Potomac Horse Fever: To Vaccinate or Not?

April Knudson, DVM, is an equine specialist with Merial Veterinary Services. She has a special interest in equine gastrointestinal health, infectious disease, and lameness. Here, she answers a question about vaccinating against Potomac horse fever.

Q: I've never heard of a case of Potomac horse fever in my area. Should I vaccinate my horse against it?

A: The best way to determine whether or not you should vaccinate your horse against Potomac horse fever (PHF) is to consult with your veterinarian. He or she should have the latest information about whether or not there have been any cases identified around the country and help you assess whether or not your horse could be at risk.

PHF is most commonly found near creeks and rivers and likely develops when horses ingest infected aquatic insects such as damselflies, caddisflies, and mayflies. Although the disease is named because the initial 1979 outbreak occurred near the Potomac River in Maryland, PHF has since been identified in 43 states, three Canadian provinces, parts of South America, the Netherlands, and France. Multiple cases in the United States were confirmed by the University of Kentucky Veterinary Diagnostic Laboratory as recently as last summer.

In determining your horse's possible exposure to PHF, you and your veterinarian should consider the horse's immediate surroundings and the local projections for this year's aquatic insect population. You'll also want to consider the conditions in any areas you might be traveling to during the peak PHF season, which spans is July through September.

PHF can have serious complications, so the decision about whether or not to vaccinate is an important one. The fatality rate in untreated cases can be up to 30%. Another devastating effect of PHF is the possible development of laminitis, which occurs in up to 40% of affected horses.

Because the disease is difficult to diagnose and has clinical signs that are subtle and mimic other diseases, early detection is key to potential recovery. Signs at the disease's onset include fever ranging from 102 to 107°F (about 39 to 42°C), decreased intestinal sounds, and diarrhea. As the disease progresses, some horses suffer from toxemia and dehydration.

Should you and your veterinarian determine PHF could pose a threat to the health of your horse, vaccination is the only way to help provide protection. Merial manufactures a PHF vaccine deigned to help protect horses against the most serious side effects of PHF, including death.

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