by hemagglutination inhibition assays. Variation in virulence among strains of PIV-3 has been reported, with types A, B, and C being characterized. Naturally circulating isolates appear to be diverging genetically from strains in vaccines, likely due to immune pressure associ­ated with vaccination.^62,63 Significant similarities between bovine PIV-3 and human PIV-3 have led to efforts to use bovine PIV-3 as a modified live intranasal vaccine for humans.

■ Clinical Signs Uncomplicated PIV-3 infections result in mild signs of disease at most. Clinical signs, if present, may include fever, cough, nasal and ocular discharge, increased respiratory rate, increased bronchovesicular sounds, and wheezes. The most important role of PIV-3 is in predisposing the respiratory tract to subsequent infection by other viruses and bacteria such as Mannheimia (Pasteurella) haemolytica. Severity of signs increases with the development of secondary bacterial pneumonia, and if death occurs it is usually the result of secondary bacterial infection. Infection with PIV-3 is widespread in sheep and goats. Only one serotype of ovine PIV-3 has been identified, and it is related to but distinct from the bovine strain. Most infections are inapparent to mild.

■ Pathogenesis Infection with PIV-3 can lead to signs referable to both upper and lower respiratory tract infection. After experimental infection of calves, clinical signs are evident by 2 days post infection, and signs peak at 4 days post infection. Virus is found in the nasal passages, trachea, and bronchiolar and alveolar epithelial cells. The virus damages the pulmonary mucociliary apparatus and depresses several important func­tions of alveolar macrophages such as Fc receptor expression, phagocytosis, and microbicidal activity, which likely predisposes infected animals to secondary bacterial pneumonia.

■ Epidemiology The widespread prevalence of antibodies to PIV-3 indicates that it commonly circulates in ruminant populations. A recent survey of unvaccinated ungulates in one U.S. state indicated that circulating PIV-3 strains may be diverging antigenically from the strains included in vaccines. Inapparent or subclinical infections with PIV-3 are common; in one report, 28 groups of calves seroconverted to PIV-3 over 8 months, but respiratory disease was seen in association with PIV-3 infection in only four of these groups. Infection appears to spread rapidly in susceptible cattle housed at high population densities and in close contact. Feedlot cattle commonly seroconvert to PIV-3 soon after feedlot arrival, and seroconver­sion has been associated with treatment for respiratory disease in some⁴⁸ but not other⁴⁷ cases. In spite of the fact that feedlot cattle frequently become infected with PIV-3 after feedlot entry, a survey of necropsy findings at 72 Canadian feedlots found PIV-3 at postmortem examination in only 4% of the cases with pneumonia.⁴⁹ The low rate of isolation at necropsy suggests that if the virus contributes to the development of pneumonia, it may no longer be present by the time the animal dies.

■ Necropsy Findings Lesions of PIV-3 infection alone are rarely seen during postmortem examination of naturally affected cases. Experimental PIV-3 infection results in conges­tion of the respiratory mucosa, swelling of lymph nodes associated with the respiratory tract, and lobular consolidation concentrated in the cranioventral lung. Bronchiolitis and alveolitis are seen histologically with both proliferative and degenerative changes in the epithelial cells of the bronchioles and alveoli. Syncytia and intranuclear and intracytoplasmic inclusion bodies may be seen. In many respects, pathologic features of experimental PIV-3 infection are similar to but more mild than those caused by BRSV

■ Diagnosis PIV-3 can be identified by virus isolation or RT-PCR of nasal swabs from infected animals. Unlike BRSV, which is also a paramyxovirus, PIV-3 is not particularly difficult to isolate. Diagnosis can also be confirmed with paired serology, with virus neutralization or hemagglutination inhibition assays most commonly used.

■ Treatment and Prevention There is no specific treat­ment for infection with PIV-3; as described for BHV-1 and BRSV, supportive care is indicated. Other management practices to prevent or control respiratory disease in calves or feedlot cattle are also appropriate.

Both inactivated and modified live multivalent vaccines containing PIV-3 with other viruses are available for parenteral administration, and modified live vaccines containing PIV-3 and BHV-1, with or without BRSV, are available for intranasal administration; these vaccines are labeled for use in cattle but not in sheep or goats. More information regarding the use of vaccines to control disease due to parainfluenza virus infection is presented in Chapter 48.

Bovine Coronavirus associated with BCoV infection are usually due to a single dominant genotype. Recent studies in Scandinavia indicate that strains circulating on farms in a region can be genetically closely related, with more than 95% similarity in the sequence of the S gene.

Antibodies to older BCoV isolates can cross-react with more recent isolates but incompletely; thus animals vaccinated with the modified live vaccine for control of calf diarrhea may not be completely protected from newer genetically divergent isolates.

■ Clinical Signs Infection with BCoV has been associated with outbreaks of calf diarrhea (Chapter 20) and winter dys­entery (Chapter 32), as well as respiratory disease in dairy and beef calves and feedlot and stocker cattle. Although it has long been known that BCoV can be identified on nasal swabs col­lected from calves with signs of respiratory disease or diarrhea, the role of BCoV as a “cause” of bovine respiratory disease has been controversial. Calves with diarrhea due to BCoV infection can shed the virus in nasal secretions as well as feces (though not always from both sites at the same time) in the absence of signs of respiratory disease. Moreover, experimental infection of seronegative naive calves can reliably cause diarrhea but inconsistently leads to respiratory disease, with respiratory signs being mild at most. This failure to clearly fulfill Koch's postulates has contributed to skepticism regarding BCoV as a cause of bovine respiratory disease. However, one group of researchers used Evans' criteria of causation to support a role for BCoV in feedlot respiratory disease. A relevant role for BCoV in bovine respiratory disease has also been inferred by identification of BCoV in multiple animals during acute respiratory disease outbreaks,^72,73 by identification of BCoV infection or seroconversion as a significant risk factor for respiratory disease in individuals or herds,^74-76 or by finding cattle with preexisting serum antibodies to BCoV to be protected from respiratory disease when seronegative cattle are not. However, other studies have failed to identify a relationshiop between BCoV infection or seroconversion and respiratory disease. At this time it seems that BCoV is likely a contributor to bovine respiratory disease outbreaks in some cases, but finding the virus may not mean that it is the primary cause of disease. Variation in respiratory virulence among BCoV strains may be important, but if so the features that make a strain more or less likely to cause respiratory disease have not yet been defined.

Clinical signs of respiratory disease attributed to BCoV are nonspecific and may include fever, nasal discharge, rapid respiratory rate, and coughing. Rectal temperatures may be high (up to 107° F [41.7° C]). Although mortality rates in uncomplicated outbreaks are usually low, secondary bacterial infection due to Mannheimia haemolytica, Histophilus somni, or other opportunistic respiratory pathogens can lead to fatal disease in multiple individuals.^78,79 Coinfection with other respiratory viruses may also occur.

■ Pathogenesis The lack of a reliable experimental model of respiratory disease following BCoV infection, as well as the fact that fatal, naturally occurring cases are typically affected by secondary bacterial infection, makes it difficult to clearly define the pathogenesis of respiratory disease specifically due to BCoV infection. The virus infects respiratory epithelial cells, presumably inducing a host response that leads to clinical signs of airway inflammation. Infection of respiratory epithelial cells may increase susceptibility of cattle to secondary bacterial infection through inhibition or the mucociliary elevator.

■ Epidemiology Respiratory disease has been attributed to BCoV infection in dairy calves, preweaning beef calves, and feedlot and stocker cattle. Coronaviruses similar to BCoV have also been identified in wild ruminants, cervids, and camelids,8⁰ suggesting that cross-infection among different species may be possible. In dairy calves and preweaning beef calves the virus appears to contribute to enzootic pneumonia as well as to epizootics (outbreaks) of variable morbidity and usually low mortality. Two naturally occurring outbreaks of respiratory disease (shipping fever) attributed to BCoV in feedlot cattle were described in a detailed report. In these outbreaks, characterized by high morbidity and mortality, BCoV was isolated from the nasal passages of more than 80% of the cattle in the early stages of the outbreaks. No other respiratory viruses were identified in most of the cattle, but M. haemolytica was also isolated from a majority of cattle as the two epizootics progressed. In another report, calves with low antibody titers to BCoV at feedlot entry were more likely to be treated for respiratory disease than calves entering the feedlot with higher titers.⁷⁷ Other researchers have commonly found the virus when cattle are sampled soon after feedlot entry. Moreover, it is common for a majority of cattle to have serum antibody titers to BCoV at feedlot arrival or to seroconvert soon after arrival; therefore it is clear that cattle are often infected with BCoV shortly before or after feedlot entry.^81-83 However, seroconversion or viral shedding has not always been significantly associated with treatment for respiratory disease.

Surveys of antibodies in pooled milk samples or bulk tank milk on dairies indicated that large herd size, geographic location, and closer proximity to neighboring herds were risk factors for BCoV exposure.^46,84 Dairy herds positive for BCoV antibodies in bulk tank milk were more likely also to be positive for antibodies to BRSV Genetically identical isolates can be isolated from different dairies in a region in the same period, suggesting that the BCoV may transfer between some farms by the sale of cattle or on fomites.⁷¹ However, it has also been shown that farms in an area may remain free from BCoV infection when the virus is circulating on nearby farms.

■ Necropsy Findings Following experimental infection, BCoV can be found in epithelial cells of the nasal passages and lung. Grossly, small areas of cranioventral lobular collapse may be seen. In naturally occuring cases of fatal disease, pathologic changes consisent with secondary bacterial infection will likely obscure any changes primarily attributable to BCoV.

■ Diagnosis BCoV is most commonly identified by PCR in respiratory secretions or tissues collected at postmortem. The virus is not easy to isolate; standard cell lines used for isolation of other respiratory viruses are often not permissive to BCoV, but the virus can be isolated on human rectal tumor- 18G cell lines.

■ Treatment and Prevention As for other respiratory viruses, there is no specific treatment for BCoV infection; supportive care as described for BHV-1 and BRSV is indicated. Although licensed vaccines labeled for the prevention or control of respiratory disease due to BCoV are not currently marketed in the United States or Canada, vaccines labeled for control of calf diarrhea due to the virus are available. A research trial tested the extralabel intranasal administration of a modified live bovine coronavirus and rotavirus vaccine (Calf-Guard [Zoetis, Parsippany, N.J.]) to control respiratory disease in feedlot cattle. Vaccination significantly decreased the subse­quent rate of treatment for respiratory disease in cattle that had relatively low serum antibody titers against coronavirus at arrival. Recently a modified live coronavirus vaccine was approved for intranasal adminstration to aid in the reduction of enteric disease due to BCoV (Bovilis Coronavirus [Merck Animal Health, Madison, N.J.]). Anecdotal reports indicate that this vaccine is also administered to control or prevent outbreaks of respiratory disease, an extralabel use; no published clinical trials have yet described benefits of this application.

Infectious BCoV can be identified on boots and clothing of personnel 24 hours after they have handled infected calves, indicating that fomites could be a means of transmission. Thus boots and other fomites should be disinfected or changed after examining calves or cattle likely to be infected with BCoV. Other management practices to prevent or control respiratory disease in calves or feedlot cattle are also appropriate.

Influenza Viruses

Influenza viruses are enveloped RNA viruses in the family Orthomyxoviridae. Currently there are four genera in this family: Alphainfluenzavirus, Betainfluenzavirus, Gammainfluenzavirus, and Deltainfluenzavirus, which contain the influenza A, B, C, and D viruses, respectively. Influenza A viruses (IAV) circulate widely in various avian reservoir hosts and cause important seasonal disease in humans; IAV also cause important disease in poultry, swine, horses, and dogs. With IAV, influenza B viruses (IBV) contribute to seasonal disease in humans; influenza C viruses (ICV) primarily cause respiratory infection in young children. Humans appear to be the primary reservoir hosts for IBV and ICV, but infection with either IBV or ICV has occasionally been identified in other species. Antibodies to IAV, IBV, and ICV have been identified in cattle, indicating that cattle can be infected with these viruses.^90-92 Seroconversion to IAV has followed outbreaks of respiratory disease in cattle when no other respiratory pathogens could readily be found, suggesting that IAV infection may rarely cause disease in cattle. However, only IDV have been commonly identified in surveys of respiratory viruses in cattle.

Influenza D Virus survey testing 35 people who have regular contact with cattle and 11 people who do not found 91% of the people with cattle contact to be seropositive versus 18% of the people without cattle contact.¹⁰¹ Thus the information available to date suggests that humans are not a reservoir for IDV, but they are susceptible to infection, which may occur commonly in individuals with frequent contact with cattle. This is relevant because of the known possibility of species to share influenza viruses, with reassortment of gene segments leading to development of new, possibly more virulent variants. The importance of this in the ecology of IDV is not yet known.

The importance of IDV as a contributor to bovine respiratory disease is still under investigation. Seroconversion has been associated with development of bovine respiratory disease,⁹⁴ and IDV has been found in materials collected from cattle with respiratory disease.^94,95,102 Viral metagenomic sequencing of nasal or nasopharyngeal swabs identified IDV as significantly associated with respiratory disease in dairy calves,¹⁰³ and IDV tended to be associated with respiratory disease in feedlot cattle.¹ When IDV is found in respiratory secretions or tissues of cattle with naturally acquired disease, it is common to find other respiratory pathogens also present, making it difficult to particularly ascribe disease to IDV alone. IDV and BCoV, another virus commonly isolated from cattle during respiratory disease outbreaks, can both use the same cellular receptor, 9-0-acetyl-V-acetylneuraminic acid (Neu5,9Ac2), to attach to and enter cells. The significance of this in the ecology of these viruses in cattle populations is not clear.

Experimental challenge of seronegative cattle with IDV alone has induced disease that is mild at most, with viral replica­tion and inflammation generally confined to the nasal passages and trachea.^104,105 In this regard, IDV is also similar to BCoV, which is commonly identified in cattle with respiratory disease but causes little if any respiratory disease when seronegative cattle are purposely exposed to the virus alone. This is currently interpreted to indicate that infection with IDV alone is unlikely to cause identifiable respiratory disease in cattle, but IDV infection can contribute to disease when it occurs concurrently with other factors that that impair or challenge respiratory defense. For example, IDV infection of nasal and trachea epithelial cells may impair function of the mucociliary elevator, increasing the likelihood of secondary bacterial infection by M. haemolytica or other opportunists.

No vaccine for IDV is currently marketed. An experimentally developed inactivated vaccine decreased clinical signs and virus shedding following experimental challenge of cattle.¹⁰⁵

Bovine and Ovine Adenovirus

Adenoviruses are nonenveloped DNA viruses of the family Adenoviridae. At least 10 serotypes of bovine adenovirus (BAV) have been identified. BAV infection is widespread and frequently subclinical; adenoviruses are also often isolated in association with other viruses and bacteria, making it difficult to assign causation in naturally occurring disease outbreaks. However, a recent survey of viral metagenomes in dairy calves with respiratory disease recently identified adenoviruses as one of the viruses significantly associated with respiratory disease.¹⁰³ Adenovirus infection has also been associated with enteritis, conjunctivitis, keratoconjunctivitis, and “weak calf syndrome.”

At least six antigenic types of ovine adenovirus and two types of caprine adenovirus have been identified. It appears likely that these viruses cause widespread infection. The majority of isolations of adenovirus have been from young lambs, and it has been isolated in association with respiratory and enteric disease. Experimental infections result in mild disease with anorexia, pyrexia, increased respiratory rates, coughing, and diarrhea. Ovine adenovirus serotype 6 has been shown under experimental conditions to act synergistically with M. haemolytica in the production of pneumonia in lambs.

Bovine Rhinitis Virus

Bovine rhinitis virus is a nonenveloped RNA virus in the family Picornaviridae. There are two serotypes of bovine rhinitis virus: bovine rhinitis A virus and bovine rhinitis B virus. A recent surveys of viral metagenomes from nasal or nasopharyngeal swabs from dairy calves with and without clinical signs of respiratory disease found bovine rhinitis A virus to be signifi­cantly associated with disease.¹⁰³ A similar study that evaluated viral metagenomes in U.S. and Mexican feedlot cattle with and without signs of respiratory disease found the viruses to be present in nearly all feedlots sampled, but their presence was not significantly associated with clinical signs of respiratory disease.¹

Bacterial and Chlamydial Agents

■ Mannheimia haemolytica

Definition and Etiology

M. haemolytica is a gram-negative aerobic bacteria of the family Pasteurellaceae. There are at least 12 serotypes of M. haemolytica, and some ruminant isolates are untypable with currently available laboratory tools. Some serotypes of M. haemolytica are pathogenic, whereas others are nonpathogenic commensals of the ruminant nasopharynx, and the serotypes that are pathogenic for cattle are not the same as those that are pathogenic for sheep or goats. Serotype A1 is the most common isolate from pneumonic lungs of cattle, and serotype A6 is the next most common; serotype A2 is the most common isolate from the nasopharynx of normal cattle. Serotype is also relevant to pathogenicity of isolates from sheep and goats; serotype A2 is the most common isolate from sheep and goats with pneu­monia, and serotypes A7, A9, and several others have also been associated with disease.

■ Clinical Signs Cattle, sheep, or goats infected with M. haemolytica display a dull or depressed attitude and lose interest in eating. Fever, tachypnea, and depression are often the only abnormal signs early in the course of infection; coughing is not a prominent sign in the acute stage of disease unless co­infection with another agent such as BHV-1 or BRSV is present. Animals may display evidence of thoracic pain, such as standing with elbows abducted or catching the breath before expiration; these signs are a result of the painful fibrinous pleuritis caused by M. haemolytica. Because M. haemolytica is a gram-negative organism, endotoxin (lipopolysaccharide) is produced by the agent, and thus cattle with mannheimiosis may show signs of endotoxemia, including fever, tachycardia, tachypnea, salivation, respiratory distress, pale or dark mucous membranes with prolonged capillary refill time, and cool extremities. Thoracic auscultation may reveal harsh or loud bronchovesicular sounds consistent with pulmonary consolidation, particularly over the cranioventral lung. Disease after M. haemolytica infection can be fatal, particularly if it is not treated; it can also lead to chronic pneumonia associated with secondary invasion with agents such as Pasteurella multocida, Mycoplasma bovis, or Trueperella pyogenes.

It is important to remember that naturally occurring disease caused by M. haemolytica is commonly preceded by infection with a viral agent such as BHV-1, BRSV, or BCoV; therefore the clinical signs described previously for these or other respiratory viruses may also be present in animals infected with M. haemolytica.

■ Pathogenesis M. haemolytica is a normal inhabitant of the nasal and pharyngeal mucosa and is considered an opportunistic pathogen. Ruminants appear to become infected at an early age and carry Mannheimia as a minor part of the upper respiratory tract flora. Only a small percentage of nasal swabs yields positive results for M. haemolytica serotype A1 in healthy, unstressed calves,¹ but if several areas of the nasal mucosa are cultured at necropsy, it is often possible to isolate M. haemolytica from animals that previously had negative findings on nasal swabs.² The stress of transportation or viral infection causes a breakdown of the defense mechanisms that limit normal bacteria flora, resulting in a rapid proliferation of virulent M. haemolytica serotype A1. The number of calves yielding a positive nasal swab M. haemolytica culture increases after transport, and there is a large increase in the numbers of M. haemolytica in positive samples.¹ BHV-1 and PIV-3 viruses have been shown to have the same effect as transportation on Mannheimia populations of the nasal mucosa. M. haemolytica has been demonstrated in the tracheal air of stressed, healthy calves harboring the organism on their nasal mucosa. Some of these inhaled organisms are deposited deep within the lung and normally are cleared within hours. However, under conditions of impaired pulmonary defenses caused by insults to the upper respiratory tract, preexisting viral infection, or other factors, M. haemolytica is able to proliferate rapidly within the lung and, with the aid of its virulence factors and toxins, to produce a severe lobar necrotizing fibrinous pleuropneumonia. Calves infected with respiratory viruses or Mycoplasma species have increased susceptibility to severe bronchopneumonia when exposed to M. haemolytica. Similarly, infection of lambs with PIV-3 or adenovirus followed in several days by M. haemolytica causes severe pneumonia.³

Once M. haemolytica becomes established in the lungs, interactions between the bacteria and the host defenses result in tissue damage and elimination of the bacteria. A major virulence factor of M. haemolytica is an exotoxin that is lethal to ruminant leukocytes, the leukotoxin. Leukotoxin, which is produced by M. haemolytica during the logarithmic phase of growth, causes cytolysis of ruminant platelets, lymphocytes, macrophages, and neutrophils.⁴ There is diversity in the gene encoding the leukotoxin molecule among M. haemolytica serotypes, and genetic diversity is related to differences in toxicity. Leukotoxin binds to cells via CD18, the beta subunit of the β₂ integrins CD11a∕CD18 (LFA-1), CD11b∕CD18 (Mac-1), and CD11c∕CD18 (CR-4); although leukotoxin can bind to cells from nonruminant species, only ruminant cells are killed by the toxin. Contact with leukotoxin increases expression of LFA-1 on ruminant leukocytes, making the cells even more sensitive to injury by leukotoxin. At low concentra­tions, leukotoxin induces leukocyte death by apoptosis, whereas at higher concentrations the toxin causes cell lysis.⁵ M. hae­molytica expressing a mutant nontoxic leukotoxin induced less lung pathology in calves as compared with M. haemolytica producing functional toxin, but clinical signs were not different between the two groups of calves, indicating that leukotoxin is not the only virulence factor of importance in causing disease.⁶

Another virulence factor that contributes to disease resulting from M. haemolytica is lipopolysaccharide, or endotoxin. As in other species, exposure of ruminants to endotoxin from gram-negative bacteria induces a multitude of responses leading to inflammation, including initiation of the complement and coagulation cascades, activation of endothelial cells and recruit­ment of neutrophils, and activation of neutrophils and alveolar macrophages, leading to their production of proinflammatory cytokines, which further amplify the inflammatory response. The proinflammatory cytokines TNF-α, IL-1β, and IL-8 are expressed in the lungs of cattle within 48 hours of M. haemolytica infection.⁷ Calves exposed to a preparation containing M. haemo- lytica endotoxin develop leukopenia, fever, tachypnea, diarrhea, and dyspnea as early as 2 hours after exposure.⁸ Endotoxin potentiates the effects of leukotoxin by inducing increased expression of β₂ integrins on leukocytes, which contain CD18, the receptor for leukotoxin. Bovine alveolar macrophages first exposed to endotoxin were susceptible to death induced by concentrations of leukotoxin too low to cause cell death alone, and macrophages exposed to both endotoxin and leukotoxin produced more of the proinflammatory cytokines IL-8 and TNF-α than did macrophages exposed only to leukotoxin.⁹ These data indicate that leukotoxin and endotoxin work in synergy to cause disease in ruminants. The production of IL-8 is of particular importance because IL-8 is a major inducer of neutrophil chemotaxis. The massive influx of neutrophils induced by IL-8 and other inflammatory mediators is a key factor in lung tissue destruction caused by M. haemolytica. Calves experimentally depleted of neutrophils can be com­pletely protected from gross and microscopic lesions of the severe fibrinonecrotic bronchopneumonia that is induced by intratracheal inoculation of M. haemolytica in calves with normal neutrophil levels.¹⁰ Lysis of neutrophils results in the release of lysosomal products, including elastase, collagenase, and reactive oxygen intermediates. These chemicals are bactericidal but also capable of destroying the neutrophils themselves and surround­ing tissues. Neutrophil-mediated damage to the endothelial cells results in exudation and thrombosis, which produce the classic lesions of necrosis and fibrinous exudation.

In addition to leukotoxin and endotoxin, M. haemolytica possesses other factors that contribute to its virulence. The bacteria have a polysaccharide capsule that aids in attachment and prevents phagocytosis by neutrophils and iron-regulated outer membrane proteins (IROMPs) that bind transferrin and alter the function of neutrophils. The bacteria produce adhesins that mediate attachment to host cells, and neuraminidase produced by the bacteria may aid in host colonization by decreasing the viscosity of respiratory mucus and decreasing the repellant negative charge on host cells by cleaving sialic acid residues.

■ Epidemiology In many studies over several decades, M. haemolytica has been found to be a common bacterial isolate from feedlot cattle with fatal fibrinous bronchopneumonia.^11,12 M. haemolytica was isolated postmortem from 25% to 30% of cattle that died or were euthanized because of pneumonia in two studies evaluating causes of mortality in feedlot cattle.^12,13 An assessment of bacteria isolated from transtracheal aspirates of live feedlot cattle with respiratory disease and healthy cattle found that M. haemolytica was significantly more likely to be isolated from aspirates from cattle with BRD, compared to healthy controls.¹³ In contrast, an older study that used BAL to identify bacteria present in the lungs of feedlot cattle before antimicrobial treatment for acute pneumonia found that M. haemolytica was not identified more frequently in the lungs of cattle with pneumonia than in normal controls, whereas P. multocida was significantly associated with bronchopneumonia.¹⁴ In this study, none of the calves died or were euthanized because of the naturally occurring disease, indicating that disease was not severe. Because M. haemolytica is relatively more likely to cause acute fatal pneumonia than other bacterial pathogens of the ruminant lung, it may be overrepresented in necropsy surveys in which only fatal cases of disease are sampled. Seroconversion to M. haemolytica or M. haemolytica leukotoxin has been significantly associated with treatment for respiratory disease or “undifferentiated fever” in feedlot cattle in some^15,16 but not others.¹⁷

In contrast to the situation in feedlot cattle, M. haemolytica is less commonly associated with pneumonia in dairy calves. In one study, M. haemolytica was less frequently isolated from guarded nasopharyngeal swabs collected from dairy calves with acute respiratory disease than P. multocida or Mycoplasma spp.¹⁸ Isolation of M. haemolytica on nasopharyngeal swabs was not significantly associated with clinical signs of respiratory disease or pulmonary consolidation identified by transthoracic ultra­sound in dairy calves in another study.¹⁹ Magnitude of serum antibody titers, or changes in serum antibody titers, to M. haemolytica were not significantly associated with treatment for respiratory disease in a third report.²⁰ Although M. hae- molytica is not as commonly associated with respiratory disease in dairy calves as other bacteria, it can be a cause of clinical and sometimes fatal bronchopneumonia in dairy calves, as well as in preweaning beef calves, adult dairy or beef cows, sheep, or goats.

■ Necropsy Findings Infection with M. haemolytica causes grossly evident fibrinopurulent bronchopneumonia (Color Plate 31.4). The infection is aerogenous, so disease occurs primarily in the cranioventral lung, but in severe cases caudoventral and even dorsal lung may be affected. Affected areas of lung are dark red to purple or gray-brown, firm, and heavy; discolored areas may be wedge-shaped owing to thrombosis of a vessel supplying the affected region during the severe inflammatory response. The interlobular septa are expanded by clear to yellow gelatinous material that represents proteinaceous fluid that has leaked from blood vessels in the lung. The inflamed areas of lung may be covered with yellow fibrin that may adhere to the pleura of the thoracic wall, and the pleural cavity usually contains straw-colored fluid, which may be voluminous. In cases that have been ongoing for a few days, firm, gritty lumps that are dry and crumbly on sectioning may be identified; these represent areas of necrotic tissue. M. haemolytica may also produce pneumonia characterized by firm, dark red pulmonary consolidation without fibrinous pleuritis; this is especially common in dairy calves or sheep and goats. Animals that survive the acute stage of pneumonia caused by M. haemolytica may have multiple abscesses and pleural adhesions; in these cases other bacteria such as P. multocida, Mycoplasma bovis, or Arcanobacterium pyogenes may be contributing to pathology.

Histopathologic evaluation of tissues from cattle with pneumonia due to M. haemolytica reveals that alveoli are filled with edema and fibrin and that there is massive infiltration of neutrophils and macrophages. Foci of coagulative necrosis may be present; hemorrhage is also often present both within and between alveoli. Bronchioles are filled with leukocytes and may have foci of epithelial necrosis.

■ Diagnosis Diagnosis of pneumonia due to M. haemolytica is most reliably made by culture of lungs with typical gross pathology at postmortem evaluation of animals that die or are euthanized as representative cases in outbreaks. Once bacteria colonies are isolated, MALDI-TOFMS or traditional biochemi­cal methods can be used to identify the bactera; MALDI- TOFMS can also be used to identify the serotype.²¹ Some laboratories offer PCR testing to identify the bacteria, which may be more sensitive than culture.²²

A presumptive antmortem diagnosis in individual animals with clinical signs of bacterial pneumonia can be made by identification of the bacteria by aerobic culture of nasal swab, guarded nasopharyngeal swab, transtracheal aspirate (TTA), or BAL. There is good to very good agreement between the results of culture of nasal or nasopharyngeal swabs and TTA or BAL at the level of the genus and species in calves with respiratory disease.^18,23 Because M. haemolytica is a commensal of the upper respiratory tract, finding the bacteria does not necessarily mean it is the only or even the most important cause of disease. However, if the agent is found in respiratory materials collected from an animal with clinical signs consistent with bacterial pneumonia, it is appropriate to assume that M. haemolytica is contributing to disease in the patient. Cytologic evaluation of TTA or BAL samples would be expected to reveal septic purulent inflammation characterized by large numbers of neutrophils that may be degenerate, exhibiting toxic change, and possibly containing intracellular bacteria. Thoracocentesis can be used to identify fluid accumulation in the thorax; fluid collected shows a high percentage of neutrophils with a high (>3 g/dL) total protein content if bacterial pleuropneumonia is the cause of fluid accumulation. If an ultrasound machine is available, transthoracic ultrasound evaluation can be used to confirm the presence of consolidated lung tissue, possibly with pleural effusion and fibrin. Thoracic radiography, if possible, is expected to show evidence of consolidation of the cranial lung and possibly pleural fluid accumulation. Evidence of pneumonia identified by either radiography or transthoracic ultrasound evaluation has been shown to correlate strongly with findings on postmortem examination of calves with bacte­rial bronchopneumonia.²⁴

Serologic tests can be used to identify serum antibodies to M. haemolytica, and seroconversion identified by paired serologic testing could be used to confirm infection with M. haemolytica in one or more ruminants. However, these tests are most commonly used for research applications, and they may not be available at all diagnostic laboratories. Moreover, seroconver­sion to M. haemolytica has not been consistently associated with disease.

■ Treatment and Prevention Treatment of M. haemolytica requires administration of an antimicrobial that is effective against the organism. Many antimicrobials are currently labeled for the treatment of M. haemolytica (see Table 31.15). The decision to choose any one of the many approved products is based on multiple factors, including regional susceptibility of M. haemolytica isolates, the number of times it is possible to treat animals, withdrawal times, and cost. Recently, M. hae- molytica that are resistant to multiple antimicrobial classes (multidrug resistant, or MDR) have been identified in lung collected at postmortem from feedlot and stocker cattle in the United States and Canada^25-27 and by guarded nasopharyngeal swabs of some populations of stocker and feedlot cattle.^28-30 Currently, evidence-based guidelines for management of cattle with bronchopneumonia attributed to MDR M. haemolytica are lacking; anecdotally it has been reported that cattle can recover despite the presence of M. haemolytica resistant to the antimicrobial with which they are treated. This may indicate that the host response can resolve infection and repair pathology in at least some cases of MDR infection.

In addition to appropriate antimicrobial therapy, treatment to prevent the adverse effects of endotoxin should be considered for ruminants suspected or confirmed to have pneumonia caused by M. haemolytica. The NSAID flunixin meglumine can ameliorate the inflammatory response to endotoxin, and treat­ment with flunixin meglumine has been shown to improve outcome in individual animals infected with M. haemolytica. However, administration of NSAIDs may not be cost-effective in the treatment of large numbers of cattle with respiratory disease. Thus a consideration of the cost versus expected benefit is appropriate before NSAID therapy is administered to an entire group.

Prevention of infection and disease caused by M. haemolytica is approached by three avenues: administering antimicrobials prophylactically to animals at high risk of disease; increasing host immunity by ensuring adequate passive transfer and administering vaccines against M. haemolytica; and minimizing factors such as viral respiratory tract infection, mixing of animals from various sources, and long-distance shipment, all of which increase the susceptibility of animals to disease caused by pathogenic serotypes of M. haemolytica.

Administration of antimicrobials effective against M. hae­molytica to an entire group of calves at high risk for disease because of recent weaning, uncertain immune status, mixing with cattle from a variety of sources, and long-distance shipment (“metaphylaxis”) has been a reliable means of decreasing morbidity and mortality associated with respiratory disease in such populations. Administration of tilmicosin to groups of such high-risk calves either before shipment or on arrival at feedlots decreased the proliferation of M. haemolytica serotype A1 in the nasopharynx of calves and was associated with decreased treatment for respiratory disease as compared with calves not treated with tilmicoson.³¹ In a later study, administra­tion of florfenicol at arrival decreased colonization of the nasopharynx with M. haemolytica A1 and delayed treatment for respiratory disease.¹ However, recent reports have described a high prevalence of MDR M. haemolytica following metaphy- lactic antimicrobial administration.^28,30 The degree to which MDR M. haemolytica negatively affect outcome in infected cattle is not yet clear, but dissemination of MDR M. haemolytica in cattle populations may begin to decrease the benefit of metaphylaxis. Further discussion of the role of metaphylactic administration of antimicrobials to control bovine respiratory disease can be found in the Metaphylactic Antimicrobial Therapy section later in this chapter.

Mixing recently weaned calves from multiple sources and shipping them long distances is a well-known precursor to outbreaks of fibrinous pneumonia³²; thus efforts to minimize stresses and improve immunity for calves moving through the marketing chain should lessen disease caused by M. haemolytica. Preconditioning, wherein vaccination and stressful procedures such as weaning and castration are carried out well in advance of mixing and shipment, can decrease costs associated with fibrinous pneumonia in feedlot cattle.³³ However, precondition­ing should not be considered a guarantee against all respiratory disease; severe disease can sometimes occur in preconditioned calves. Further discussion of the role of preconditioning to prevent bovine respiratory disease can be found in the Pre­conditioning section later in this chapter.

Vaccination can be used as part of a plan to decrease disease caused by M. haemolytica. Bacterins, toxoids, and at least one modified live vaccine are currently available in the United States and Canada for the prevention and control of respiratory disease due to M. haemolytica in cattle; additional information is presented in Chapter 48. At the time of this writing, it appears that only one vaccine containing M. haemolytica and P. multocida is specifically labeled for vaccination of sheep and goats in the United States and Canada (Mannheimia Haemolytica-Pasteurella Multocida Bacterin [Colorado Serum Company, Denver, Colo.]). Vaccination of sheep or goats with vaccines labeled for use in cattle may not reliably provide protection against disease, as serotypes of M. haemolytica that most commonly cause disease in sheep and goats are not included in vaccines for cattle. However, a recent study found that one licensed cattle vaccine decreased disease in sheep following experimental challenge of sheep with PIV-3 followed by challenge with serotype A2 M. haemolytica.

■ Definition and Etiology

Pasteurella multocida

P. multocida is a gram-negative aerobic bacteria of the family Pasteurellaceae. Like M. haemolytica, P. multocida can be found in the nasopharynx of healthy ruminants. Although P. multocida is regularly isolated from the lungs of ruminants with bron­chopneumonia, there has been debate over the years as to whether this species is a primary pathogen (i.e., capable of causing disease alone) or whether some other primary stressor or insult is required to occur before this agent can participate in disease. As with M. haemolytica, experimental challenge of calves with P multocida alone does cause clinical signs and pathologic change in the lung similar to that seen in natural outbreaks of disease,^34-36 but large numbers of the bacteria must be administered in a way that bypasses the upper respira­tory tract (most commonly by intratracheal or intrabronchial instillation). Alternatively, exposure to a viral respiratory pathogen a few days before exposure to P. multocida makes ruminants more likely to develop disease. These findings indicate that some insult that weakens respiratory defense is necessary for most if not all cases of naturally occurring disease to occur.

P. multocida is a diverse species of bacteria that is classified into five serogroups (A, B, D, E, and F) based on antigenic differences in the bacterial capsule. Serogroups B and E cause hemorrhagic septicemia, a disease predominantly seen in Asia and Africa; these serogroups are rarely described in North America. Serogroup A is the predominant serogroup associated with ruminant respiratory disease,³',³⁸ although serogroups D and F may be common in some regions, particularly in sheep.³⁹ In addition to the alphabetic serogroup designation, isolates may also receive a numeric designation based on cell wall antigen types (for example, P. multocida A3 is commonly isolated from cases of bovine pneumonia). A report characterizing 153 P. multocida isolates from cases of pneumonia and mastitis in cattle in England and Wales indicated that relatively few strains of P. multocida cause the majority of disease in cattle. Moreover, although a few strains isolated from cattle had also been associated with disease in swine and poultry, the majority of strains were uniquely associated with their host species of origin.³⁸ Other research indicated that certain strains of P multocida had a predilection for the respiratory tract of sheep, whereas others were associated with the reproductive tract.³⁹ Taken together, these data suggest that differences among strains of P multocida are related to the type of disease caused and the host likely to be affected. Therefore merely isolating P. multocida without further characterizing the isolate could make it difficult to know with confidence whether the bacteria isolated was actually contributing to disease. It has been sug­gested that strain variation is related to differences in severity of respiratory disease occurring in ruminants infected with P multocida.³⁵

■ Clinical Signs Calves infected with P multocida display clinical signs of fever, increased respiratory rate, and sometimes depression, coughing, and mucoid to mucopurulent nasal discharge. Loud or harsh bronchovesicular sounds may be heard over the cranioventral lung fields owing to pulmonary consolidation, and coarse crackles may be heard as a result of air moving through exudate in large airways. Because P. multocida is a gram-negative organism that produces endotoxin, signs of endotoxemia (such as tachypnea, tachycardia, and/or fever) may also contribute to signs that accompany infection with the organism. Compared with calves infected with M. haemo­lytica, calves infected with P multocida tend to have less severe signs, and signs of disease last for a shorter time.

■ Pathogenesis Little is known regarding the pathogenesis of P. multocida in ruminant respiratory disease. In addition to lipopolysaccharide (LPS), the organism has a capsule that allows it to resist phagocytosis. Outer membrane proteins, particularly IROMPs, are likely to contribute to the ability of the bacteria to establish and proliferate within the host.^40,41 The prominent role of P. multocida in dairy calf pneumonia relative to its role in acute fatal bronchopneumonia of feedlot cattle suggests that prolonged impairment of the respiratory defense mechanisms may be necessary for this organism to establish in the lungs in sufficient numbers to cause pathology. P multocida has been shown to overgrow M. haemolytica in challenge studies using large doses of pure cultures of M. haemolytica.³ This may be related to the fact that P multocida can inhibit growth of M. haemolytica in vitro.⁴² Thus when P multocida is identified in lung tissue or other materials from cattle with respiratory disease, it is possible that in at least some cases, the agent has displaced M. haemolytica that actually initiated disease.

Epidemiology P. multocida is commonly isolated from the lungs of dairy calves that die or are euthanized because of bronchopneumonia, with mycoplasmas being the only bacteria isolated more often in surveys of farms experiencing outbreaks of calf pneumonia.^43,44 Recent studies evaluating respiratory pathogens in individual live dairy calves have also found P multocida to be the agent most commonly identified.^18,19 Although M. haemolytica is more commonly isolated than other bacteria from lung tissue of feedlot cattle that die of acute bronchopneumonia,¹¹ it is interesting to note that two different studies comparing the bacteria identified in transtracheal aspirates or BALs collected from live feedlot cattle with respira­tory disease, and also from normal control cattle, found P. multocida to be most strongly associated with respiratory disease.^13,14 This may indicate that P multocida is an important contributor to feedlot respiratory disease but M. haemolytica is more likely to lead to disease that ends in death, causing M. haemolytica to be overrepresented in surveys of bacteria in tissues collected at postmortem.

Necropsy Findings P. multocida produces a purulent bronchopneumonia with plum-colored cranioventral consolida­tion and purulent exudate on cut section within the airways; when calves are experimentally infected with P. multocida alone, the lesion is usually not extensive and is typically confined to the cranioventral lung lobes. Histologically, bronchopneumonia characterized by infiltration of neutrophils and macrophages into alveoli and airways is seen. Microscopic evidence of abscesses may be present. Fibrin deposition with expansion of lymphatics and interlobular septa with edema, and focal areas of coagulative necrosis, can be seen in calves infected with P multocida, but this lesion is more typical of infection with M. haemolytica.

Diagnosis Bronchopneumonia caused by P multocida is diagnosed as described for M. haemolytica. The species is most commonly identified by culture of lung lesions identified at postmortem examination of affected animals or from guarded nasopharyngeal swabs or other respiratory materials collected antemortem. Because of the association of P multocida with chronic or ongoing pneumonia, identification of this agent may indicate that the animal has been affected with broncho­pneumonia for several days to weeks.

Treatment and Prevention Treatment of P multocida is as described for M. haemolytica. Several antimicrobials are labeled specifically for the treatment of P. multocida, and products labeled for the treatment of M. haemolytica are also likely to be effective against this organism (see Table 31.15). Because P. multocida seems to be associated with chronic or ongoing pneumonia, effective treatment of infection may require longer therapy than the 3 to 5 days historically recommended for ruminants with bronchopneumonia, but this recommendation has not been tested in controlled studies and would be an extralabel use of some antimicrobials. Although antimicrobial resistance has historically been uncommon in P multocida associated with ruminant respiratory disease, recent surveys of bacteria recovered from lung tissue or other respiratory materials collected from feedlot cattle have revealed P. multocida isolates that are resistant to multiple classes of antimicrobi­als.^25,26,29 MDR in P multocida as well as M. haemolytica has been attributed to genetic elements called integrative conjugative elements (ICEs), which are integrated into the bacterial genome, include multiple genes encoding resistance to multiple anti­microbial classes, and can be transferred between bacteria of different genera.⁴⁵ The clinical significance of MDR P. multocida is not clear at this time, but these findings suggest that respira­tory disease due to P multocida may become more difficult to treat effectively if resistant bacteria become widely distributed in cattle populations.

Because prior insult to respiratory defense mechanisms appears necessary for P multocida to cause disease in cattle, prevention of disease is likely to be aided by undertaking efforts to prevent other primary respiratory injury. This could include efforts to prevent infection with viral respiratory pathogens and to establish management practices that help minimize respiratory tract disease.

Relatively little is known about protective immunity that limits respiratory disease due to P multocida in ruminats. Modified live vaccines can protect calves from disease caused by experimental challenge.^46,47 Antibody responses to several outer membrane proteins were associated with protection in one study, and these were induced by live but not inactivated vaccine.⁴⁷ Several commercial vaccines are marketed for administration to cattle that contain P. multocida in combination with M. haemolytica, with or without other agents. It appears at present that one vaccine is licensed for administration to sheep and goats (Mannheimia Haemolytica-Pasteurella Multocida Bacterin [Colorado Serum Company, Denver, Colo.]). At this time no properly designed clinical trials have been reported that allow identification of any beneficial effect of the P. multocida component of available vaccines for ruminant respiratory disease in the field. More information about vac­cination against P. multocida is presented in Chapter 48.

■ Definition and Etiology

Histophilus somni

H. somni is a gram-negative aerobic bacteria of the family Pasteurellaceae. H. somni can be found on the genital and upper respiratory mucosa of normal ruminants,^48-51 but it can also cause a variety of diseases, including septicemia, thrombotic meningoencephalitis (TME), endometritis, abortion and infertility, pneumonia, pleuritis, laryngitis, otitis, conjunctivitis, myocarditis, mastitis, and polyarthritis.⁵² Only pneumonia and pleuritis resulting from this organism are considered here. Two bacteria very similar to H. somni, Haemophilus agni and Haemophilus ovis, have been isolated from sheep with septicemia, meningitis, mastitis, and reproductive abnormalities; DNA hybridization studies have indicated that these three organisms should all be classified as H. somni. Although specific serotypes of H. somni have not been associated with disease as is the case for M. haemolytica and P. multocida, differences in virulence among different strains have been demonstrated in experimental challenge studies.⁵³ Moreover, differences in expression of molecules considered to be virulence factors have been dem­onstrated when strains from healthy animals are compared with strains from diseased animals.⁵⁴ Thus it appears that some isolates of H. somni are more likely to cause disease than others, but more research is needed to confirm the characteristics that define a pathogenic isolate.

■ Clinical Signs H. somni can cause disease in a variety of organ systems, and the clinical signs of infection will depend on the organ system affected. The possibility of concurrent infection of multiple systems should be considered in patients suspected to have disease caused by H. somni. In a Canadian retrospective study from the early 1990s, the majority of animals presented to a regional diagnostic laboratory for necropsy as a result of haemophilosis had disease in more than one organ system.⁵⁵ Signs of respiratory tract infection can range from mild to severe and can include fever, tachypnea, cough, nasal discharge, and depression. Severe cases of disease can be fatal.^53,56 Harsh or loud bronchovesicular sounds may be heard on thoracic auscultation because of pulmonary consolida­tion, particularly over the cranioventral lung fields. Affected animals may have signs of pain owing to fibrinous pleuritis,⁵⁶ including reluctance to move, standing with elbows abducted, and catching the breath before expiration. The cell wall of H. somni contains lipooligosaccharide (LOS), which induces inflammatory responses identical to those induced by LPS from E. coli⁵¹∙; therefore animals with pleuritis or pneumonia caused by H. somni could also have clinical signs referable to endotoxemia, including tachypnea, tachycardia, dark or pale mucous membranes with prolonged capillary refill time, salivation, or dyspnea.

■ Pathogenesis H. somni can live on respiratory or genital mucous membranes without causing disease, and it is not entirely clear what factors related to the pathogen or host must change for disease to occur. The physical and immunologic barriers presented by the upper respiratory tract are apparently of major significance; calves exposed to H. somni by aerosol did not develop disease,⁵⁸ whereas instillation of the bacteria directly into the trachea or bronchi led to disease that could be severe.^53,56 As has been shown for M. haemolytica and P. multocida, primary infection by a viral respiratory pathogen is likely a predisposing factor that allows H. somni to advance and establish in the lower respiratory tract in many cases. Calves infected with BRSV before infection with H. somni had disease of greater severity than that seen in calves infected with either BRSV or H. somni alone.^56,59

H. somni has many features that help the bacteria escape the immune response.⁶⁰ The bacteria have outer membrane proteins that bind to the Fc region of antibody, allowing them to escape opsonization.⁵⁴ When the bacteria are ingested by neutrophils or macrophages, they are able to resist being killed.⁶¹ The bacterial LOS induces inflammatory responses similar to the LPS of other gram-negative bacteria,⁵⁷ and H. somni is able to periodically change the structure and antigenicity of its LOS, which is likely to be important in evading the host immune response.⁶²

An important aspect of the pathology caused by H. somni is the formation of vasculitis and vascular thrombi. H. somni can induce programmed cell death (apopotosis) of vascular endothelial cells, stimulate endothelial cell tissue factor activity, and disrupt intercellular junctions.⁶³ These effects lead to exposure of the vascular basement membrane, activation of platelets and the coagulation cascade, and thrombosis. These processes presum­ably contribute to pathology through release of inflammatory mediators by activated platelets and endothelial cells and through impaired blood flow due to microscopic thrombi.

Another important aspect of H. somni is the propensity of infection to induce IgE production by the host.^59,64 Infection with BRSV before infection with H. somni led to production of high levels of H. somni-specific IgE, which was associated with disease of increased severity as compared with control calves.⁵⁹ Cattle vaccinated with four different H. somni bacterins were all found to develop serum levels of H. somni-specific IgE that were significantly higher than levels seen in control unvaccinated cattle.⁶⁴ Because IgE mediates type I (immediate) hypersensitivity, animals that produce IgE against H. somni after vaccination or primary infection could have signs of an allergic or anaphylactic response on subsequent revaccination or reinfection. Such IgE-mediated hypersensitivity reactions have been proposed to contribute to the adverse reactions sometimes seen in cattle after vaccination for H. somni.⁶⁴ In addition to inducing IgE production, H. somni directly produces histamine,⁶⁵ which could further contribute to the development of hypersensitivity-like signs during H. somni infection. Among other actions, histamine increases permeability of bronchial epithelium; this may help the bacteria move out of the airways and into the lung parenchyma.

■ Epidemiology Exposure to H. somni is common, with 25% to 100% of cattle in various populations having serum antibodies.⁶⁶ It is common for feedlot cattle to have measurable antibody titers to H. somni at feedlot entry,^16,67 indicating that cattle are commonly exposed on the farm of origin or in transit to the feedlot. Seroconversion is also common in the first few weeks after feedlot entry,^16,17 indicating that cattle also continue to be exposed to H. somni in the feedlot. Several Canadian reports have indicated that H. somni can be a significant contribu­tor to the development of respiratory disease or undifferentiated 136870

fever in feedlot cattle., A causative association is typically inferred either by the association of seroconversion with respira­tory morbidity and mortality during the period of study or by the association of H. somni vaccination with decreased respiratory morbidity and mortality. By these measures it appears that most disease resulting from H. somni occurs within the first 2 months of the feeding period,⁷⁰ and perhaps even within the first 2 weeks.¹⁷ High-quality field research on the role of H. somni in feedlot respiratory disease in recent years has come almost entirely from Canada; it is not clear whether this indicates that H. somni is a less significant contributor to disease in the United States or the research has just not been done.

H. somni is usually less prevalent than P multocida and Mycoplasma spp. in surveys of the causes of pneumonia in dairy 194471

calves,,, but it can cause bronchopneumonia that is sig­nificant; one necropsy survey found pneumonia in 12 of 15 calves younger than 8 weeks of age submitted because of disease resulting from H. somni.⁵⁵

■ Necropsy Findings H. somni usually produces lung lesions similar to those due to P multocida; rarely it may produce lesions that resemble M. haemolytica, including fibrinous pleuropneumonia.^52,53,56 Grossly, plum or red to brown con­solidated lobules are seen in the cranioventral lung, sometimes with abscesses containing brown-red fluid material. Interlobular septa can be distended with edema and fibrin (Color Plate 31.5), and hemorrhage may be grossly visible. Purulent material is seen within airways on the cut surface of lung. The surface of the pleura may be flecked with fibrin, or in some cases fibrin deposition may be extensive, with varying amounts of straw­colored fluid in the pleural cavity. Bronchial lymph nodes may be enlarged, with ecchymotic hemorrhages on the cut surface.

Histologically, inflammatory cells predominately made up of neutrophils infiltrate the alveoli and airways. Edema, hemor­rhage, and fibrin can be seen in alveoli and interstitial spaces, and areas of coagulation necrosis surrounded by inflammatory cells may be found. Interlobular septa are expanded with fibrin, and thrombi are seen in blood vessels.

A massive fibrinous pleuritis with pleural effusion sometimes results from septicemic spread of H. somni. This condition can be differentiated from the fibrinous necrotizing lobar pleuropneumonia of shipping fever caused by M. haemolytica because the H. somni lesion involves only the pleural surface, not the lung.

■ Diagnosis Bronchopneumonia or pleuropneumonia caused by H. somni is diagnosed as described for M. haemolytica. The bacteria can be difficult to isolate, so samples collected for culture should be transported to the diagnostic laboratory without delay, and the diagnostic bacteriology laboratory should be specifically requested to attempt to isolate H. somni if involvement of the agent is suspected. PCR tests for the bacteria are available at some laboratories, and these may be more sensitive than culture.²² Samples are ideally taken from animals before antimicrobial treatment, as H. somni is usually susceptible to a wide variety of antimicrobials and treatment makes it difficult to isolate the bacteria.⁵² Serologic tests have been used to identify antibodies to H. somni for research,^16,17 and seroconversion could be used to confirm infection in a group of cattle, but these assays are unlikely to be available at most diagnostic laboratories. IHC has been used to identify H. somni in association with lesions in formalin-fixed tissues,⁷² and this may be an additional test available at some diagnostic laboratories.

■ Treatment and Prevention Historically H. somni has been found to be susceptible to a variety of antimicrobials, including tetracycline, penicillin, and sulfonamides, as well as newer antimicrobials, many of which are labeled for use in the treatment of H. somni (see Table 31.15). However, recent surveys of bacteria recovered from lung tissue or other respira­tory materials collected from feedlot cattle have revealed some H. somni isolates that are resistant to multiple classes of antimicrobials, although MDR H. somni have not been as prevealent as MDR M. haemolytica or P. multocida in these reports.^25,26,29 The impact of MDR on the efficacy of treatment of H. somni is not yet clear.

Several H. somni vaccines for use in cattle are commercially available; all are killed whole bacteria preparations (bacterins). More information about vaccination to control disease due to H. somni is presented in Chapter 48.

■ Definition and Etiology

Mycoplasma bovis

Mycoplasma bovis is a member of the genus Mycoplasma of the class Mollicutes, and as such it is among the smallest free-living organisms. M. bovis was first identified in 1961 in the United States, where it was isolated from a case of mastitis.⁷³ It has since spread worldwide. The bacterium was originally considered a subspecies of Mycoplasma agalactiae, and the two agents can be difficult to distinguish. However, M. agalactiae is a pathogen of sheep and goats and is rarely isolated in the United States, whereas M. bovis is regularly isolated in cases of pneumonia, respiratory disease, arthritis, tenosynovitis, and other disorders of cattle. M. bovis can rarely cause disease in sheep, goats, and other species.

■ Clinical Signs The clinical syndromes associated with M. bovis and the approach to their treatment and prevention have been reviewed.⁷⁴ Respiratory infection with M. bovis causes fever, tachypnea, inappetence, and sometimes respiratory distress.⁷⁵ Coughing and nasal discharge are reported in some cases.^76,77 Respiratory disease caused by M. bovis can occur in outbreaks; in young calves, a subset of calves affected often develops otitis, characterized by unilateral or bilateral drooping of ears with purulent aural discharge, possibly with vestibular signs such as head tilt, nystagmus, and ataxia.^76-78 In weaned beef calves and cattle entering feedlots, a subset of affected animals may develop arthritis and tenosynovitis^79-81; this syndrome is sometimes referred to as chronic pneumonia and polyarthritis syndrome.^81,82 Young calves can also develop arthritis or tenosynovitis.⁷⁷ A typical complaint by the producer experiencing a respiratory disease outbreak involving M. bovis is that cattle do not respond to therapy as expected, and a significant proportion of affected animals remain chronically ill and unthrifty for weeks after the onset of disease.^79,80 In addition to respiratory disease, M. bovis can also cause mastitis, arthritis and tenosynovitis, conjunctivitis, otitis, sinusitis, and myocarditis and/or pericarditis. The bacteria can also be isolated from aborted fetuses and from semen, linking the agent to reproductive failure; infected semen can also be a vehicle for introduction of M. bovis into a herd.⁸³ North American bison (Bison bison) can develop severe and sometimes fatal disease following M. bovis infection, with mature adults often being predominantly affected.⁸⁴

The ability of M. bovis to act as a primary respiratory pathogen has been debated. Classically the agent has been understood to be an opportunist, establishing itself after primary infection with viral pathogens or other bacteria. However, experimental challenge of gnotobiotic calves with M. bovis alone induced respiratory disease with clinical signs of fever, tachy­pnea, and inappetence and grossly evident lung lesions.⁷⁵ These findings indicate that M. bovis can cause disease while acting alone; however, as for other bacterial respiratory pathogens of cattle, it is likely that natural infection and disease often follows another primary insult.

■ Pathogenesis Much is still unclear about the mechanisms by which M. bovis causes disease. In vitro studies suggest that pathogenicity is associated with the ability of M. bovis to attach to host cells.⁸⁵ Antibodies against several of the variable surface proteins (VSPs) expressed by M. bovis were able to partially but not completely block attachment, indicating that the VSPs play a role in attachment.⁸⁵ M. bovis is able to migrate between ciliated respiratory epithelial cells,⁸⁶ which likely facilitates transfer from the respiratory mucosa to deeper sites such as joints and tendon sheaths. M. bovis was found to colonize the tonsils of calves fed colostrum contaminated with the bacteria⁸⁷; thus infected tonsils may serve as a reservoir leading to otitis media and possibly respiratory disease in calves fed milk or colostrum containing M. bovis. Caseous necrosis, often found in the lungs of calves with pneumonia associated with M. bovis infection, appears to be due at least in part to tissue damage induced by oxygen and nitrogen radicals released by leukocytes attempting to clear the infection.⁸⁸

M. bovis has a variety of ways to evade the host immune response, which likely contributes to persistent infection and poor response to therapy. VSP expression has been shown to be an important pathogenic mechanism.⁸⁹ M. bovis expresses at least three VSPs—VspA, VspB, and VspC—and isolates have been found that express some or all of these proteins.⁹⁰ M. bovis isolates show extensive variability in VSPs at both the genetic and the antigenic level. In addition, M. bovis can directly impair the activity of neutrophils⁹¹ and monocytes⁹² and can kill lymphocytes by inducing them to undergo apoptosis (programmed cell death).⁹³ Induction of proinflammatory cytokine production by the host may also contribute to disease caused by M. bovis.⁹⁴

■ Epidemiology M. bovis can be isolated from the respira­tory tracts of normal cattle^95,96 as well as those of cattle with

9799

respiratory disease.⁹' Surveys of nasopharyngeal swabs taken from dairy calves with no clinical signs of respiratory disease found M. bovis in 0% to 34% of the animals sampled.^99,100 A survey of multiple-source weaned beef calves sampled soon after arrival at nine different backgrounding or stocker opera­tions identified nasal shedding of M. bovis in 0% to 6% of animals at each operation.¹⁰¹ M. bovis can also be found in bovine lungs without evidence of disease at postmortem examination.¹² The fact that M. bovis can be isolated from animals with no clinical or pathologic signs of pneumonia has led some to question whether the agent is a true respiratory pathogen. However, a consistent association of M. bovis with a clinical syndrome of chronic nonresponsive pneumonia with or without otitis, arthritis, or tenosynovitis and with a pathologic syndrome of bronchopneumonia with multifocal caseous necrosis has led to general acceptance that M. bovis contributes importantly to significant morbidity and mortality in some situations.

M. bovis has been identified in dairy calves and feedlot cattle with respiratory disease in multiple reports. For example, in a survey of pathogens in calves in dairy herds in Quebec, only M. bovis was significantly associated with higher odds of clinical signs of respiratory disease, lung consolidation identified by transthoracic ultrasound, and decreased weight gain.¹⁹ A survey of causes of death in cattle in Ontario feedlots identified caseonecrotic pneumonia caused by M. bovis as a cause of more fatalities within the first 60 days of the feeding period than acute fibrinosuppurative pneumonia typical of disease caused by M. haemolytica or H. somni.¹² A large proportion of cattle can be infected with M. bovis in the first weeks after feedlot entry,^102,103 and infection can be associated with decrased weight gain.¹⁰² M. bovis infection of the respiratory tract appears to persist for weeks in cattle, but data are mixed in terms of whether cattle clear one infection only to be infected with a new isolate¹⁰³ or a single isolate persists for several weeks.¹⁰⁴ In 49 chronically sick cattle from a single feedlot, M. bovis was identified in the lungs of 82% of the cases and in the joints of 45% of the cases.¹⁰⁵ BVDV was also present in 39% of the cases from which M. bovis was isolated. A later retrospective study by the same authors found BVDV in the lungs of 44% of cattle subjected to necropsy with a final diagnosis of pneu­monia caused by M. bovis; these data led the authors to suggest that immunosuppression resulting from BVDV may predispose a subset of animals to chronic pneumonia and/or arthritis caused by M. bovis. In contrast, others found that BVDV infection was not more common in feedlot cattle with caseonecrotic lesions typical of M. bovis infection.⁹⁸

Currently available data indicate that M. bovis can spread from a few animals carrying the bacteria to others until a large proportion of a group is infected; transmission in these cases is most likely via respiratory infection by direct contact or short-distance aerosol. ’ ’ Long-term persistence of bacteria in pneumonic lung of a few individuals may facilitate transmis­sion to others in the herd.¹⁰⁶ Feeding milk infected with M. bovis appears to be an important source of infection for dairy calves.^77-79 Multiple reports have indicated that outbreaks can be caused by a single genotype of M. bovis,^ιm,l°^s although more than one genotype of M. bovis can also be isolated during outbreaks.¹⁰⁹

■ Necropsy Findings Grossly, lungs of cattle with pneumonia caused by M. bovis have dark red, firm consolidated lobules of the cranioventral lung. Raised white to yellow, firm nodules that range from 0.5 to several centimeters in diameter are often but not invariably seen clustering in the cranioventral lung (Color Plate 3 1.6).^75,82,110 These nodules may appear to be abscesses, but in most cases they are actually foci of coagula­tion (caseous) necrosis (Color Plate 3 1.7).^75,98 Differential diagnoses for gross lesions of abscessing pneumonia in cattle include infection with Trueperella pyogenes and Fusobacterium necrophorum (both of which are more likely to be associated with a foul odor than M. bovis) and Mycobacterium bovis (tuber­culosis). Other gross lesions include enlargement of interlobular septa by edema and fibrin. Fibrinous or fibrous pleural adhesions are unusual and if present may signal co-infection with M. haemolytica or H. somni. Animals with pneumonia typical of M. bovis may also have arthritis, tenosynovitis, myocardial abscesses, and/or otitis. Microscopic evaluation of affected lung reveals purulent pneumonia and bronchiolitis, with extensive infiltration of neutrophils in airways, and peribronchiolar cuffing with lymphocytes and mononuclear cells. Foci of eosinophilic coagulation necrosis are surrounded by a rim of dark pyknotic inflammatory cells and, farther out, a region of primarily macrophages and some plasma cells.^75,82 IHC staining for the organism may reveal large numbers surrounding the periphery of areas of coagulation necrosis as well as in association with bronchiolar and alveolar epithelial cells.^82,111

■ Diagnosis Confirmation of respiratory disease caused by M. bovis is most reliably made by identification of the agent by culture or PCR of respiratory materials collected antemortem from cattle with clinical signs of bronchopneumonia or in lung tissue with typical gross lesions collected postmortem. For antemortem diagnosis, materials are ideally collected from the lower respiratory tract (TTA or BAL). The reliability of nasal swabs or guarded nasopharyngeal swabs for confirmation of disease due to M. bovis is less certain; the agreement between the results of nasal swab culture and the presence of lung disease caused by M. bovis was good in one study⁹⁷ and only moderate to poor in two other studies.^14,112 A recent report indicated very good agreement between between the results of nasal swabs or guarded nasopharyngeal swabs and the TTA if mycoplasma culture was followed by PCR to specifically identify M. bovis but not when culture alone was used.¹⁸ The reliability of nasal swabs or guarded nasopharyngeal swabs for identification of M. bovis in respiratory disease outbreaks may be greater if multiople animals in a group are sampled and found to be positive. When samples are submitted for identifica­tion of M. bovis, it is important to request that the diagnostic laboratory specifically identify this agent; diagnostic laboratories often characterize mycoplasmas only to the level of the genus based on results of mycoplasma culture. Because other myco­plasma such as Mycoplasma bovirhinis, Mycoplasma dispar, and Mycoplasma alkalescens can also be isolated from the respiratory tract of cattle with pneumonia, a report of “Mycoplasma spp.” is not equivalent to a diagnosis of M. bovis. Species-specific identification of M. bovis is most commonly made by PCR testing, which is available on request at many veterinary diagnostic laboratories. MALDI-TOFMS can also be used to identify bacterial species once colonies are isolated by culture. Although either mycoplasma culture followed by PCR or direct PCR (without mycoplasma culture first) can be used to identify M. bovis, the agreement between both tests used in the same animal may not be strong.¹⁰¹ One recent report found culture to be more likely positive than PCR when nasal swabs were tested, whereas PCR was more likely positive than culture when milk was tested.¹¹³ Although available data suggest that M. bovis is less commonly isolated from healthy cattle than are other mycoplasmas, because the organism can be found in nasal swabs and lung tissue of apparently normal animals, isolation of the bacteria in the absence of evidence of disease is of uncertain significance.

Serologic assays have been used to identify seroconversion in epidemiologic studies or to identify evidence of recent or past infection, but the tests are not widely available, and their results have not been shown to reliably indicate active infec- tion.¹¹⁴ Serology is most commonly used to determine whether M. bovis is a likely cause of mastitis in dairy herds; the value of serology in the diagnosis of respiratory disease is not well established. A review article discussing the diagnosis of myco­plasmas in cattle in detail was recently published and should be consulted for more information.¹¹⁵

■ Treatment and Prevention Several antimicrobials, including tulathromycin (Draxxin [Pfizer Animal Health, Parsip- pany, N.J.]), florfenicol (Nuflor Gold [Merck, Madison, N.J.]), enrofloxacin (Baytril 100 [Bayer Healthcare LLC, Pittsburgh, Pa.]), and gamithromycin (Zactran [Boehringer Ingelheim, Ingelheim am Rhein, Germany]), are currently labeled for treatment of disease due to M. bovis. However, multiple authors report that cattle with pneumonia caused by M. bovis respond poorly to antimicrobial therapy.^80,81,114 Because M. bovis can be seen in association with foci of coagulation necrosis in lungs of affected cattle,^82,98 it may be that antimicrobials are not maximally effective in this environment, or it may be that insufficient duration of treatment is carried out in at least some cases. Controlled trials indicating the duration of antimicrobial therapy needed for effective treatment of pneumonia caused by M. bovis are lacking, but anecdotal reports suggest that early treatment is critical for success^77,116 and that treatment should be continued for at least 10 to 14 days. Failure of antimicrobials that are predicted to be efficacious against M. bovis may also be related to an inability of the immune response to effectively clear the organism due to immunosuppressive mechanisms described previously. Some veterinarians report that outbreaks of M. bovis cannot be stopped with any treatment but that outbreaks eventually terminate when chronically affected animals die or are shipped to slaughter.

Recently, multiple European reports have described increas­ing rates of antimicrobial resistance based on in vitro assessment

of M. bovis isolates.¹¹⁷ Although clinical trials under conditions typical of U.S. and Canadian cattle operations that confirm the efficacy of treatment in a field setting are lacking, antimi­crobial resistance may begin to make treatment of cattle with disease due to M. bovis even more problematic.

Research indicates that vaccination can theoretically afford some protection against experimental challenge with M. bovis.^118,119 Vaccines for control of disease due to M. bovis vaccines are licensed for sale in the United States; for vaccines to be fully licensed, they must show evidence of safety and efficacy in experimental challenge studies. However, properly designed field trials provide better evidence of usefulness of vaccines in the field, and reports of field trials to test the efficacy of currently licensed M. bovis vaccines are sparse. Results from two trials testing the effect of M. bovis vaccines on respiratory disease in dairy or veal calves have been reported.^120,121 In one trial there was no difference in respiratory morbidity or mortality in the first 90 days of life for vaccinated calves, as compared to unvaccinated controls.⁴⁷ In the second trial, which evaluated the effect of two different commercially available M. bovis vaccines on lung lesions at harvest of veal calves at approximately 145 days of age, lung lesions were decreased in one vaccine group but not in the other group, as compared to unvaccinated controls.⁴⁸ Autogenous vaccines have been used for years, but no reports with large numbers of animals and appropriate controls are available to provide information about efficacy. More properly designed field trials are needed to determine the value of M. bovis vaccination as used in the field in different types of cattle populations. Issues contributing to the lack of an unequivocally effective vaccine to prevent disease due to M. bovis have recently been reviewed.¹²²

■ Definition and Etiology

Mycoplasma mycoides subsp. capri

Caprine pneumonias that are not contagious among adults can be caused by several species of Mycoplasma. However, M. mycoides subsp. capri (Mmc; formerly M. mycoides subsp. mycoides Large Colony type) can be a very serious cause of mortality among goat kids, and it is so in North America.^123,124 Mycoplasma mycoides subsp. mycoides Small Colony type (MmmSC), which is related to Mmc, is the causative agent of contagious bovine pleuropneumonia, a serious cause of sickness and death in cattle in Africa. MmmSC has been eradicated from North America.

■ Clinical Signs In herds with Mmc infections, goat kids usually appear clinically normal until 2 to 8 weeks of age, when the following three clinical syndromes occur:

1. A peracute illness characterized by high fevers (41.1° to 42.2° C [105.8° to 107.9° F]) and death within 12 to 24 hours;

2. A central nervous system syndrome with opisthotonos and death within 24 to 72 hours; and/or

3. An acute to subacute syndrome with high fevers, multiple hot swollen joints, and pneumonia.

The most common manifestations are swollen joints, lame­ness, and recumbency. About one half of affected kids have increased lung sounds on expiration and elevated respiratory rates. During an outbreak, 80% to 90% of kids die or are euthanized because of permanent recumbency. Mmc infection in adult is also life threatening.¹²⁵

■ Necropsy Findings The most common necropsy finding in goat kids that die of Mmc infections is a fibrinopurulent polyarthritis. Approximately half of field cases have pneumonia. One or more lung lobes have areas of patchy to diffuse red consolidation that is sometimes covered with a fibrinous exudate. Clear, golden-yellow to serosanguineous fluid is found in the thorax in half of the cases. In some patients there are fibrinous adhesions between the lungs and thoracic wall. Affected lungs have microscopic evidence of bronchopneumonia or interstitial pneumonia. Other common lesions include pericarditis, peritonitis, and enlargement of the kidneys, liver, and spleen.

■ Diagnosis The definitive diagnosis of Mmc infection in individuals requires isolation of the agent from milk, joint fluid, blood, urine, or tissue. Infected goat herds can be readily identified by culturing bulk tank milk, because infected does shed up to 10¹⁰ Mmc organisms per milliliter of milk. Inapparent carriers can be identified by milk culturing, but false-negative results are a risk because organisms are shed intermittently. Goats can carry Mmc in their external ear canal, and PCR of auricular swabs has been shown to be a sensitive means of diagnosis of chronic carriers.¹²⁶

■ Treatment and Prevention Conventional antibiotic therapy for goats with Mmc infections is almost always unsuc­cessful. Tylosin or tetracyclines have been used; newer-generation antibiotics effective against Mycoplasma bovis could also be tried, although this would be an extralabel use of these drugs. A low percentage of kids make a clinical recovery from the septicemic illness but often have arthritis by the time they freshen. Does that recover from mastitis become chronic carriers.

Prevention is based on maintaining herds free of Mmc infection. Purchased does should originate only from herds that have no history of mortality in kids from arthritis and pneumonia and that have negative bulk tank cultures for Mmc. Purchased individuals should be held separate from the milking herd until they have an Mmc-negative milk culture result; treatment for ear mites may also be prudent. A vaccine is not commercially available.

Control of Mmc outbreaks centers on prevention of the systemic infection in kids and mastitis in milking does. Rapid prevention of new cases in kids can be expected from a program of feeding heat-treated goat colostrum (56° C [132.4° F] for 1 hour) or cow colostrum at birth, pasteurized milk up to 1 month of age, and pasteurized milk or a high-quality milk replacer from 1 month to weaning. All kids with swollen joints should be culled. Milking hygiene should be improved to prevent transmission of infection during milking. Milk samples from all does in the milking herd should be cultured to identify carrier does. Infected does should be kept in a separate string and milked last, or culled. Colostrum of dry does should be cultured as they freshen, and the does should be hand-milked separately until their milk is found to be free of Mmc. Monthly cultures of the bulk tank milk from the noninfected string should be performed to ensure that it is free of Mmc infection. The goal of control procedures is eradication of Mmc from the herd.

Other Mycoplasmas

Mycoplasmas are isolated, usually in combination with other pathogens, from 50% to 90% of beef and dairy cattle pneumonias.^44,127-129 Mycoplasmas have been associated with peribronchial and peribronchiolar lymphoid hyperplasia, which is sometimes referred to as a “cuffing pneumonia,” a common lesion in calves that die or are euthanized because of pneumonia.^130,131

Mycoplasmas have also been recovered from lesions of acute and chronic bronchopneumonia in which a cuffing pneumonia was not apparent.

Other than Mycoplasma bovis, the species of mycoplasmas (see Box 31.4) prevalent in North America are generally considered to be mild respiratory pathogens, mainly causing subclinical infections unless coupled with environmental stresses or infections by other pathogens. Tracheobronchial aspiration performed on dairy calves at random found that calves with both Mycoplasma spp. and Pasteurella spp. present in the aspirate were at significantly greater risk of developing pneumonia than calves with only one organism or no organisms. A study of feedlot cattle found seroconversion to M. alkalescens to be significantly associated with undifferentiated fever, a clinical definition similar to undifferentiated respiratory disease.¹⁶ Effects such as immunosuppression and inhibition of the mucociliary transport mechanism, which can be mediated by mycoplasmas, suggest that they may play an important contribu­tory role in the pathogenesis of bovine pneumonia. Mycoplasma ovipneumoniae is often isolated from pneumonic lungs of sheep and goats, usually accompanied by M. haemolytica. On its own, M. ovipneumoniae is capable of causing mild, subacute to chronic bronchiolitis or bronchopneumonia that probably predisposes to M. haemolytica infections.

Bibersteiniia trehalosi

Bibersteinia trehalosi causes pneumonia or systemic disease with multiorgan infection in sheep. B. trehalosi has also been reported in association with severe peracute to acute bronchopneumonia in cattle and recently weaned calves.¹³² Affected cattle died before they were noticed to be ill, and antimicrobial therapy did not seem to be very effective despite the fact that B. trehalosi isolates were susceptible to many antimicrobials in vitro. However, an effort to induce disease by experimental challenge of calves was not successful,¹³³ making it unclear whether the bacteria alone can be assumed to be the primary cause of respiratory disease in field cases. In cases where B. trehalosi appears to be contributing to disease, therapy with antimicrobial drugs effective against M. haemolytica should be attempted. Vaccines against Mannheimia haemolytica may help limit the problem.¹³⁴ At this time it is not known what factors increase the risk of pneumonia due to B. trehalosi in cattle or domestic sheep.

Truepereiia pyogenes

Trueperella pyogenes (formerly Arcanobacterium pyogenes) is a gram-positive rod-shaped species of bacteria that is a common cause of internal or SC abscesses in ruminants. T. pyogenes is occasionally isolated from the lungs of ruminants with pneu­monia, typically from animals that have chronic pneumonia, possibly from grossly visible abscesses. This species is viewed as an opportunist that contributes to respiratory disease as a secondary or possibly “tertiary” invader after viral pathogens and other bacterial respiratory pathogens have become estab­lished. T pyogenes has a variety of virulence factors, including a cytolytic toxin called pyolysin, as well as several molecules that aid in adherence to host cells.¹³⁵ Diagnosis of this pathogen is usually made as an incidental finding at necropsy of an animal with significant respiratory disease from other primary causes. Finding T. pyogenes during a respiratory necropsy should not induce efforts to directly treat or prevent this pathogen but rather should induce efforts to prevent other primary or secondary causes of pneumonia and to treat animals in a timely manner with appropriate therapy for an adequate duration in order to prevent chronic pneumonia.

Chlamydial Agents (Chiamydophiia Species)

Chlamydial agents, most commonly Chlamydophila abortus, Chlamydia psittaci, and Chlamydophila pecorum, have been associ­ated with a variety of disease syndromes in cattle and small ruminants, including abortion and infertility, conjunctivitis, polyarthritis, encephalomyelitis, and enteritis.¹³⁶ These agents are also occasionally identified in pneumonic ruminant lungs. Because chlamydial agents can also be identified in ruminants with no signs of disease and because seroprevalence is high, the importance of chlamydial agents in ruminant respiratory disease has been debated. Although experimental infection of calves with high numbers of C. psittaci via a route that bypasses the upper respiratory tract can lead to clinically evident respira­tory disease that may be severe,¹³⁷ in the natural setting these agents are most likely to participate as one of mutiple factors necessary to induce important ruminant respiratory disease. Research indicates that calves are infected with chlamydial agents relatively early in life and that crowding enhances the likelihood of a high proportion of animals being infected.¹³⁸