Corynebacterium pseudotuberculosis is a gram-positive, pleomorphic, intracellular, facultative anaerobic rod with worldwide distribution. The bacterium causes ulcerative lymphangitis, external subcutaneous abscesses, and internal abscesses (systemic infection) in horses. The North American geographic distribution of disease is concentrated in the western and southwestern regions of the United States, but cases of C pseudotuberculosis infection have been reported throughout the United States. In recent years, disease caused by this organism has increased in prevalence in the Northwest and Midwestern United States and Western Canada.
Two species-specific biotypes of C pseudotuberculosis have been identified on the basis of differences in nitrate reduction, and DNA fingerprinting techniques have revealed multiple strains. Biotypes isolated from small ruminants are nitrate-reduction negative, whereas those from horses are nitrate positive. From the results of DNA studies, the terms biovar equi for nitrate-positive and biovar ovis for nitrate-negative strains were proposed. Natural cross-species transmission does not seem to occur between sheep and horses; however, cattle can become infected with either biotype.
The biologic reservoir for C pseudotuberculosis is soil. The equine biovar is able to survive and multiply in different types of soil under a wide range of environmental conditions.
The portal of entry of this soil-borne organism is thought to be through abrasions or wounds in the skin or mucous membranes. Epidemiologic studies suggest that the disease can be transmitted through horse-to-horse contact or from infected to susceptible horses by insects, other vectors, or contaminated soil; the incubation period is variable, estimated at 3 to 4 weeks. Many insects have been incriminated as vectors for transmission of the disease to horses, including Haematobia irritans, Musca domestica, and Stomoxys calcitrans. The regional location of abscesses suggests that ventral midline dermatitis is a predisposing cause of infection.
The incidence of disease fluctuates considerably from year to year, presumably because of environmental factors, such as drought and temperature and herd immunity. To date, the definitive environmental factors supporting the spread of infection have not been determined. The highest incidence is observed in the summer and fall, although cases may be seen all year.
Most horses experience a single episode of infection, becoming resistant to infection in subsequent years. In one retrospective study, 9% of the study horses had recurrent episodes of infection over subsequent years, and 8% had systemic infection. Horses with systemic infection and horses with more complicated cases of infection are often detected 1 to 2 months after having external infection or after there has been an outbreak on the premises. No breed or sex predisposition has been documented. Horses of all ages may be affected, although the low incidence of disease in foals younger than 6 months suggests possible colostral protection in foals born to mares in endemic areas.
Clinical and Clinicopathologic Features
External abscesses may arise anywhere on the body but develop most frequently in the pectoral region and along the ventral midline of the abdomen. This form of infection is commonly known as pigeon fever, because of the large size of the pectoral abscesses giving the contour or appearance of a pigeon’s breast, or dryland distemper, because of its prevalence in arid geographic regions. Abscesses contain tan, odor-free, purulent exudate and are usually well encapsulated. Additional sites with a predilection for abscess formation include the prepuce, mammary gland, axilla, triceps musculature, limbs, and head. Less common areas are the thorax, neck, parotid gland, guttural pouches, larynx, flanks, umbilicus, tail, and rectum. Septic joints and osteomyelitis have been reported. Horses may have an abscess involving a single site or involving multiple regions of the body. It is common to observe multiple subcutaneous abscesses coursing along a suspected lymphatic vessel.
The clinical signs most frequently associated with external abscesses are regional edema, fever, and nonhealing wounds. Other clinical signs include lameness, ventral dermatitis, weight loss, depression, anorexia, and mammary gland or preputial swelling. Generally, horses with external abscesses do not develop signs of systemic illness, although one fourth will develop fever. If signs of systemic illness are present, further diagnostic testing to rule out internal infection is warranted. In horses with external abscess formation, a large area of edema is often observed in the region. As the abscess matures, the area becomes hard and painful. Abscesses can become quite large, particularly in the pectoral region. Abscesses typically have a thick capsule and can cause severe lameness if located in the axillary, triceps, or inguinal region. Maturation can be slow and drainage difficult to establish if the abscess lies deep to muscle.
After drainage is established, either by spontaneous rupture or lancing, most horses recover within 10 to 14 days without complications. The abscesses may contain 5 to 400 mL of thick, tan, purulent exudate. Ultrasonography may aid in determining the best location for establishing drainage in external abscesses. The case fatality rate for horses with external abscesses is very low (0.8%); recovery generally is complete within 2 to 4 weeks, although rarely horses develop persistent or recurrent infections lasting for more than 1 year.
Horses with severe acute or chronic lameness usually have abscesses deep in musculoskeletal structures associated with the limbs that should be surgically drained, rather than being managed conservatively by waiting for the abscess to mature. Affected horses typically have an inflammatory leukogram, with anemia and hyperglobulinemia. High synergistic hemolysis inhibition (SHI) titers are common, and ultrasonography may be useful in assisting with localization of lesions and facilitating surgical drainage to alleviate lameness. Rarely, osteomyelitis or septic arthritis develops, carrying a poor prognosis.
Regardless of the form of C pseudotuberculosis infection, clinicopathologic abnormalities that may be observed include anemia of chronic disease, leukocytosis with neutrophilia, hyperfibrinogenemia, and hyperproteinemia. These hematologic changes can occur with both internal and external abscesses but are more consistently observed with internal abscesses.
Approximately 8% of affected horses develop internal infection, which is associated with a case fatality rate of 30% to 40%. In a retrospective study, infection was localized to a specific organ or organs in 90% of horses (27 of 30). Involvement of multiple internal organs was identified in 37% of horses (10 of 27). The organs most often involved were the liver and lungs, with the kidney and spleen being affected less often. Abdominal ultrasonography was a useful diagnostic tool for identifying specific affected abdominal organs.
Diagnosis of internal infection is made on the basis of clinical signs, clinicopathologic data, serology, diagnostic imaging, and bacterial culture. The most common clinical signs are concurrent external abscesses, decreased appetite, fever, lethargy, weight loss, and signs of respiratory disease or abdominal pain. Other signs observed in horses with internal abscesses include ventral edema, ventral dermatitis, ataxia, hematuria (caused by renal abscesses), and infrequently, abortion. The median ages in two studies were 7 and 8 years, with a range of 1 to 23 years. Anemia of chronic disease, leukocytosis with neutrophilia, and high fibrinogen are common features of infection, particularly in horses with internal abscesses. Leukocytosis with neutrophilia was seen in 36% and 76% of horses with external and internal abscesses, respectively. Hyperproteinemia, caused by increased serum globulin concentrations, was observed in 38% and 59% of horses with external and internal abscesses, respectively.
Peritoneal fluid is frequently abnormal in horses with abdominal abscesses. However, peritoneal fluid analysis may be unremarkable if abscesses are located retroperitoneally in the kidneys without involvement of other abdominal structures. Corynebacterium pseudotuberculosis was isolated from 32% of samples of peritoneal fluid from affected horses in one study; however, failure to isolate the organism from peritoneal fluid does not rule out the disease. The organisms could be located retroperitoneally, sequestered within a thick capsule, or suppressed by local factors or nucleated cells.