chestnut horse standing in stream
Potomac horse fever (PHF) is a common emergency in equids in the northeastern United States, though it’s been reported in other locations. The disease is caused by the obligate intracellular bacterium Neorickettsia risticii. Other Neorickettsia species have also been isolated in PHF cases. Equids acquire the Gram-negative bacteria through a complicated transmission cycle: Neorickettsia colonize within a trematode (flatworm) host that infects aquatic snails and insects (most commonly mayflies and caddis flies). Horses typically and inadvertently consume the infected insects on pasture, near a water source. Proximity to a lake, pond, or river increases the risk of bacterial exposure. Once ingested, the bacteria colonize within cells of the horse’s gastrointestinal tract.

Potomac horse fever caseload is highest in late summer and throughout fall, when the insect population increases. Veterinarians have also reported cases in winter and spring.

Signs of disease

A fever and “pipe-stream” diarrhea are the characteristic signs of PHF. Early in the disease process, clinical signs include a lethargic attitude and decreased appetite. These findings are caused by a fever, or a temperature of at least 101.5 degrees F. Elevated body temperature can also increase heart rate and/or respiratory rate. Diarrhea then develops within several days of the initial clinical

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