You notice your horse isn’t eating, or maybe he’s running a fever. While these two signs can be indicative of a number of problems, they are also signs of Potomac horse fever (PHF), a potentially deadly disease.
“PHF is a serious disease, especially this time of year, and if you haven’t vaccinated your horse for it, it’s important to understand the causes and signs as well as how to diagnose it,” said Hoyt Cheramie, DVM, an equine specialist with Merial Large Animal Veterinary Services.
He offers the following information to help you protect your horse:
PHF is a disease caused by the bacteria Neorickettsia risticii. The bacteria exist in flukes (flatworms) that develop in aquatic snails and release into bodies of water. Aquatic insects such as damselflies, caddisflies, and mayflies might pick up infected immature flukes, and horses grazing near freshwater creeks, rivers, or on irrigated pasture can ingest the insects carrying PHF.
Clinical Signs
In the beginning, the signs of PHF might be subtle. The horse could have:

  • Loss of appetite;
  • Fever;
  • Depression;
  • Decreased intestinal sounds;
  • Diarrhea; and
  • Mild colic.

Any combination, though not necessarily all, of the signs might be present. Some horses might also develop laminitis, and pregnant mares are at risk for abortion. Other complications include toxins in the blood, decreased intestinal motility, more severe colic signs, and dehydration.
Veterinarians consider polymerase chain reaction tests the most sensitive tests for PHF, as they look for and identify the bacteria DNA in the white blood cells or manure. Because other diseases can have similar clinical signs (such as colitis, primary colic, or peritonitis), veterinarians recommend testing for other potential causes of illness in addition to PHF.

Vaccination can protect horses from contracting PHF. One study found that 90% of unvaccinated horses became sick with PHF when exposed and 20% of those infected horses died. The goal of an immunization program should be to time vaccination so the horse has the highest level of immunity before and during highest exposure to disease. For PHF, this is just before peak insect hatch—usually mid- to late summer into early fall.