Dryland Distemper

One of the most commonly diagnosed bacterial problems in California (and several other western states) is dryland distemper, otherwise known as pigeon fever. 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 and midline. Clinical signs can include lameness, fever, lethargy, and weight loss. Dryland distemper 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, DVM, PhD, Dipl. ACVIM, of the University of California, Davis. The disease is seasonal, with the majority of cases seen in the late fall, but sporadic cases can "pop up" during other times of the year.

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. This causes swelling that looks like a puffed-out pigeon breast, thus giving the name pigeon fever to the disease.

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

The disease occurs in three forms--external abscesses, internal abscesses, or limb infection known as ulcerative lymphangitis. The external abscess "form" is the most common, said Spier.

For external abscesses, the use of antibiotics is controversial, and timing is important. The use of antibiotics for external abscesses might actually prolong the infection.

Antibiotics do need to be used for internal abscesses or for infections involving the limbs (ulcerative lymphangitis), said Spier.

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.

(Editor's Note: There have been multiple e-mail questions regarding this disease in recent weeks. If you suspect your horse has dryland distemper, contact your veterinarian for a diagnosis and a proper treatment regimen. Some information for this article was taken from Equine Internal Medicine by Stephen M. Reed, DVM, Dipl. ACVIM, and Warwick M. Bayly, BVSc, PhD, Dipl. ACVIM.)

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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