LEXINGTON, Ky.-- 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 Neoriketssia 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 infects the larvae of mayflies and caddis flies in fresh water. The larvae then mature into adult flies and are ingested by horses when they inadvertently consume the insects while grazing or in feed. Horses kept near fresh-water streams or ponds are more likely to be at risk for getting the disease. The Kentucky filly was kept on a farm with several small streams and a pond.

What Happened?

The two-year-old filly went off her feed on June 5, displaying the first of an array of symptoms that puzzled the owner, farm veterinarian (who treated the horse for the seven days prior to referral), and referring veterinarian Bonnie Barr, VMD, of Rood and Riddle Equine Hospital in Lexington, which admitted the filly to its isolation facility on June 11. Barr explained that the filly, "had the history of having diarrhea on the farm, she was depressed, showed signs of colic, anorexia, and dehydration. Colic isn't something you see all of the time (with PHF), but you do see it in about a third of the cases. She showed signs suggestive of laminitis or founder.

"You kind of go through your head the different diseases that would cause the horse to have these symptoms-Salmonella and Clostridium--and although Potomac horse fever is in the back of your mind, you wouldn't think of it in this area."

The owner described the filly's demise after several days in the clinic. "She had vasculitis, or extreme swelling. She was a pretty diminutive filly, and her face, belly, and legs blew up the size of a draft horse. At first they thought it was the result of self-inflicted injuries (when the horse had a violent episode of abdominal discomfort prior to referral). They did every test under the sun--spinal taps, X rays, you name it-but could not figure out what it was," the owner said. "She was there eight days before the decision was finally made to put her down (June 18)."

Barr explained that what usually debilitates the horse in a PHF case is founder, which is what happened to this filly. "You can treat the PHF, but if the horse founders, you have a whole lot of things to deal with. If you're suspicious, you treat them with the oxytetracycline." She said that one can test the blood of the horse for PHF, but often by the time the results are returned the horse is either on the road to improvement or beyond treatment.

"I was surprised when Dr. Williams (Neil M. Williams, DVM, PhD, of LDDC) called and told me the results," said Barr. "Maybe this case will make us a little more aware. Sometimes things come in a cycle, and maybe we'll see a few more cases."

Williams, who did his doctoral studies on PHF, said in Kentucky, "We only see one or two cases per year. The number of cases have 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. This disease is very sporadic; usually there's only one case on a farm."

The first signs that are typically seen with PHF would be lethargy, fever, loss of appetite, and possibly diarrhea, said Williams. Any time those symptoms are encountered, he added, you should seek veterinary assistance.

"The literature describes it as being only 5-20% fatal," said Williams. "Obviously, we would only deal with the fatal cases (here at the LDDC). Blood samples could also be sent in from live horses for testing purposes (no vet clinics in the Lexington area test for PHF in-house). That is the best way to practice surveillance, since fortunately most of the horses don't die.

"While there is a vaccine, we're not necessarily recommending vaccinating all horses with it. It's available and you may want to consider it, but that would be best discussed with your veterinarian," added Williams. "The vaccine has been reported to be not completely protective."

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 and did the first experimental reproduction of the disease by feeding horses caddisflies. Researchers at The Ohio State University have since reproduced the UC Davis study and confirmed the initial finding of transmission by ingestion of insects.

"It's time to recall this very important disease," said Madigan. "There are some new and effective DNA diagnostics available, and the disease is not a mystery any more--if the disease is found on a premise, a person can go in and do some control measures to potentially reduce the risk of getting the disease. At River Downs in Southern Ohio on the Kentucky border (where a large outbreak occurred in 1980s), they filled in all the ditches (and areas of fresh water) and the outbreak went away. Madigan suspects the disease is much more common and lack of testing and vaccine protection have reduced the awareness and detection. It is also a cause of abortion in mares and not easy to detect in aborted fetuses, Madigan explained.

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

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