by William V. Bernard, DVM, MS
mare recently aborted in late gestation. My veterinarian
mentioned leptospirosis as a possible cause. What is
leptospirosis and what can you tell me about how horses
get it, what it does to them, and how to treat it?
Leptospires are bacteria. They are quite different
from other bacteria in that they are spiral-shaped and
are motile. As a result of their shape, they are
classified as spirochetes. Other spirochetes of medical
importance are Treponema and Borrelia. Treponema pallidum
causes syphilis in humans. Borrelia burgdorferi is the
bacteria that causes Lyme disease.
Leptospires are grouped into two complexes: L.
interrogans and L. biflexa. L. interrogans are the
disease-causing leptospira and are further divided into
serovars and serogroups. Serovars consist of closely
related serogroups. The serovars of interest to our
discussion are pomona, grippotyphosa, hardjo, bratislava,
canicola, and icterohaemmorhagiae. All of these serovars
can cause disease in the horse. However, abortion in the
horse is most frequently associated with L. pomona
infection. L. bratislava is thought to be "host
adapted" to the horse. In other words, it can exist
in the horse without causing disease. This was first
shown by a researcher in Ireland who cultured L.
bratislava from horses which did not have disease, and it
was substantiated by other researchers in Canada and the
United States who found a high frequency of titers
(exposure) to L. bratislava with the absence of disease.
Leptospires are very common bacteria which can be
found in a variety of domestic and wild animals. When
leptospires are shed into the environment and come into
contact with the mucous membranes of a potential host,
then disease can ensue. The spirochetes invade mucous
membranes and/or damaged skin and migrate to various
organs in the body. In the case of the mare which
aborted, the leptospires localized in the uterus/placenta
and subsequently in the fetus, resulting in the death of
Other problems in horses can be caused by leptospires
besides abortion, including renal (kidney) or ocular
(eye) problems. Leptospires also can result in the birth
of stillborn or sick foals. The kidney infections and
neonatal disease caused by leptospirosis are not common.
However, the eye problems-termed recurrent uveitis or
moon blindness-are suspected to be more frequent. The
recurrent uveitis (which is an inflammation of the eye)
can become a chronic problem and can result in blindness.
The renal infections can occur in any age horse. The
disease in foals causes weak foals that might be term or
premature. They might have fevers, clinical evidence of
infection, and/or hematuria (bloody urine).
There is not an approved vaccine for the horse against
leptospirosis. Maintaining as hygienic an environment as
possible, and isolating mares with confirmed leptospiral
abortions, might prevent spread of the disease. However,
I should mention that the disease is not common and
varies in distribution through the country depending on
Antibiotics can be effective in treatment of some
forms of leptospirosis. Newborn foals with leptospirosis
have been treated successfully with penicillin
intravenously. Older foals and adult horses with renal
disease have also been effectively treated with
antibiotics. The ocular disease is not likely a result of
direct infection in the eye, therefore antibiotics might
not be effective. Treatment of uveitis usually consists
of topical, symptomatic therapy that will vary depending
on ocular examination by your veterinarian.
Unfortunately, mares infected with leptospirosis
usually abort without premonitory signs. Mares which
abort often have very high titers to one or more
leptospira serovars. These mares often shed the organism
in urine, which is a possible source of infection for
other individuals, as is the aborted fetus and placenta
(mares walk up and sniff the contaminated aborted fetus
or placental fluids and become infected through the
The definitive method of diagnosis of leptospirosis is
by direct identification of the organism through culture
or immunoflourescence (staining). When these techniques
are not successful or feasible, then serology (a blood
test for antibodies) can provide a presumptive diagnosis.
Paired serology-a four-fold change in titer-is more
suggestive of active or recent infection. However,
interpretation of serology must be cautious and
conservative as individual positive titers are not
definitive for leptospiral infection.
Leptospirosis is a zoonotic disease, which means it
can affect many types of animals, including humans. Human
exposure to uterine fluids and to urine should be
minimized when leptospiral infection has been identified.
To my knowledge, there has been no transmission of
leptospira from horses to man. However, transmission from
cattle to man has been reported.