Potomac horse fever was an equine disease shrouded in mystery. Scientists knew what caused the illness–the bacterium Ehrlichia risticii–but its source in nature could not be found. No one knew how horses became infected. Until now.

An article that appears in the June issue of the Journal of Clinical Microbiology offers strong evidence that freshwater snails and the larvae of flukes (parasitic flatworms) play key roles in Potomac horse fever’s transmission. Based on research conducted at the University of California at Davis, the groundbreaking information shatters the popular theory that the disease is spread by biting arthropods such as ticks. It also suggests that Potomac horse fever can be prevented by keeping horses away from the streams, ponds, and other aquatic habitats in which snails and flukes live.

“This really changes the whole concept of Potomac horse fever,” said Dr. John E. Madigan, one of the article’s authors. “A lot of good entomologists had looked for the organism (that causes the disease) in flies, mosquitoes, and ticks, but nobody had ever come up with it.”

Potomac horse fever is most prevalent in the Eastern United States, but it also occurs in California and other areas of the country as well as in Canada and Europe. Its symptoms include loss of appetite, fever, depression, diarrhea, colic, and dehydration. Laminitis is a complication in a significant number of cases, and the disease is potentially fatal