The death of at least one Oklahoma horse has been definitively linked to Potomac horse fever, a disease rarely found in the state, and two of his 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. Kentucky had a brief encounter with PHF last summer, when at least five horses were stricken with the illness, which can cause depression, fever, a pounding digital pulse, reduced gut sounds, a low white blood cell count, and edema (swelling) of the limbs. Many horses develop diarrhea and laminitis; the latter often leads to the demise of horses with PHF.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. 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 ranch is about a mile from the Illinois River.Positive PHF test results were received on blood and fecal samples on Aug. 22, the same day that horse was euthanized. A laboratory at the University of California, Davis has a specialized and highly accurate test that costs about $30, and results can be returned in 24 hours. According to Jack Carson, spokesman for the Oklahoma Department of Agriculture, Food, and Forestry, PHF in Oklahoma is "extremely rare. In fact I was talking to our state vet about it and he said he hadn't heard of it in over two years." Sheets said that he doesn't recall any PHF cases in the state in recent years, but added that the farm where the horses were found was "on a river bottom."There is a PHF vaccine, but its efficacy has been questioned by veterinarians.As with discovery of any disease symptoms, Carson said that horse owners should contact their veterinarians. 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 official veterinarian. A veterinarian must catch the clinical signs early to have much luck with treatment.