Potomac Horse Fever (PHF), a potentially fatal equine disease, is being reported among horses in the St. Louis area in unusually high numbers this summer. Dr. Philip Johnson, a veterinarian at the University of Missouri College of Veterinary Medicine 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 flies that live near rivers. Exposure in horses often occurs when flies – stoneflies, mayflies, dragonflies, damsel flies and caddis flies – pick up the infection in the river environment and then spread out. 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 pregnant mare abortions. Without treatment, the disease is often fatal.

Fortunately, 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 said. When combined with observation of clinical signs, PCR testing is a far more accurate process to diagnose Potomac Horse Fever than old-fashioned blood tests, he said.

Once Potomac Horse Fever has been diagnosed, it can be treated using oxytetracycline, a medicine not commonly prescribed to horses due to the potential for side effects (including diarrhea, paradoxically one of the signs of Potomac Horse Fever), but that is effective in treating this disease.

If you suspect that your horse is exhibiting symptoms of Potomac Horse fever, no matter where you live, contact your veterinarian and request a PCR test. If you live in Missouri, you can also call the College of Veterinary Medicine’s Equine Clinic at 573-882-3513.