Missouri Reports Uncommonly High Incidence of Potomac Horse Fever This Summer

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.

Potomac Horse Fever in Saratoga? Vet Clinic Warns Local Horse Community

I've just returned from Saratoga Springs, New York, where several people mentioned to me that there is or was an outbreak of Potomac Horse Fever (PHF)in the region. This was news to stop me in my tracks.

At one farm that I visited in Ballston Spa, I was told that one of the horses there had recovered from PHF-like symptoms.

An announcement from Saratoga Equine Clinic has been posted to help put horse owners in upstate New York both on the alert and at ease. There does seem to be a problem with sick horses in the area, and the horses need medical attention, but it might be less severe than the horrors of true Potomac Horse Fever.

Any illness is disturbing and this one comes at a time when horses are at their most mobile. The Northway was lined with horse vans bringing horses to the racetrack at Saratoga after Belmont Park's closing on Monday. Polo activity is accelerating. The County Fair was in full swing in Ballston Spa, with horses coming and going daily. And horse owners were out enjoying their horses on a perfect summer weekend.

Saratoga Equine reports that so far, no horses to their knowledge have died from the unknown illness. They remarked in their announcement, "Although the prevailing clinical signs are very typical of PHF (Potomac Horse Fever), diagnostic testing to confirm a PHF infection has been unrewarding."

The vets there are crediting vaccination with helping many horses in the area: "Although the efficacy of PHF vaccines in preventing clinical disease is very controversial, our clinical observation is that vaccinated horses have a better chance surviving the disease. Therefore, non-PHF-vaccinated horses should receive immunization."

Principle symptoms of PHF are general lethargy and fever, sometimes accompanied by colic-like symptoms, diarrhea and other alarming medical signs that may look like other diseases. If left untreated,the disease may escalate into a true medical emergency. Many severe cases have included disastrous laminitis as a side effect.

In Texas, three cases of Potomac Horse Fever, which is not a regulatory disease, have been confirmed in Kerr County by the Texas Veterinary Medical Diagnostic Laboratory. Potomac Horse Fever is not typically found in Texas, but the recent flooding there is being blamed for the cases.

Horses with PHF usually live near rivers, streams, ponds or canals. The infection involves tiny flukes that are parasites of water snails. The flukes hatch their offspring into the water, and these are then picked up by aquatic insects that molt into flying insects, including caddis flies and mayflies.

Horses can become exposed to Potomac Horse Fever when they eat or drink anything contaminated with the insects.

I'll keep you posted on this story; thanks to Saratoga Equine Clinic for keeping the horse world informed.