Potomac Horse Fever Fatality in Kentucky

A Thoroughbred filly in central Kentucky recently succumbed to Potomac horse fever (PHF), a disease that is detected only once or twice per year in the Commonwealth. The cause of death was a mystery until test results were received from the University of Kentucky Livestock Disease Diagnostic Center (LDDC) on June 27.

Potomac horse fever first hit the horse industry in the mid-1980s with an outbreak in the Potomac River area of Maryland. Researchers were able to link the causative agent (a bacteria named Ehrlichia risticii, which has been re-named Neorikettssia risticii), to parasites of freshwater snails. Scientists at the University of California, Davis (UC Davis), were able to complete the natural transmission cycle of PHF in 2000 when they found that the bacteria also infects the larvae of mayflies and caddis flies in fresh water. The infected larvae then mature into adults and are inadvertently ingested by horses while grazing or eating feed. Horses kept near fresh-water streams or ponds--as the Kentucky filly was--are at higher risk for getting the disease.

Neil M. Williams, DVM, PhD, of the LDDC, who did his doctoral studies on PHF, said that in Kentucky, "We only see one or two cases per year. The number of cases has really fallen off dramatically since the 1980s. The fact that it is not that prevalent, and that the signs can vary greatly from case to case, makes it a diagnostic challenge."

Typically, the first signs of PHF are lethargy, fever, loss of appetite, and possibly diarrhea, said Williams. Any time those signs are encountered, he added, you should seek veterinary assistance. Often what debilitates the horse in a PHF case is laminitis, which is what happened to the Kentucky filly which was euthanized at Rood and Riddle Equine Hospital on June 18. There is a PHF vaccine, but its efficacy has been questioned by veterinarians.

John Madigan, DVM, MS, Dipl. ACVIM, of UC Davis, has been involved in PHF research for nearly 10 years and was part of the research group that linked caddis flies and mayflies to the life cycle of PHF in 2000, demonstrating that they carry the infectious agent. They also did the first experimental reproduction of the disease by feeding horses infected caddis flies. Researchers at The Ohio State University have since reproduced the UC Davis study and confirmed the initial finding of transmission by ingestion of insects.

Madigan suspects the disease is much more common, and that lack of testing and vaccine protection have reduced awareness and detection. He says PHF is also a cause of abortion in mares, but that it is not easy to detect in aborted fetuses.

"If a horse comes in with a fever, colic, and/or diarrhea, and he lives close to bodies of water, it is prudent to get him tested, or be thinking of prompt treatment in areas where the disease has been confirmed. If you think about it, Potomac horse fever can often be treated; otherwise it's a killer," says Madigan.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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