In the USA and Canada, serovars pomona and grippotyphosa are the most common causes of equine leptospirosis. The prevalence of leptospirosis in horses is unknown, but serologic evidence indicates a higher incidence than is apparent clinically. Antibodies to serovar Bratislava are reported frequently in the USA, and in Europe horses are thought to be a maintenance host for this organism. Clinical leptospirosis in horses is most commonly associated with abortions, systemic illness in foals, and recurrent uveitis.

Leptospirosis is responsible for 2–4% of all equine abortions annually, although high rainfall and flooding may result in abortion outbreaks. Abortions typically occur 1–3 wk after a mild, nonspecific clinical illness in the mare characterized by fever, anorexia, depression, and in rare cases, icterus. Lesions in aborted foals are similarly nonspecific and are often compromised by autolysis. Leptospires can be detected in the placental and fetal organs by immunohistochemistry, immunofluorescence, PCR, or culture. Aborting mares typically have very high leptospiral antibody titers at the time of abortion.

In foals, acute leptospirosis is typical with severe forms including hemolysis and vasculitis with petechial hemorrhages on mucosal surfaces, hemoglobinuria, anemia, icterus, conjunctival suffusion, depression, and weakness. Renal failure and hepatopathy may also occur.

The precise role of leptospirosis in the development of recurrent uveitis (see Equine Recurrent Uveitis) remains controversial, although the condition has been reproduced experimentally in horses infected with serovar pomona. Typically, uveitis develops 2–8 mo following the initial infection. Leptospires have been identified in aqueous humor of >50% of horses with recurrent uveitis in several studies, but have not been detected in similar cases by other investigators. It is not clear whether the uveitis is due to persistent or repeated intraocular infection or is immune-mediated. Uveitis is treated symptomatically to reduce inflammation and prevent synechiae although in severe, chronic cases, intraocular antibiotics or vitrectomy have been used.

Horses infected with serovar pomona may shed the organism in their urine for 3–4 mo after infection. Antibiotics have not been shown to significantly reduce the shedding period. In acute disease, systemic antibiotics are useful, and antibiotics are also used to prevent further abortions in co-housed or exposed pregnant mares. There are no leptospiral vaccines for horses.

Last full review/revision March 2012 by Carole Bolin, DVM, PhD