Many horse owners in central California may have heard of “Pigeon Fever” — and if not, now’s the time to learn about it. 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 more arid parts of our state are hit harder by Pigeon Fever. Some years the prevalence seems to be much higher than others, with almost a cyclic nature.
Pigeon Fever is caused by a bacteria called Corynebacterium pseudotuberculosis that can survive for long periods of time in the soil. The bacteria is likely to penetrate the skin through abrasions, small wounds, or enter by fly bites (most common). Once the bacteria has entered the horse, it proliferates in these warm, moist cutaneous environments.
Pigeons actually have nothing to do with the disease, but it historically causes abscesses and dramatic swelling in the pectoral region of the horse, making the horse’s chest resemble that of a pigeon. Other names for the condition include “Dryland Distemper,” “Pigeon Breast Fever,” “Dryland Strangles,” and its bacterial name, Corynebacterium pseudotuberculosis.
The typical signs of Pigeon Fever include abscesses anywhere along the ventral midline (i.e., especially where flies bite!) including the chest, sheath/mammary glands, or in small chains along the lymphatic channel of a leg. However, abscesses can develop anywhere on the horse (see facial abscess image). Large plaques of ventral edema are also frequently present adjacent to the abscesses (ventral edema causes sponge-like swelling on the underbelly of horses)(see edema image). Pigeon fever abscesses usually take several weeks to mature, open, and drain, but rarely the infection may be long-lasting and recurrent for over a year.
Pigeon Fever should be considered at the top of the differential list in any horse that has a swelling or abscess in a typical location. The diagnosis can be confirmed by culturing fluid taken from an abscess and growing the bacteria in the lab.
Uncommonly, C. pseudotuberculosis can instead cause an internal abscess (less than 3% of all cases). Horses with internal abscesses present with fever, weight loss, depression, and sometimes lameness. If a veterinarian suspects an internal abscess, there is also an antibody blood test that can be performed to help rule in or rule out the disease. This blood test is performed at the University of California at Davis and is very affordable.
Treatment of Pigeon Fever can vary depending on the severity and the body system(s) involved. Draining the abscess is the mainstay of treatment (see drainage image) but should not be performed until the abscess is mature. If done prematurely, drainage can be painful and the abscess is more likely to recur.
The maturity of an abscess can be assessed by palpation (they generally get soft in the middle when they are mature) and/or by ultrasound. Ultrasound is also helpful in determining if there are multiple pockets of fluid and in identifying deep abscesses.
Once opened, the abscess cavity should remain open and it should be flushed daily with an antiseptic solution like povidone-iodine or chlorhexidine. Most horses will be completely over the disease within 3 weeks of the abscess being drained. Again, rare horses will have recurrent abscesses. Use of antibiotics is controversial in horses with Pigeon Fever. Many veterinarians recommend against administering antibiotics for external abscesses because of the potential to delay abscess maturation. However, in cases involving internal abscesses, those involving the lymph channels of a leg (“lymphangitis”), or with very deep abscesses that are difficult to drain and are causing the horse extreme discomfort, long-term antibiotics are generally prescribed. While horses affected with external abscesses have an excellent prognosis, internal abscesses have a more guarded prognosis.
Once a Pigeon Fever abscess matures and the condition resolves, over 90% of horses will remain immune to the disease in the future. In those rare instances where the disease recurs, it is unknown whether recurrence is due to re-infection or relapse of the original disease. It is possible it is more likely to recur, or even occur in the first place, in horses with somewhat compromised immune systems.
Prevention is centered on good sanitation practices and fly control. Quarantine of affected individuals is not generally recommended due to the long distances that insects carrying the bacteria can travel. A vaccine has recently become available from Boehringer-Ingelheim and may be advisable in naive horses at high-risk for disease. The use of this vaccine should be a discussion between each owner and his/her veterinarian.