Dry weather throughout large portions of the United States seems to have increased the incidence of a disease in horses called pigeon fever or dryland distemper. The bacteria that cause the disease, Corynebacterium pseudotuberculosis, survives best in the environment under hot and dry conditions. C. pseudotuberculosis can spread from animal to animal via flies, so unusually large fly populations may also be playing a role.
Pigeon Fever is typically a problem in the Western United States. Recently however, the Louisiana Department of Agriculture and Forestry announced that at least 100 cases of pigeon fever were reported in that state in 2011. More than 30 of those cases occurred late in the year. In the spring of 2012, several cases of pigeon fever were reported in northwestern Florida, which led to the cancellation of the Dixie Gulf Arabian Horse Association Show to protect against the spread of the disease.
Horses infected with Corynebacterium pseudotuberculosis often develop abscesses in their chest muscles, causing this area to swell and giving them a “pigeon breast” appearance, which explains how the disease got its name. Affected horses may have a stiff gait and be sore but usually continue to eat and don’t seem to feel all that bad.
Pigeon fever less commonly affects other parts of the body. When abscesses form in internal organs or the infection involves the lymphatic system (a condition called ulcerative lymphangitis), horses can become very sick exhibiting high fevers, lethargy, and loss of appetite.
Treatment and Prevention
Horses that have only external abscesses tend to recover uneventfully once the abscesses drain – either after rupturing on their own or being lanced by a veterinarian. The use of horse antibiotics is somewhat controversial in these cases. It does not appear to improve outcomes in horses with only external abscesses and is therefore usually reserved for those cases where internal involvement is suspected. Treating horses with internal or lymphatic involvement is more difficult and carries with it a guarded prognosis.
Pigeon fever is contagious. When pus from an abscess drains into the environment, the bacteria that are released can spread to other horses (or cattle) via direct contact, flies, people’s hands, shoes, clothing, etc. Lancing an abscess is preferable to having it rupture on its own because this procedure allows the pus to be collected and discarded and the environment disinfected which limits the chances that the disease will spread.
Use a fly ointment around wounds, fly spray, and other fly control measures (i.e. truck manure off site) to reduce the risk of pigeon fever making an appearance on your farm. A preventative vaccine is not available at this time. Horses diagnosed with pigeon fever should be isolated as they recover.