Potomac horse fever (PHF), a somewhat regional rickettsial disease, causes acute diarrhea and leads to death in up to 30% of affected horses. In an effort to understand the disease better, Sandra Taylor, DVM, PhD, Dipl. ACVIM, of Purdue University’s school of veterinary medicine, performed a retrospective study in which she and colleagues looked for specific factors that were favorable for survival among PHF cases. She presented the results at the 2012 American Association of Equine Practitioners’ (AAEP) Convention, held Dec. 1-5 in Anaheim, Calif.

Potomac horse fever is caused by the intracellular bacterium Neorickettsia risticii, which impacts specific white blood cells and cells lining the colon. The disease was first identified in the early 1980s, and while its mode of infection was largely mysterious in the early years, researchers now understand that the reservoir for this bacterium is a fluke, which parasitizes water-borne snails, aquatic insects, birds, and bats. Larval stages of aquatic insects such as mayflies or caddis flies become infected. Birds and bats can eat the aquatic insects and pass the bacteria in their feces. A horse can be exposed by inadvertently ingesting aquatic insects infected with flukes carrying the bacteria (in contaminated forage or feed), or by drinking flukes directly from rivers or streams.

A horse doesn’t require access to rivers or streams to become infected since the flying insects can land anywhere nearby, including in hay or drinking water. Clinical signs begin to appear 12-15 days post-exposure. Vaccines for PHF are mostly ineffective, with an 89% failure rate in preventing disease.