Summer and fall are key seasons for the appearance of Potomac horse fever, which is caused by the bacterium Neorickettsia risticii. Don’t be fooled by the name; Potomac horse fever originally was identified in 1979 as a sporadic disease affecting horses residing in the eastern United States near the Potomac River, but since that time it has been diagnosed throughout the United States and Canada.

“Don’t ignore fevers,” warned Nathan Slovis, DVM, Dipl. ACVIM, of Hagyard Equine Medical Institute in Lexington, Ky. “If you have an animal in a pasture that spikes a fever (102-105°F), rule out other causes and test it for Potomac horse fever.”

Slovis has seen several cases this year already in Kentucky, although he stressed this is not an outbreak, just the normal course of events in hot summer weather. One horse came from the racetrack and others were show or pleasure horses at pasture.

Research has shown that up to 80% of confirmed Potomac horse fever cases eventually die, some because of secondary laminitis