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.

Taylor and her colleagues examined the records of 50 horses diagnosed with PHF over 15 years (1996-2011) and noticed that most cases were seen in August, with some occurring June through December. Blood testing with PCR confirmed positive diagnosis in most horses, while veterinarians found high blood antibody levels against the bacterium in others.

Researchers recorded presence of diarrhea in 66% of horses, fever in 48%, lack of appetite in 42%, depression in 40%, and colic in 38%. Laminitis developed in 32% of the cases; 88% of these horses were affected in all four feet. In general, 76% of all PHF cases survived to discharge but those with laminitis were less likely to survive.

While PHF has a serious clinical impact on affected horses, some parameters are associated with survival. The outcome was correlated to the severity of colon disease, which was reflected by the amount of specific electrolyte losses from the bowel. Blood chloride, sodium, magnesium, and calcium concentrations were all decreased in nonsurvivors. Blood creatinine, blood urea nitrogen, packed cell volume, and anion gap were all increased in nonsurvivors; these parameters indicate significant dehydration secondary to diarrhea.

The use of antibiotic oxytetracycline improved survival odds twelvefold. Taylor’s take-home message was that if there is any suspicion of PHF infection (based on location, time of year, and presence of diarrhea) prior to a confirmed diagnosis, it is prudent to start a horse on oxytetracycline and intravenous fluid therapy right away.