by William V. Bernard, DVM, MS

My 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 the fetus.

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 environmental conditions.

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 mucous membranes).

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.

>William V. Bernard, DVM, MS, Diplomate American College of Veterinary Internal Medicine, is a partner with the Rood and Riddle Equine Hospital near Lexington, Ky.

Courtesy of the AAEP