Potomac Horse Fever

Potomac horse fever (PHF) or equine monocytic ehrlichiosis is a disease caused by a bacterial organism called Ehrlichia risticii (E. equi). In horses, the clinical signs of PHF include fever, lethargy, diarrhea and occasionally, mild colic. Laminitis (founder) can be a serious complication that limits a horse's future performance.

Until recently, researchers did not know how horses were being exposed to this organism. Dr. John Madigan and his research team have recently identified freshwater snails and the larvae of flukes (parasitic flatworms harbored by the snails) as possible carriers of this disease. This research is exciting because until now, it was thought that PHF was spread by ticks. Though more research is underway, prevention may be as simple as keeping horses away from the streams, ponds and other aquatic habitats where these snails and flukes reside.

Dr. Madigan's research team also identified E. risticii as the disease causing organism for the "Shasta River crud," a disease reported in horses living near California's Shasta and Klamath rivers. The first cases of Potomac horse fever were identified in 1979, but the Shasta River illness has been reported for at least 25 years and causes the same clinical signs as PHF. When researchers compared the DNA of the disease-causing organism, they found that it was the same disease.

The disease causing agent of PHF is closely related to another bacterial organism associated with an aquatic habitat,     Neorickettsia helminthoeca, which causes "salmon poisoning," a fatal condition in dogs that eat infected salmon, as well as Ehrlichia sennetsu, an agent of human illness in Japan and Malaysia. The disease in humans causes signs of chronic mononucleosis and occurs in people that have waded in particular waterways.

Potomac horse fever currently is treated with tetracycline antibiotics, however, the success rate of treatment varies depending on the severity of the infection and the timing of the treatment. Diagnosis of PHF is based on the history, clinical findings and the knowledge of E. risticii activity in the immediate area. Accurate, reliable diagnostic testing for PHF is currently unavailable.