Dryland Distemper Common in Western U.S.

One of the most commonly diagnosed bacterial problems in California (and several other Western states) is dryland distemper, commonly known as pigeon fever. This disease is caused by Corynebacterium pseudotuberculosis and is seen worldwide, writes Editor Kimberly S. Graetz in the December edition of The Horse.

Dryland distemper 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. The disease can occur in any age, sex, or breed of horse, but most cases are seen in young animals (less than five years of age), according to Dr. Sharon Spier of the University of California, Davis.

Dryland distemper is seasonal, with most cases occurring in late fall, but sporadic cases can pop up during other times of the year. Some years have more cases than others, but researchers don't know why. Outbreaks can occur when herd immunity wanes or naïve horses are exposed.

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

After the horse is infected, it might take weeks or months for abscesses to develop fully. This means that a horse 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 the name pigeon fever.

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

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, Spier said.

For external abscesses, the use of antibiotics is controversial and timing is important. The use of antibiotics to treat a horse with external abscesses might actually prolong the infection. Antibiotics do not need to be used for internal abscesses or for infections involving the limbs (ulcerative lymphangitis), Spier said.

While prognosis is generally 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.