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

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

Potomac horse fever’s causative agent, a bacterium named Neorickettsia risticii, has been linked to parasites of freshwater snails. The parasites are called cercariae, and they 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 which inadvertently consume the insects 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. The first horse died within 48 hours of clinical signs appearing; the second horse was treated at Oklahoma State University and was euthanized after about 30 days of treatment. He said that the ranch is about a mile from the Illinois River