Unusually High Number of 'Pigeon Fever' Cases Reported in Kentucky

By Kimberly S. Herbert
An unusual outbreak of the bacterial disease "pigeon fever," also known as dryland distemper, has been occurring in Kentucky over the past three weeks, according to Dr. Doug Byars, a specialist in internal medicine and equine critical care. He said 15 cases have been confirmed with bacterial cultures in the last three weeks by Hagyard-Davidson-McGee veterinary associates in Lexington. He said about 15 more cases have been clinically diagnosed. Byars said there were mini-clusters of two or three horses on some farms.

"Pigeon fever has been reported east of the Mississippi before, but in solitary cases," said Byars. "We found it in one individual about seven years ago (in Kentucky). I know Florida has reported increasing numbers in recent years. It should be going away when the winter weather sets in."

Pigeon fever is one of the most commonly diagnosed bacterial problems in California (and several other western states). This disease is caused by Corynebacterium pseudotuberculosis and is seen worldwide. It usually is associated with very deep abscesses and multiple sores along the chest (hence the name pigeon fever, as the chest swells up and resembles a large pigeon breast). Occasionally there will be sores on the midline and abdomen, or even in aberrant places such as the back. Byars said there also can be times when the bacteria causes an ulcerative lymphangitis (which causes the hind legs swell and "bust out" in crusts). Horses also can suffer from internal abscessation.

While horses affected by pigeon fever usually have a good prognosis since the bacteria is very sensitive to antibiotics, noted Byars, the one Kentucky horse diagnosed with internal abscesses died.

Clinical signs can include lameness, fever, lethargy, and weight loss. Pigeon fever can occur in any age, sex, or breed of horse, but most cases occur in young animals (less than five years of age), according to Sharon Spier, of the University of California at Davis. The disease is seasonal, with the majority of cases seen in the late fall in California, but sporadic cases can "pop up" during other times of the year, she said. Some years have more cases than other years, but researchers don't know why. Similar to strangles, outbreaks can occur when herd immunity wanes or naive horses are exposed.

The causative bacteria live in the soil and can enter the horse's body through wounds or broken skin, and through mucous membranes. It can be transmitted by various flies, including house flies and probably horn flies.

Dryland distemper might take weeks or months for abscesses to develop fully after the horse is infected. This means that horses might be transported to a region where dryland distemper is unknown, develop active abscesses or sores, and because of the scarcity of the disease in that region, not be diagnosed properly, or at all. Abscesses usually form deep in muscles, such as the pectorals (chest). 

These abscesses can be very large and might require hot poultices, lancing, flushing, or draining. Some cases might require surgical intervention to promote drainage.

While prognosis generally is good for a complete recovery, some horses might have recurrence of abscesses or sores once treatment is stopped. Other horses might seem to be cured, only to develop more clinical signs in a matter of months.

It is recommended that contaminated stalls, paddocks, and utensils be thoroughly cleaned and disinfected where possible. Because flies can carry the bacteria, pest control can serve as a deterrent to spread or continuance of the disease.