Acute leptospirosis in horses is characterized by fever of 103-105° Fahrenheit for two to three days, depression or dullness, loss of appetite, and jaundice. Abortion can occur several weeks after the fever in pregnant mares, and uveitis can strike months later. Leptospires that cause leptospirosis are bacteria.

However, says William Bernard, DVM, Diplomate ACVIM, of Lexington, Ky., they are different from other bacteria in that they are spiral shaped and are motile. Bernard, a practitioner at Rood and Riddle Equine Hospital, has authored a paper on leptospirosis that was published in Veterinary Clinics of North America: Equine Practice.

Leptospires are grouped into two complexes–L. interrogans and L. biflexa. The dangerous group is L. interrogans because those are the bacteria that cause disease.

L. interrogans are divided into serovars and serogroups. The prime serovars that can cause disease in the horse are pomona, grippotyphosa, hardjo, bratislava, canicola, and icterohaemorrhagiae. Evidence indicates that pomona is the serovar most often implicated in equine abortions in the United States.

Believed to be one of the least dangerous is bratislava. L. bratislava, says Bernard, is thought to be "host adapted" to the horse. This means that it can exist in the horse without causing disease.

However, that viewpoint might be changing. John Timoney, MVD, PhD, of the Gluck Equine Research Center in Kentucky, says that information presented at the 8th Equine Infectious Disease Conference in Dubai this past March points to bratislava as more of a culprit than previously bel