Potomac horse fever, a potentially fatal disease, has been reported among horses in the St. Louis, Mo., area in unusually high numbers. Philip Johnson, BVSc, MRCVS, MS, Dipl. ACVIM, a veterinarian at the University of Missouri College of Veterinary Medicine who is specializing in equine medicine and surgery, said he has treated six cases of Potomac horse fever this summer and is aware of a dozen additional cases treated by St. Louis-area veterinarians. Johnson said the disease is uncommon in the Midwest.

“Potomac horse fever crops up as mini-epidemics when conditions are right,” he said. “We don’t usually see it in Missouri, and we’ve seen a lot of it this year on both the Illinois and Missouri sides of the Mississippi River.”

Johnson said the wet summer and subsequent flooding would favor promulgation and dissemination of the infectious agent that results in disease.

Potomac horse fever is caused by Neorickettsia risticii, an infectious agent found in snails, swallows, bats, and some flies that live near rivers. Exposure in horses often occurs when these flies–stoneflies, mayflies, dragonflies, damsel flies and caddis flies–pick up the infection in the river environment and then spread. When they die, their bodies can fall onto pastures or water troughs where horses unknowingly consume them. The resulting bacterial infection of the large intestine can result in fever, colic, diarrhea, toxemia, laminitis, and abortion. Without treatment, the disease is often fatal.

A new type of testing is now available to diagnose Potomac horse fever. Polymerase chain reaction (PCR) testing involves examining an ailing horse’s blood and feces employing scientific processes similar to that used in DNA fingerprinting, Johnson explained. When combined with observation of clinical signs, PCR testing is a far more accurate process to diagnose Potomac horse fever than simple blood tests