The recent confirmation of Potomac horse fever (PHF) in an Oklahoma horse was possible because of relatively new–and inexpensive–testing techniques that help speed diagnose of the disease. This allows horse owners to be alerted to the active presence of the disease’s causative agent in their area. Recognition of the disease in areas unused to seeing PHF is important. This burden falls to treating veterinarians, horse owners, and diagnostic laboratory officials who take or request blood or tissue samples from horses with clinical signs of PHF.

The Lucy Whitier Core Molecular Laboratory at the University of California, Davis (UC Davis) School of Veterinary Medicine, reported the Oklahoma equine case on Aug. 22 after tests were completed that day on blood and feces.

Potomac horse fever’s causative agent, the bacterium Neorickettsia risticii, has been linked to parasites of freshwater snails. The parasites are called cercariae, and they also infect the larvae of mayflies and caddis flies in fresh water. When the infected fly larvae mature into infected adult flies, they can be ingested by horses inadvertently consuming the insects while grazing or eating feedstuffs.

John Madigan, DVM, MS, Dipl. ACVIM, of UC Davis, has been involved in PHF research for nearly 10 years. He was part of the research group that linked caddis flies and mayflies to the life cycle of PHF in 2000, demonstrating that they carry the infectious agent. That group also did the first experimental reproduction of the disease by feeding horses infected caddis flies. Researchers at The Ohio State University have since reproduced the UC Davis study and confirmed the initial finding of transmission by ingestion of insects