Potomac Horse fever (PHF) season is now underway in New York. “There are no actual numbers for the (amount of) disease because it is spread out,” according to Jenny Gold, DVM, an equine veterinarian in the large animal clinic at Cornell University College of Veterinary Medicine in Ithaca, N.Y. Some veterinarians say they have heard of at least 30 horses infected and three dead from the disease in northern New York, and there is a case under treatment at Belmont Park racetrack in Elmont, N.Y.

Gold has treated at least six horses with PHF. “The clinical signs of the horses I have seen have been depression, dropping off feed, and a fever between 103ºF-106ºF,” she says. “Most owners are bringing their horses in before they begin having diarrhea.” The sooner the horses are brought in to receive treatment, such as fluids to prevent dehydration and antibiotics, the greater their chances of survival are, says Gold.

Some of the horses Gold has seen have been vaccinated against PHF. “The vaccination is not 100% effective against the disease, but it does decrease the severity of it,” says Gold. As the severity of the disease is lessened, the horse’s chance of survival is greatly increased because there is less risk the horse will die from complications, such as laminitis. Horse owners are encouraged to call their veterinarian as soon as their horse begins to show clinical signs of PHF, which can include profuse diarrhea, fever, depression, and shock. Because PHF may resemble colitis caused by Salmonella or Clostridium bacteria, it is difficult to diagnose simply by the clinical signs.

Ed Dubovi, PhD, a New York State Diagnostic Lab serologist, says, “The lab is receiving 20-30 tests per week to be tested for the disease (PHF). About 50% of them are positive values.” Dubovi speculates that submissions for PHF testing will continue at this rate until the end of summer. “We are right in the middle of PHF season. The number of tests being run (for PHF) are about the same as they are every year.” However, Dubovi says these numbers do not always reflect what is occurring out in the field and the situation could be worse than what is seen in the lab. Veterinarians might choose not to submit blood for testing and just begin treatment for PHF immediately