Known as pigeon breast, dryland distemper, and Colorado strangles, this disease is unpredictable, contagious and, thankfully, curable in most cases.
You go out to the pasture to check on some horses you haven’t looked at for a few days and notice one of them has swelling over the pectoral muscles (front of the chest) or along the underside of the belly, and is walking stiffly. Your first assumption is bruising from a kick, since horses at pasture are always playing around, and the swelling is in a location for a possible kick wound. But when your veterinarian examines the horse, the diagnosis is not trauma; instead, it’s an infection deep in the tissues, commonly called pigeon fever – since swellings on the front of the horse look like the protruding breast of a pigeon. Originally thought to be a disease that only crops up sporadically in a few dry regions of the West, it has been quietly expanding its territory. The past five years have seen an alarming increase in cases, in regions where it was never seen before.
For instance, Fred Robinson, D.V.M, of Riverside Veterinary Clinic, Pendleton, Ore., says his county had an extensive outbreak in 2005, with cases showing up over a large area of northeastern Oregon – a state that historically had not experienced this disease.
“For some reason, we seem to be a hot spot,” Robinson said.
This bacterial infection is caused by Corynebacterium pseudotuberculosis and characterized by deep intramuscular (and sometimes internal) abscesses in horses. It has also been called pigeon breast, dryland distemper, and Colorado strangles. According to Sharon J. Spier, D.V.M, Ph.D., Dipl. ACVIM, and professor in the Department of Medicine and Epidemiology at UC Davis School of Veterinary Medicine, this disease was first reported in horses in San Mateo County of California in 1915. Since then, it has been recognized in many Western states and, in recent years, has spread to other areas, including Kentucky and Florida.
Spier has been doing research on this disease for more than 20 years. There are actually three forms. Most common is the appearance of external abscesses in the pectoral region (breast muscles) or ventral abdomen (along the midline of the belly). Much less common is involvement of internal organs, with abscesses appearing in lungs, liver, kidneys or spleen. Least common in North America is infection of limbs (ulcerative lymphangitis, with swelling and multiple draining lesions on the hind legs). In a study of infections in California, Spier found that external abscesses made up about 81 percent of cases, internal abscesses about 8 percent, and ulcerative lymphangitis about 10 percent of infections. Some veterinarians who deal with pigeon fever have never seen the lymphangitis form of the disease, however, even though they occasionally see cases with internal abscesses.
Sheep and goats are sometimes infected with the same organism, but it’s a different biotype; horses can’t get pigeon fever from sheep or goats. But cattle can be infected with either type, and horses could theoretically get the disease from cattle and vice versa, particularly if they live in the same pastures. These bacteria occur worldwide, and live in the soil. The organism has been shown to survive for up to two months in hay or bedding (straw, shavings) and more than eight months in soil samples.
Incidence of disease fluctuates from year to year, possibly due to herd immunity and environmental factors such as temperature and rainfall, with the highest numbers of cases occurring during dry months – especially fall and early winter. Since flies are thought to be involved in the spread of the disease, and since it takes awhile for abscesses to develop after a horse is infected, most cases start showing up several weeks after fly season begins, and may keep cropping up for weeks after flies are gone in late fall.
Horses kept outdoors or with access to an outdoor paddock seem to be at higher risk than stabled horses, and foals less than 6 months old seem at least risk, which suggests that passive transfer of antibodies via the dam’s colostrum may offer protection to foals born in regions where the disease exists. Horses with internal abscesses are generally diagnosed one to two months after the peak number of cases with external abscesses, according to Spier.