“Pigeon Fever” is a disease about which horse owners in Central California should be aware. Pigeons have nothing to do with pigeon fever other than the disease can cause a pronounced swelling on the horse’s chest making them look like a pigeon. Pigeon fever is caused by a gram positive bacterium called Corynebacterium Pseudotuberculosis. This bacterium is found in the soil and is thought to be transmitted to horses by flies feeding on abrasions or small wounds already present in the skin. Although also seen throughout the southern United States, California is particularly well known for having a high incidence of this disease, especially during the late summer and early fall. The disease tends to be more common in the more arid regions of the state. Like so many other diseases, there are some years in which the prevalence seems to be much higher than others.
There are several different forms of the disease with the most common being external abscesses on the chest or along the ventral midline extending to the sheath or mammary gland. Large plaques of ventral edema may be present prior to the abscess being visible. The second form is called ulcerative lymphangitis which occurs in the lymphatics in the legs. In this form you may notice heat, pain, and swelling along the limb with small abscesses up and down the leg. In approximately 8% of cases the disease can also spread internally, causing abscesses in the abdominal or thoracic cavity. In these cases the horse may experience fever, weight loss, depression and lameness.
The diagnosis of pigeon fever is often presumptive, based on finding an abscess in a typical location. To confirm the diagnosis purulent material collected from an abscess can be cultured. If a positive culture is not obtained or if your veterinarian suspects an internal abscess, there is also a blood test that can be performed to help diagnose the disease by looking at an antibody titer. Treatment of the disease may vary depending on the severity of infection and on which body systems are involved. Draining the abscess is the mainstay of treatment but should not be performed until the abscess is mature. If done prematurely, the abscess is more likely to recur. Abscess maturity can be determined by palpation (they generally get soft in the middle when they are mature) and by ultrasound. Ultrasound is also helpful to determine if there are multiple pockets of fluid and to identify deep abscesses. Once opened, the abscess cavity should be flushed daily with an antiseptic solution such as betadyne or nolvasan. Most horses will be completely over the disease within 2-3 weeks of draining the abscess. Once the abscess matures and the condition resolves, over 90% of horses will remain immune to the disease in the future. In some cases horses continue to form new abscess for several months before clearing the disease. Horses that have excessive difficulty clearing the infection should be evaluated for possible causes of immune suppression (such as Equine Cushings Disease). Horses affected with external abscesses have an excellent prognosis, but internal abscesses can be more devastating requiring extensive antibiotic treatment and are fatal in up to 30-40% of cases. The use of antibiotics for treatment of external abscesses is controversial. Many veterinarians advise against administering antibiotics for external abscesses because of the potential to delay abscess maturation and the prolonged period of time a horse may need to be treated to clear the abscess up. However, in cases of internal abscesses or ulcerative lymphangitis long-term antibiotics are necessary for treatment.
Pigeon fever is not a contagious disease in the classical sense. If horses are actively draining they should kept confined to their pen. The purulent materials present in an abscess will contain large amount of the bacterium which increases the soil contamination. Horses are not contagious to other horses if they are not draining purulent material. If they have an internal abscess or an abscess that has not matured yet they cannot infect other horses. Dilute bleach solution can be used to clean up an area where an abscess was lanced or flushed out.