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, gender, 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
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
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.
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.