Chronic Progressive Pneumonias of Sheep and Goats

Jeanne Lofstedt • Emily John

Chronic progressive pneumonias are diagnosed with some frequency in mature small ruminants. In sheep, ovine progressive pneumonia (OPP) and Corynebacterium pseudotuberculosis-induced mediastinal lymph node and lung abscesses are the chief causes of chronic progressive pneumonia.

Ovine Progressive Pneumonia (Maedi-Visna)

■ Definition and Etiology OPP and maedi-visna (MV) are North American and European terms, respectively, for multisystemic diseases of adult sheep characterized by chronic, progressive, debilitating pneumonia; wasting; indurative mastitis; arthritis; and occasionally neurologic signs.^1-5 Synonyms are Marsh progressive pneumonia, Montana sheep disease, zwoegerziekte, la bouhite, and Graaff-Reinet disease.² The OPP virus (OPPV) and MV virus (MVV) are nononcogenic exogenous retroviruses that belong to the genus Lentivirus, which includes the human, simian, feline, and bovine immunodeficiency viruses; equine infectious anemia (EIA) virus; and CAEV^4,5 These enveloped, single-stranded RNA viruses contain the enzyme RNA- dependent DNA polymerase or reverse transcriptase (RT). With this enzyme the viruses use cellular machinery to transcribe a segment of DNA from a template of single-stranded viral RNA; the DNA strand or provirus is then incorporated into the host genome.

■ Clinical Signs and Differential Diagnosis The incubation period of OvLV infections is prolonged, and most infections are subclinical.^4,5 Approximately 25% to 30% of infected sheep eventually exhibit clinical symptoms.¹ Clinical disease is usually not evident until at least 50% of a flock is seropositive.¹ Conditions in sheep attributed to infection with OvLV include progressive emaciation (“thin ewe syndrome”), progressive respiratory failure, indurative lymphocytic mastitis (“hard bag”), posterior paresis, and chronic nonsuppurative arthritis.^1,4,5

In North America, progressive pneumonia and aseptic indurative mastitis are the most common clinical manifestations of OvLV infections.³ Clinical signs of disease are usually first observed in 2- to 3-year-old sheep, but most affected animals are 4 or 5 years old before they exhibit signs.^3,6,7 Emaciation, despite a good appetite, is one of the earliest symptoms. Tachypnea and respiratory distress develop gradually and initially are evident only on exertion.^3-5 Flared nostrils, neck extension, and open-mouth breathing are seen in advanced cases. Thoracic auscultation may reveal increased breath sounds, but crackles and wheezes are typically absent.³ Pyrexia and purulent nasal discharge are uncommon and, if present, usually indicate concurrent secondary bacterial pneumonia.⁵ Wool loss may be seen in advanced cases.⁴ Affected ewes may give birth to small, weak lambs.⁵ Death from anoxia or secondary bacterial infection ensues within 6 to 12 months of first appearance of clinical signs.

The neurologic form of OPP is accompanied by weight loss and neurologic signs such as hindlimb ataxia, stumbling, and proprioceptive deficits, which progress over a period of months to paralysis or occasionally quadriplegia^3,5 (see Chapter 35). This form of the disease is encountered sporadically in heavily infected flocks and may occur separately or concurrently in sheep with respiratory distress.^4,5 Indurative mastitis is often encountered in heavily infected flocks. The somatic cell count (SCC) in affected ewes is increased, but the milk is grossly normal; a firm udder may be detected with careful palpation.⁴ The main consequence is poor weight gains in lambs suckling affected ewes, particularly if there are twins or triplets.⁴ Arthritis, primarily affecting the carpal joints, is a rare manifestation of OvLV infection.^4,5

Chronic pneumonias of sheep that have to be differentiated from OPP include chronic bronchopneumonia caused by Mannheimia (Pasteurella) haemolytica, OPA, verminous pneu­monia, and pulmonary and thoracic lymph node abscesses caused by C. pseudotuberculosis.¹'¹ Thoracic radiographs, culture of tracheal wash material, and fecal examination by the Baermann technique are useful for antemortem differentiation. Gross and histopathologic evaluations of pulmonary tissues obtained at necropsy are helpful for diagnosing a flock problem.

■ Clinical Pathology and Diagnosis Hematologic changes in sheep with OPP are nonspecific and may include lymphocytosis early in the course of disease and mild hypo­chromic anemia and hypergammaglobulinemia in advanced cases.^3,7 A presumptive diagnosis of OPP can be made based on clinical signs of progressive weight loss and chronic pneu­monia or, in rare cases, neurologic disease in adult sheep,^4,5 but the diagnosis is often overlooked if mildly affected sheep are culled for other reasons.⁴ The majority of sheep infected with OvLV do not show obvious clinical signs and are essentially asymptomatic virus carriers serving as a reservoir of virus for other sheep in the flock.^4,5 For disease control purposes it is important to identify such animals as soon as possible after infection.

Serologic tests include the agar gel immunodiffusion (AGID) test, several ELISAs, and the Western blot assay.^3,4 The period from infection to seroconversion typically ranges from 3 weeks to several months but can take up to a year.⁴ Some infected sheep never seroconvert, whereas others with advanced disease become transiently seronegative.⁵ The AGID test has been the preferred serologic test for many years. It has good specific­ity but has gradually been replaced by a variety of ELISAs.^4,5 ELISAs are more sensitive than the AGID test and are useful when large numbers of samples need to be screened.^3-5 The use of ELISAs to detect antibodies in milk offers the option of low-cost screening in milking sheep flocks.⁴ The Western blot test is used on occasion to confirm ELISA results.⁵

Tests for virus include virus isolation in cell culture or viral genome detection by PCR.^5,8,9 These tests are usually reserved for select cases—for example, when infection is suspected in a seronegative animal or in young lambs that have received colostrum from seropositive ewes.⁵ Milk or lung lavage samples are useful substrates for this purpose. Virus isolation, which is time consuming and expensive, is accomplished by coculture of infected fresh or frozen tissue with indicator cell lines. In culture, retroviruses form characteristic multinucleated syncytia.⁷ Viral antigens can be detected in these syncytia by use of immunofluorescent staining.⁷ PCR testing is more cost-effective than virus isolation.^3,4

■ Pathophysiology OvLVs gain access to the body via ingestion of milk or colostrum or through inhalation of aerosol­ized infectious particles. Infection is then established in the monocyte or macrophage cell line and spreads via these infected cells to the lungs, lymph nodes, choroid plexus, spleen, bone marrow, mammary glands, and kidneys.⁷ The virus is able to persist in the face of humoral and cellular immunity through (1) latent infection of host cells by DNA provirus, (2) long-term nonproductive infection of blood monocytes (virus replicates only on differentiation of monocytes into macrophages), and (3) virus mutation with emergence of new antigenic variants that are not neutralized by preexisting antibody.⁷ Because of the interaction of the virus with the macrophage monocyte series, a progressive multisystem chronic inflammatory process is initiated, primarily but not exclusively in the lungs and mammary gland.⁴ Variations in genetic susceptibility, virus genotype, and the immune response of individual sheep influ­ence target organ involvement and disease progression.⁴

■ Epidemiology OPP and MV are present in many sheep­rearing countries but have not been diagnosed in Australia or New Zealand.⁵ The seroprevalence of OPP in cull ewes in the United States has ranged widely and increases with advancing age.² With the exception of West Texas, which has an infection rate of 0.5%, serologic surveys in cull ewes in other states have revealed infection rates of 30% to 67%.

■ Necropsy Findings Lesions may develop in any or all of the following tissues of OPPV-infected sheep: lungs and regional lymph nodes, brain, joints, mammary glands, and blood vessels.⁴ Changes are most obvious in the lungs, which do not collapse on opening of the thorax.^4,5,7 Vertical rib impressions may be seen on the exterior lung surface.

■ Treatment and Prevention OPP is not treatable. Antimicrobials can be used to control secondary bacterial pneumonia, but most sheep die within a year of first exhibiting clinical signs.^2,6 No effective vaccines are presently available for control of the disease. There is some evidence that vaccinat­ing lambs born to seronegative ewes early in life may delay the onset of seroconversion when these lambs are maintained in an infected herd.¹³

OPPV is difficult to eradicate from a flock once infection has been established. Uninfected flocks should maintain strict biosecurity measures to avoid introduction of the disease. This may include not sharing rams, not mixing sheep with other flocks, and maintaining segregation at shows.⁴ Breeding stock should be purchased from known OPP-free herds and quar­antined and tested to ensure that they are seronegative before being introduced into the flock. Control methods that may be economically feasible for purebred flocks with a low sero­prevalence rate include (1) a “test-and-cull” practice and (2) isolation of infected adults and artificial rearing of their off- spring.³ With the first method, all sheep are tested annually for antibody, and seropositive animals and their progeny younger than 1 year of age are culled or isolated from the negative flock; culling is preferred because of the risk of cross­contamination. New additions should be seronegative and originate from seronegative flocks. Annual testing should be performed until two consecutive negative test results are obtained to be reasonably confident that the flock is virus free. With the second approach, progeny are removed from their dams before they nurse and are fed cow's colostrum or colostrum from seronegative ewes and raised in isolation. The clean herd should be kept isolated from infected sheep and goats and from humans and equipment in contact with infected sheep. Herd additions and annual testing should be handled as described for method 1. In flocks with well-established infections and a high seroprevalence rate, control options are limited.⁴ The test-and-cull procedure and removal of lambs are impracti­cal under commercial conditions. Increasing the flock replace­ment rate by strict culling of suspect cases and not keeping ewes beyond 4 to 5 years of age will assist in keeping the flock seroprevalence down and in reducing the incidence of clinical disease.⁴

Recent research has demonstrated that variation in the gene encoding transmembrane protein 154 (TMEM154) results in decreased susceptibility of sheep to infection with SRLVs.¹⁴ Three main haplotypes of the TMEM154 gene have been identified: haplotype 3, the ancestral variant; haplotype 1, with a single amino acid substitution E35K; and haplotype 2, with a different single amino acid substitution N70I. Under natural exposure conditions, sheep possessing at least one copy of haplotypes 2 or 3 were 2.85 times more likely (95% CI 2.36-3.43) to be seropositive for OPPV than sheep possessing two copies of haplotype 1.¹⁵ This suggests that marker-assisted selection of genetically resistant sheep may help decrease prevalence of OPPV infection within flocks.

Ovine Pulmonary Adenocarcinoma

■ Definition and Etiology OPA, also known as jaagsiekte (South Africa), sheep pulmonary adenomatosis (United Kingdom), or pulmonary carcinoma (United States),¹⁶ is a naturally occurring, retrovirus-induced bronchioloalveolar carcinoma of sheep and, in rare cases, goats that has been associated etiologically with a betaretrovirus called jaagsiekte retrovirus (JSRV).^17,18 OvLVs and herpesviruses have been isolated from OPA tumors; however, they are not consistently present and do not induce OPA when inoculated experimen­tally.¹⁹ One theory is that these viruses may act as cofactors in tumor induction. Defining the role of JSRV in OPA has been complicated by the presence of 15 to 20 endogenous jaagsiekte retroviruses (enJSRVs) that are stably integrated into the genomes of sheep and goats.^20,21 Recent research has indicated that exogenous JSRV is likely not of endogenous origin. There has been speculation that the endogenous viruses may modify the genome of exogenous JSRV, either by inducing the expres­sion of an oncogene or by inactivating a tumor suppressor gene.¹⁸ Sequence analyses of exogenous JSRV and enJSRV suggest that endogenous viruses do not directly contribute to the pathogenesis of OPA through large-scale recombination events, but small-scale recombination or complementation of gene function has not been definitively excluded.²⁰ There has been speculation that expression of enJSRV in the fetal or

neonatal period may influence expression of OPA through induction of immunologic tolerance.^17,20 Endogenous JSRVs appear able to exert a protective effect on the host by blocking cellular entry of JSRV via receptor interference as well as by preventing proper cellular trafficking and, ultimately, release

22

of virus particles.²²

■ Clinical Signsand Differential Diagnosis Symptoms normally appear in mature sheep between 2 and 4 years of age, although lambs as young as 3 months of age and sheep as old as 11 years have occasionally been diagnosed with OPA.¹⁶ Signs include exercise intolerance, weight loss despite a good appetite, tachypnea, dyspnea, occasional cough, and high-pitched crackles on thoracic auscultation. A characteristic feature of OPA is the accumulation of abundant fluid in the respiratory tract that is frothy, clear, milky, or at times pink-tinged and may flow from the nostrils of affected sheep when the rear limbs are raised and the head is lowered (“wheelbarrow test”).^16-18 Up to 400 mL of nasal secretions may be collected in a day, although 10 to 40 mL/day is more usual. Rectal temperature is usually normal unless secondary bacterial infection is present. Sheep with OPA typically succumb within a few weeks to months after first exhibiting clinical signs.

Chronic pneumonias of sheep that must be differentiated from OPA include chronic bronchopneumonia caused by M. haemolytica, OPP, verminous pneumonia, and pulmonary and thoracic lymph node abscesses caused by C. pseudotuberculosis.¹'¹ Thoracic radiographs, culture of tracheal wash material, and fecal examination by the Baermann technique are useful for antemortem differentiation. Gross and histopathologic evalu­ations of pulmonary tissues obtained at necropsy are helpful for diagnosing a flock problem.

■ Clinical Pathology and Diagnosis The only reported clinical pathologic abnormality in sheep with OPA is hypergam­maglobulinemia. Advanced clinical cases in mature sheep can be presumptively diagnosed based on clinical signs of weight loss and respiratory distress accompanied by copious nasal secretions, but in many cases the disease is subclinical.^16,17 To date, sensitive and specific tests for identifying apparently healthy animals that may be infectious are lacking. The absence of JSRV-specific antibodies in infected sheep have precluded the use of serologic tests for diagnosing infection.^17,18 PCR tests, capable of demonstrating viral nucleic acids of endogenous JSRV tissues in OPA-infected animals, have been developed but have a high false-negative rate and are not useful for identifying individual animals for eradication purposes.¹⁷ The most successful approach for early diagnosis of OPA is PCR testing of bronchoalveolar fluid collected from live animals, but this is impractical for widespread testing, and saliva and nasal secretions of infected sheep often test negative for JRSV.¹⁷ Recently, rapid transthoracic ultrasound has shown high sensitiv­ity and specificity for antemortem identification of sheep with lung lesions consistent with OPA at postmortem examination, making this imaging modality potentially useful for flock-level screening for OPA.²³

■ Epidemiology OPA has been reported in sheep-rearing countries throughout the world but not in Australia, New Zealand, or the Falkland Islands.^17,18 Iceland is currently free of OPA as a consequence of implementing an aggressive slaughter policy in endemic areas in the 1950s.¹⁷ The prevalence of OPA varies among countries in which the infection occurs; it is endemic in Scotland, Peru, and South Africa, where annual losses range from 2% to 10%. In contrast, OPA is infrequently diagnosed in the United States and Canada, where totals of 11 and 43 cases, respectively, have been reported.¹⁸

OPA has been reproduced experimentally by intratracheal inoculation of lung homogenates or pulmonary lavage fluids obtained from infected animals.²⁴ Lambs between birth and 5 weeks of age have been shown to be more susceptible to experimental infection than 10-week-old lambs.²⁴ This age- related susceptibility to OPA suggests that natural virus transmission occurs in the neonatal period.¹⁷ Aerosol transmis­sion and contamination of feed and water by respiratory secretions are likely methods of disease spread, particularly in confined sheep. Provision of diets deficient in selenium had no effect on development or progression of pulmonary tumors when compared to sufficient dietary selenium intake.²⁵

■ Necropsy Findings OPA lesions in the lungs are categorized as classical OPA or atypical OPA based on their appearance.^16,17 In classical OPA the lungs are heavy (two to three times normal weight) and exude clear fluid from the cut surface.^16-18 The trachea and bronchi often contain clear, foamy fluid. Large, firm, gray masses are commonly encountered in the cranioventral regions of one or both lungs. Smaller (1- to 2-cm) nodules are occasionally visualized in the caudodorsal lung regions. In atypical OPA the tumors consist of solitary or aggregated hard, white nodules that are clearly demarcated from normal lung tissue and have a dry cut surface.^16,17

Metastasis to regional lymph nodes occurs in approximately 10% of cases, resulting in pulmonary lymph node enlarge- ment.^16,17,26 Metastases to cardiac and skeletal muscles are infrequently reported.^16,17,26 Metastasis to solid abdominal organs as well as to the digestive tract and skin is also possible, although rare.²⁷ Secondary bacterial pneumonia caused by M. haemolytica may mask the lesions of OPA.¹⁷

OPA is classified by the World Health Organization (WHO) as a bronchioloalveolar carcinoma arising from alveolar type II pneumocytes or nonciliated bronchiolar cells.²⁶ Histologic lesions in the lungs consist of columnar or cuboidal cells arranged in an acinar or papillary pattern and are similar for classical or atypical OPA but with an exaggerated inflammatory response and fibrosis in the atypical form of the disease.¹⁶

■ Treatment, Prevention, and Prognosis There are no known treatments for this disease. Antibiotics may prolong the life of OPA-affected sheep through control of secondary bacterial pneumonia.¹⁸ There are no vaccines for OPA at this time.¹⁷ Prompt culling of ewes suspected to be infected, as well as their offspring, has been used to decrease disease prevalence in endemic flocks.^16,17 OPA was eradicated from Iceland by slaughtering all sheep in endemic areas.^16-18 Contaminated areas where infected sheep were held should be left vacant for 2 months.¹⁷ OPA has reportedly been eliminated from two flocks by removing the lambs at birth, depriving them of maternal colostrum, and raising them in isolation or by using embryo transfer.^16,17 This approach is impractical for commercial flocks. Until reliable diagnostic tests for subclinically affected sheep are developed, control will remain problematic.

Caprine Arthritis and Encephalitis

CAE is a persistent lentiviral infection of goats with four possible clinical presentations: leukoencephalomyelitis in kids 2 to 6 months of age and chronic, hyperplastic polysynovitis, indurative 2830

mastitis, and interstitial pneumonia in mature goats.^28-30 Mild interstitial pneumonia, which is silent clinically, is a common postmortem finding in goat kids infected with CAEV.³¹ Occasional kids with leukoencephalomyelitis may have diffuse involvement of one or more lung lobes.³⁰ In one study, 60% of adult goats serologically positive for CAEV had lesions of severe chronic interstitial pneumonia at slaughter.³¹ In addition, chronic interstitial pneumonia, accompanied by exercise intolerance and dyspnea, was described in dairy goats originating from herds that had clinical cases of arthritis and Ieukoencephalomyelitis.³² The lung lesions in these goats resembled those of OPP and MV and were positive for CAEV.^32,33 However, CAEV has been recovered from the lungs of goats with advanced arthritis but without interstitial pneumonia. In addition, goats inoculated intratracheally with purified CAEV did not develop pulmonary lesions.³² This may indicate that there is no causal relationship between CAEV and interstitial pneumonia in goats or that an additional agent such as a helper virus is required for the lesions to develop.³⁰ The first signs of chronic progressive pneumonia associated with CAEV infection appear at times of stress such as kidding and include exercise intolerance, dyspnea, weight loss, and occasionally a cough.³⁰ Enlarged carpi may be present in some of the affected goats.³⁰ Radiographs in advanced cases demonstrate large areas of lung consolidation and sometimes bullous emphysema in the cranioventral or caudal lung lobes.³² Histopathologic examination of lung biopsy samples may assist with an antemortem diagnosis.³⁰ Serologic testing is of no diagnostic value other than identifying the presence of CAE in the herd of origin. Lesions in the lungs at postmortem are as for sheep with the pneumonic form of OPP.³⁰ CAE is discussed in detail in Chapter 38.

Caseous Lymphadenitis

CLA, caused by C. pseudotuberculosis (previously known as C. ovis), is a chronic disease of sheep and goats characterized by the development of pyogranulomatous abscesses in lymph nodes and internal organs.^34-39 The disease affects small ruminants worldwide.³⁹ CLA most commonly presents as an external form characterized by abscesses in superficial lymph nodes.^37,39 The internal, or visceral, form of CLA manifests as internal abscesses in the mediastinal and mesenteric lymph nodes as well as internal organs, primarily the lungs, but also in the liver, kidneys, and central nervous system.³⁹ Small ruminants with the visceral form of the disease rarely have concurrent abscessation of external lymph nodes.³⁸ Goats are more com­monly afflicted with external abscesses, whereas the visceral form of CLA appears to be more common in sheep.³⁴ Visceral CLA may be asymptomatic but is typically associated with progressive weight loss.^34,38,39 Specific clinical signs that may implicate visceral CLA of the lungs and associated lymph nodes include exercise intolerance, tachypnea, dyspnea, and chronic cough. Because of the encapsulated nature of pulmonary abscesses and the lack of exudate in airways, pulmonary crackles are rarely auscultated.^38,39 Nonspecific laboratory findings in sheep and goats with CLA include leukocytosis, hyperfibrino­genemia, hyperproteinemia, and hypergammaglobulinemia; however, laboratory findings are frequently normal in animals with chronic abscesses.^36,37 A definitive diagnosis of visceral CLA with pulmonary involvement may be achieved by thoracic radiographic and ultrasonographic findings of abscesses in the lungs and mediastinal lymph nodes or by isolating C. pseudo­tuberculosis from a transtracheal aspirate (TTA).^34,38 Failure to isolate the organism from tracheal wash fluid, however, does not rule out CLA as the cause of pulmonary disease. Charac­teristic postmortem findings of the respiratory form of CLA are one or more thick-walled, laminated, encapsulated, caseo- purulent abscesses in the bronchial lymph nodes, mediastinal lymph nodes, and/or lung parenchyma.³⁶ Serologic tests have been developed for the detection of asymptomatically affected animals or those with internal abscess. They include the synergistic hemolysis inhibition (SHI) test and several ELISAs.^36,37 Most serologic tests are not specific enough to justify their use in culling programs. However, an ELISA developed in the Netherlands for disease eradication purposes has recently been improved to increase sensitivity (94%) and specificity (98%).⁴⁰ CLA is discussed in more detail in relation to the hemolymphatic system (see Chapter 37).