Hagyard Equine Medical Institute in Lexington issued an alert Tuesday regarding two cases of Potomac horse fever confirmed this week in one racehorse and one broodmare. Both had diarrhea and fever.
Symptoms of Potomac horse fever include depression, anorexia, colic, diarrhea, fevers and signs suggestive of laminitis (inflammation of the feet). The horses diagnosed had a history of fevers, and when routine bloodwork was performed an extremely low white blood cell count was noted. If the symptoms are caught early, the horse can be treated with intravenous tetracycline. If caught early most horses will respond to treatment in 24 hours and have a dramatic recovery.
Potomac fever made headlines in the 1980s when an outbreak of diarrhea in the Potomac River area of Maryland drew attention to the disease. The causative agent, a bacteria named Ehrlichia risticii (recently renamed Neoricketssia ristici), has been linked to parasites of fresh water snails.
During hot weather, such as what Central Kentucky has experienced the last two weeks, the bacteria-infested juvenile parasites are released into the water and consumed by aquatic fly larvae such as mayflies, caddis flies, dragon flies and 14 other aquatic insects. These flies are now infected.
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Horses can inadvertently consume these flies while grazing or eating feedstuffs. Horses kept near freshwater streams or ponds are more likely to be at risk for the disease because of the close proximity of the aquatic insects. Horses cannot get the disease from drinking water infested with juvenile parasites or eating the snails because the juvenile parasites will easily die in the horse’s stomach acid.
However, a horse does not have to be near water to contract the fever. Some horses diagnosed last year were stabled and did not have access to a stream or pond.
There is a vaccine on the market, but its efficacy is questionable in the Midwest as there appear to be numerous strains of the fever.