Preventing Leptospiral Abortion in Horses

One cause of late abortion in mares is infection with leptospires, bacteria that are found worldwide. The infection is spread by contact with urine from affected small mammals. The urine of infected horses also contains active bacteria and can be a source of infection, such as when urine splashes onto ground-fed hay that is then eaten by another mare.

Mares infected with leptospires rarely show any signs of illness before having an abortion or giving birth to a dead or nonviable foal. Mares can be screened for infection through blood tests, but there are many positive reactions among mares without an active infection. Urinalysis can detect mares that are shedding leptospires so that those mares can be separated from the uninfected members of the herd.

Cases of leptospiral abortion vary from year to year, with wet fall weather sometimes correlated to an increase in numbers. Figures from the University of Kentucky Veterinary Diagnostic Laboratory showed peaks of 42 cases in 2007 and 31 cases in 2010. There were no cases in 2011 and only 6 cases in 2008. In the U.S., December, January, and February saw the highest number of abortions from leptospiral infection, though abortions due to leptospires were recorded from in every month from August to April.