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