Potomac Horse Fever: A Review

Potomac horse fever (PHF) is a bacterial disease that can affect horses of any age.
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By Nathan M. Slovis DVM, Dipl. ACVIM, CHT, director, McGee Medical Center, and reprinted with permission from the Kentucky Equine Education Project newsletter

Potomac horse fever (PHF) is a bacterial disease that can affect horses of any age. PHF 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 type of bacteria named Ehrlichia risticii (recently named Neoriketssia ristici) has been linked to parasites (juvenile “flukes”) of fresh water snails. These juvenile parasites are called cercariae.

During hot weather the bacteria infested cercariae are released into the water and consumed by aquatic fly larvae such as mayflies, caddis flies, dragonflies and 14 other aquatic insects (commonly noted in Kentucky from July through early September). These aquatic insects will now be infected with PHF. The cercariae will mature into metacercariae after being consumed by the aquatic insects. These metacercariae are RESISTANT to the acid pH of the horse’s stomach and therefore will not be destroyed. Horses grazing or eating feedstuffs can inadvertently consume these infected aquatic insects. Horses kept near fresh-water streams or ponds are more likely to be at risk for getting the disease, because of the close proximity of the aquatic insects (horses cannot get the disease from drinking infested cercariae water or eating the snails because the cercariae are easily digested in the stomach and are NOT RESISTANT to the acid pH of the stomach). A horse does not have to be near water to contract PHF.

Several horses diagnosed with PHF at our hospital were stabled and did not have access to a stream or pond. Aquatic insects carrying the bacteria can travel and easily congregate around a stall light that has been left on. These insects would eventually die and fall into the horse’s water bucket, feed bucket and/or hay where they can be consumed

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