13 Kentucky Potomac Horse Fever Cases Confirmed

A few cases of Potomac horse fever (PHF) occur each fall in Kentucky. This year, a few cases have been seen, but they occurred a little earlier than normal. Nathan Slovis, DVM, MS, Dipl. ACVIM, of Hagyard Equine Medical Institute, said the early occurrence probably was due to the dry weather the past month.

"It's nothing new or crazy," said Slovis. "We usually see cases this time of year (mid-August). We did, however, see cases last month (July), which is earlier than usual."

Neil M. Williams, DVM, PhD, of the University of Kentucky's Lexington Disease Diagnostic Center, who did his doctoral studies on PHF, said that at the diagnostic laboratory in Central Kentucky, "We usually have a few cases of PHF each year. For the most part, the number of cases has really fallen off dramatically since the 1980s. This year there have been 13 positive PHF cases, none in May, one in June, 10 in July, and two so far in August. The fact that it is not very commonly seen, 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.

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, 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 they are grazing or eating feed. Horses kept near fresh-water streams or ponds are at higher risk for getting the disease. 

Slovis thinks the main reason for seeing the early cases in Kentucky this year is the long lag times without rain causing small streams and water sources to dry up. Horses with access to graze pastures with those water sources will choose vegetation by streams that is more palatable, "and they probably were consuming aquatic insects," theorized Slovis. "Or, aquatic insects were flying away from dried up water sources and becoming available to horses to accidentally ingest."

Slovis said snails release the causative organism into the water with the changing weather (from hot to cool to hot). He said this year's case load looked a lot like 2002, when multiple horses were diagnosed with PHF in Kentucky. He said he knows of more than 10 cases in the Hagyard clinic and general practice. There also were multiple horses affected on one farm.

About half of the sick animals were vaccinated for PHF, noted Slovis. "The efficacy of the vaccine is still in question," he said. "Is if efficacious, or did the animals get an exposure above and beyond the vaccine's protection. Does the vaccine cross-protect against different strains? (There are several strains that can cause illness in horses.)"

Slovis said owners and managers should watch out for high fevers (104-105 degrees Fahrenheit) of unknown origin in adult horses that come on out of the blue. "Look for lethargy and inappetence," he added. "They don't necessarily have diarrhea, about 50% didn't have it with the fever."

His advice: "If you can't find cause (for the fever), your veterinarian might want to use tetracycline and test for Potomac horse fever until you know what is causing the problems."

Slovis also recommended keeping stall lights off at night. "Bugs are attracted at night by the light," he explained. "If you have feed or water troughs near light sources, remove them to avoid dead insects. If you have an increased incidence (of PHF) in a pasture, take animals out of that pasture for the time being. Those insects can fly pretty far."

John Madigan, DVM, MS, Dipl. ACVIM, of UC Davis, has been involved in PHF research for nearly 14 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.

In a previous article on PHF, Madigan said he suspected the disease is much more common than horse owners think, 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 (for PHF), or be thinking of prompt treatment in areas where the disease has been confirmed," said Madigan. "If you think about it, Potomac horse fever can often be treated; otherwise it's a killer."

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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