Potomac Horse Fever in Oklahoma

The death of at least one Oklahoma horse has been definitively linked to Potomac horse fever (PHF), a disease rarely found in the state. Two of her stablemates likely died of the same illness. The horse manager at the farm with the confirmed case said 11 other horses in the area have died with similar clinical signs, but blood and tissue samples from those horses were not submitted for diagnosis.

The actual number of PHF cases in Oklahoma is not known since it is a disease not typically seen or reported by equine practitioners in the state.

Potomac horse fever's causative agent, the bacterium Neorickettsia risticii, has been linked to parasites of freshwater snails (cercariae). The parasites 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 while grazing or eating feedstuffs. Horses kept near fresh-water streams or ponds are more likely to be at risk for getting the disease because of the close proximity of the aquatic insects. There is a PHF vaccine, but its efficacy has been questioned by veterinarians.

Rocky Carroll, horse manager at Black Fox Ranch in Cherokee County, Okla., reported that three horses from his farm died with similar clinical signs between June 29 and Aug. 22. He said the ranch is about a mile from the Illinois River. Already this year, "Several horses along the Illinois River have died prior to ours with reasons to suspect PHF," said Carroll.

Mike Sheets, DVM, of Northside Animal Clinic in Stillwell, Okla., treated the third horse from Black Fox Ranch. Sheets first saw the horse at the farm on Aug. 11, and the horse was admitted to his clinic on Aug. 12. The horse was "initially depressed, acutely febrile, and had a pounding digital pulse, a lack of gastric motility (no gut sounds), but no lameness," he said. The horse had a low white blood cell count and developed edema in the limbs. "I started treating with tetracycline IV, and it was about 72 hours before it developed laminitis (what often leads to the demise of horses with PHF). After the laminitis, it started showing pain in its feet and broke with diarrhea, but only for about 24 hours." The diarrhea subsided, but the laminitis continued. The horse was euthanized late Aug. 22.

Also on Aug. 22, Sheets received positive polymerase chain reaction (PCR) test results from the University of California, Davis. "The U Cal-Davis turnaround is pretty remarkable," he said. "I sent (fecal and blood) samples in, they got it on Friday, and they called and left me word Friday night." Sheets was already treating the case as if it were PHF since he had heard of other suspect cases in the area.

John Madigan, DVM, MS, Dipl. ACVIM, of UC Davis, has been involved in PHF research for nearly 10 years, and his laboratory performs the PCR testing. (For more information on this new, inexpensive test, see article #3380 online.)

According to Jack Carson, spokesman for the Oklahoma Department of Agriculture, Food, and Forestry, PHF in Oklahoma is "extremely rare." He said horse owners should contact their veterinarians if a horse has any disease signs. If the veterinarian makes the initial diagnosis of PHF or finds anything else "out of the ordinary," then the vet will contact a state or federal veterinarian.

Catching PHF early is important. "You need to treat it in the first 72 hours of clinical signs or you're not going to have much luck," said Sheets.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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