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