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By Sharon Spier, DVM, Ph.D., DACVIM

Infection caused by Corynebacterium pseudotuberculosis (commonly known as pigeon fever or dryland distemper) assumes three forms: external abscesses, internal infection, and ulcerative lymphangitis (limb infection). The most common clinical form of the disease, characterized by external abscesses in the pectoral or ventral abdomen, is often called “pigeon fever,” due to the swelling of the horse’s pectoral region resembling a pigeon’s breast. Veterinarians also call it “dryland distemper,” reflecting the prevalence in arid regions of the western United States. There appears to be no breed or sex predilection for the development of any of these forms. Portals of entry for this soil-dwelling organism are thought to be abrasions or other skin wounds, or mucous membranes. Studies have shown that insects such as the horn fly, housefly, and stable fly can act as mechanical vectors for the transmission of this disease to horses. The disease could also be transmitted through horse-to-horse contact or from infected to susceptible horses via insects, other vectors, or contaminated soil.

The first deep intramuscular abscesses caused by C. pseudotuberculosis were reported in California in 1915, and the disease has since been considered one of the most frequently diagnosed equine infectious diseases in the western United States. Evidence exists that infection is increasing in incidence, possibly associated with climate change. Unprecedented epidemics in the past decade have affected thousands of horses in Colorado, Idaho, Kentucky, New Mexico, Oregon, Utah, and Wyoming, all states that historically had low prevalence of the disease. High temperatures and drought conditions preceded all reported outbreaks of disease. The incidence of pigeon fever fluctuates considerably from year to year, presumably due to herd immunity and environmental factors such as rainfall, temperature, and insect populations. Disease incidence is seasonal, with the highest number of cases occurring during the dry months of the year, although cases might be seen all year.

External abscesses can occur anywhere on the body, but they most frequently develop in the pectoral region and along the ventral midline of the abdomen. Generally, horses with external abscesses do not develop signs of systemic illness, although 25% will develop fever. If signs of systemic illness are present, further diagnostics to rule out internal infection are warranted. A diagnosis of internal infection is based on clinical signs, serology, diagnostic imaging, and bacterial culture. The most common clinical signs are concurrent external abscess, decreased appetite, fever, lethargy, weight loss, and signs of respiratory disease or abdominal pain. Ulcerative lymphangitis is the least common form of pigeon fever. Signs include limb swelling (hind limbs are affected more commonly), cellulitis (skin infection), and draining tracts that follow lymphatics in the horse’s body. Affected horses often develop a severe lameness, fever, lethargy, and anorexia.

The veterinarian must individualize the treatment regimen for a horse’s external abscesses, depending on the severity of disease. Establishing drainage is the most important treatment and ultimately leads to faster resolution and return to athletic performance. Antimicrobials are indicated for horses with internal abscesses and for those with ulcerative lymphangitis.

Until a protective vaccine is developed for horses, we can only suggest that horse owners in endemic areas practice good sanitation and fly control and avoid unnecessary environmental contamination from diseased horses. Feeding supplements containing insect growth regulators and using fly parasites and adult fly traps also reduce pest populations. Proper sanitation, disposal of contaminated bedding, and disinfection might reduce the incidence of new cases. Proper wound care (including avoiding fly bite dermatitis on the ventral midline) is also important to prevent contamination from the horse’s environment and resulting infection. 

About The Author

Sharon Spier, DVM, PhD, Dipl. ACVIM, is a professor in the Department of Medicine and Epidemiology at the University of California, Davis.

Article reprinted Courtesy of The Horse: Your Guide to Equine Health Care, an AAEP Media Partner.