Mastitis in Small Ruminants

General Concepts

The udders of sheep and goats differ from those of cows in that they have only two mammary glands, which are logically referred to as halves. With the exception of the relative proportion of the cisternal cavities, anatomic structures, milk secretion, and milk letdown of small ruminants are similar to cattle.

Just as in dairy cows, mastitis of small ruminants is a bacterial disease that occurs in either a subclinical or a clinical state. Subclinical mastitis is an important and underrecognized disease in small ruminants. In two separate studies, an Israeli research group compared milk production and composition of healthy and NAS-infected half-udders in ewes.⁵²⁸ These studies demonstrated large increases in SCCs and 30% to 50% reduc­tions in milk yield of infected half-udders. Subclinical mastitis is generally defined by the migration of neutrophils into the mammary gland in response to bacterial infection. This response occurs in all dairy species, but the magnitude of the response and the distribution of cell types in the healthy mammary gland differ considerably among species.⁵²⁹ In both sheep and cows, a significant increase in somatic cells occurs almost exclusively in response to bacterial infection of the mammary gland. The SCC response in dairy goats is not as specific to infection, and thus different interpretation criteria are necessary. The proportion of neutrophils (PMNs) and the number of cytoplasmic particles present in milk are very different in milk produced by goats as compared to milk produced by ewes or cows. Part of this difference is generally attributed to different milk secretion mechanisms. Both goats and sheep are thought to produce milk using a largely apocrine process, where the apical portion of the secretory cell is excreted into the milk.

Enumeration of SCC of goat milk must be performed using DNA-specific methods.⁵³⁰ The CMT is based on reaction of the detergent with DNA in cells and is also considered accurate. When enumeration of SCC in goat milk is properly performed, IMI is a well-known cause of increased SCC, but the threshold used to determine infection must be determined relative to stage of lactation.⁵³¹ Milk samples obtained from infected udder-halves of goats generally exhibit SCC values greater than 500,000 cells/mL (first 90 days in milk) and greater than 1,000,000 cells/mL (later stages of lactation).⁵³¹ Important factors that must be considered when evaluating SCCs of goats include parity, stage of lactation, breed, and estrus.^532,533 In does, the ability of estrus to stimulate increased SCCs in the absence of IMI has been demonstrated in a controlled study using induced estrus.⁵³⁴ In one part of the trial, the day after inducing estrus, SCC values were 1,778,000 cells/mL for does in estrus versus 363,000 cells/mL for does in the control group; both values have been converted from the reported log values. These physiologic increases were not associated with IMI or with decreased milk production, but the mechanism behind the increase was not elucidated. Overall, while several noninfec- tious causes for increased SCC are observed in goats, IMI remains an important cause of increased SCC.

The types and proportions of cells present in sheep milk are more similar to dairy cows than goats, and standard methods used to count somatic cells in cows' milk are considered accurate for counting somatic cells in ewes' milk. Evaluation of SCC data is considered an effective tool for diagnosing IMIs in dairy sheep.^535,536 In an uninfected half-udder, the SCC is generally lower than 200,000 to 400,000 cells/mL.^531,536 Higher counts are almost always associated with bacterial infections and indicate the presence of subclinical mastitis.⁵³⁷ Individual gland SCC values increase in response to IMI in ewes, and thus bulk tank SCC values are an indication of the quality of milk and increase when the prevalence of subclinical mastitis increases.⁵³⁷ Dairy sheep producers should monitor bulk tank SCC and manage the flock to maintain a SCC of less than 300,000 cells/mL.

Mastitis in Dairy Sheep

In North America, most sheep are kept for production of meat, and most research literature discusses symptoms of mastitis occurring in ewes that are nursing lambs. In this population, only severe clinical mastitis is likely to be diagnosed. A lon­gitudinal study of meat sheep conducted in Great Britain reported that acute mastitis affected 2% to 3% of ewes per year, but there was tremendous variability among flocks.⁵³⁸ These researchers also reported that IMI occurred in 11% of lactating ewes and resulted in reduced weight gain of suckling lambs.⁵³⁸ Udder conformation is a known risk factor associated with increased risk of mastitis in ewes.⁵³⁹

A lack of emphasis on milking ewes has led to an overem­phasis on the occurrence of clinical mastitis and a lack of appreciation for subclinical mastitis.

In almost all instances, mastitis is caused by a bacterial infection. While most bacteria can cause both clinical and subclinical mastitis, S. aureus, Pasteurella haemolytica, and various yeasts and molds are organisms frequently reported to be recovered from milk samples of ewes affected with clinical signs.⁵⁴⁰ Bluebag (clinical mastitis with a hard, cold, swollen udder) is typically caused by either P haemolytica or S. aureus. NAS are considered to be minor pathogens in dairy cows but behave as major pathogens in dairy sheep and have been frequently reported to be the most commonly isolated pathogens recovered from cases of subclinical mastitis in dairy ewes.^535,541 Subclinical infection caused by NAS and other mammary pathogens has been associated with increased SCC.^535,536 Other pathogens typically recovered from subclinical mastitis infections in ewes include Corynebacterium spp., yeast, Streptococcus spp., Enterobacteriaceae spp., and S. aureus. Yeast and mold infections in ewes are often associated with nonhygienic administration of intramammary treatments, and great care must be taken when these treatments are used.⁵⁴²

In rare instances, the lentivirus that causes ovine progressive pneumonia (OPP) has been associated with mastitis in sheep,⁵⁴³ but there is no evidence that this virus has influence on the SCC of sheep milk.⁵³¹ Mammary gland symptoms are associated with lesions in secretory tissue. While it is known that this virus has an affinity for mammary glands, the disease is slowly progressive and results in weight loss, greatly reduced milk production, and other clinical signs that make it unlikely to become widespread in flocks used for dairy production.

Risk factors for subclinical mastitis are not well defined for intensively managed milking sheep in North America. European research in Mediterranean countries has indicated that most of the variation in mastitis is associated with differences in herd management.⁵⁴⁴ In the same study, higher producing breeds were at greater risk of mastitis, and the use of dry-off treatment resulted in less mastitis.^545-547 Practices that improve udder hygiene and reduce teat exposure to bacteria are likely to result in less mastitis.

Ewes that develop clinical mastitis are often seriously ill and should be treated immediately according to protocols that have been developed in consultation with the flock veterinarian. Most treatments for severe clinical mastitis are administered systemically, and the ewe may require supportive therapy. There are no antibiotic compounds approved for treatment or preven­tion of mastitis in milking sheep. Drugs used for these purposes are considered by the FDA to be administered in an “extralabel” manner, and this usage must be prescribed and supervised by a licensed veterinarian. Administration of a drug that is approved for treatment of another sheep disease, such as ceftiofur for treatment of pneumonia, to treat mastitis is also considered extralabel usage. It is important to recognize that systemic administration of ceftiofur will not achieve effective inhibitory levels in the mammary glands of cows, sheep, or goats.

The use of intramammary dry-off treatment has been shown to positively influence milk yield and SCC in the subsequent 542547

lactation and is recommended., However, administration of intramammary treatments does increase the risk of mastitis caused by yeast bacteria, and selective dry-off treatment can be recommended in flocks that have a relatively low prevalence of subclinically affected ewes. Milk samples obtained from ewes with three or more monthly SCCs of 400,000 cells/mL or greater in the previous lactation were six to eight times more likely to be positive for mastitis pathogens in the next lactation, as compared to milk samples obtained from ewes with SCCs below that threshold.

Mastitis in Dairy Goats

Similar to ewes, the incidence of clinical mastitis is generally reported to be less than 5% of lactating does per year.⁵³¹ A study that surveyed about 300 goat dairy farms in Holland reported an incidence of clinical mastitis of 2% per year, and about two thirds of the farms culled the majority of affected does rather than treat them.⁵⁴⁸ One interesting study conducted in Spain linked the incidence of clinical mastitis to selenium deficiency.⁵⁴⁹ Spanish researchers reported that for does consum­ing a deficient diet, the incidence of clinical mastitis was 3.8% and 15.4%, respectively, for does that had been treated with slow-release barium selenite or were enrolled in a nonsupple­mented control group.⁵⁴⁹

There are neither national surveys nor comprehensive reviews that describe the prevalence of subclinical mastitis in dairy goats in the United States or Canada. Review of existing data about the prevalence of subclinical infection is further complicated by the lack of a uniform SCC threshold and the influence of intervening factors (such as estrus) on SCC. When recovery of bacteria from milk samples is used as the gold standard to identify subclinical mastitis, several studies have indicated that half-udder prevalence of subclinical mastitis varies between about 15% and 40%.^550,551 When using a SCC threshold of 500,000 cells/mL for defining subclinical mastitis, researchers have estimated sensitivity (probability of recovery of pathogens when the SCC is higher than the threshold) as ranging from 0.69 to 0.90 and specificity (probability of not recovering pathogens when the SCC is higher than the threshold) of about 0.35 to 0.77.⁵⁵⁰ Although some high SCC values are likely associated with physiologic changes, some reflect IMI, and it is likely that the prevalence of subclinical mastitis in many goat herds is greater than commonly recognized.

Similar to dairy sheep, researchers have consistently reported that NAS are responsible for the greatest proportion of subclini- 531551552

cal mastitis infections occurring in this species.,, Infections with NAS are especially prevalent in goats at parturition, with recovery of NAS from up to 17% of goats reported.⁵³¹ Similar to ewes, the early lactation spontaneous cure rate is only about 50% for IMIs caused by NAS, and up to 25% of does may remain infected 6 weeks after parturition.⁵³¹ SCC values of infected half-udders of does are always significantly greater than SCC values of healthy half-udders. Other pathogens frequently recovered from goats with subclinical mastitis include Corynebacterium spp., Streptococcus spp., and S. aureus. The relationship between lentiviral infections (caprine arthritis encephalitis virus [CAEV]) and SCC has been reviewed,⁵³¹ and herds with greater prevalence of seropositive does have been shown to have greater SCC values. However, this relationship is considered weak and may have been a result of immunosup­pression caused by CAEV infection.

Clinical mastitis in goats is often associated with infection by S. aureus, Streptococcus spp., Pseudomonas⁵⁵ or miscellaneous pathogens such as yeast. In many regions of the world, IMIs are associated with infection by a variety of Mycoplasma spp., and milk samples obtained from goats with chronically increased SCCs should be submitted for Mycoplasma culture. When clinical mastitis cannot be explained by bacterial or fungal infection, Mycoplasma infection should be suspected. A variety of Myco­plasma species can cause clinical mastitis outbreaks in small ruminants. Mycoplasma mycoides subsp. mycoides (large-colony type) is an important mastitis pathogen in goats in the United States, especially in California.^554-557 Mycoplasma putrefaciens is involved in some outbreaks,⁴⁶⁴ and Mycoplasma agalactiae is isolated on rare occasions.⁵⁵⁶ Does with Mycoplasma mastitis often exhibit severe clinical illness and reduced milk yield, in addition to an inflamed udder and visibly abnormal milk; however, subclinical IMIs can occur. Adult goats and kids in herds with Mycoplasma mastitis may experience polyarthritis and pneumonia, and pregnant does may abort. Morbidity and mortality can be very high, with more than 90% of goats dying or euthanized in some herds.⁵⁵⁶

Because all mastitis treatments involve extralabel drug usage, treatment of clinical mastitis should be performed using protocols developed by a veterinary practitioner who has a valid veterinary-client-patient relationship. Treatment of systemically ill animals should focus on supportive care and appropriate antimicrobial therapy. Treatment of animals with local signs of clinical mastitis generally involves administration of commercial intramammary products and should be accom­panied by microbiological assessments of at least some cases. Treatment of subclinical mastitis is unlikely to be pursued by most farms, and aggressive culling of affected animals has been shown to be associated with herds that have lower bulk tank SCCs.⁵⁴⁸ At least one study has demonstrated that treatment of subclinical mastitis in early lactation based on CMT reaction resulted in increased bacteriologic cure but was not economically beneficial.⁵⁴⁸ Thus treatment of subclinical infections during lactation is not currently recommended. However, the use of dry-off therapy has been shown to effectively cure NAS infec­tions and result in lower SCCs in early lactation.⁵⁵⁸ As with sheep, producers should be taught to use extreme care when disinfecting teat ends to prevent the iatrogenic development of IMI caused by yeast.

Mastitis is an important disease of small ruminants used in dairy production, and its prevalence varies with management. Most mastitis occurs in a subclinical form, and producers who do not routinely measure individual animal SCCs will be unable to determine the impact of subclinical mastitis on production and milk quality. Most subclinical mastitis in small ruminants is caused by NAS, which should be considered as major mastitis pathogens in these species. Prevention of infection is the key to control of mastitis, and good hygienic housing and milking practices are a necessity to minimize the impact of this disease. As in all dairy species, exposure of the teat end to bacteria is the mechanism for development of mastitis, and control programs are based on principles that improve hygiene and reduce exposure to potential pathogens. The prevalence of subclinical mastitis has been shown to be decreased in goat herds that practice good teat dipping and premilking teat sanitation.⁵⁵⁹

Mastitis in Horses

Mares have a uniquely different anatomic structure of the udder. Similar to the bovine udder, the equine udder is com­posed of four independent glands; however, the pairs of glands on each side of the midline share a single teat and are referred to as halves.⁵⁶⁰ Mastitis occurs uncommonly in horses, in part because of the relatively small size and inguinal location of the udder, which minimizes exposure to potential pathogens. Little research has been done on the prevalence of subclinical mastitis in horses, but it is not recognized to be a serious problem and a longitudinal study estimated the prevalence to be 5% to 10%.⁵⁶¹ SCCs in uninfected glands have been reported to be relatively low (in national mastitis council, Atlanta, GA, 2000, National Mastitis Council.

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