Potomac Horse Fever Confirmed in New Hampshire Horse

An attending veterinarian in New Hampshire confirmed that seven horses were exposed and one Rockingham County horse tested positive for Potomac horse fever (PHF) on Aug. 22. The affected 30-year-old Quarter Horse gelding began showing signs of fever, inappetence, and acute laminitis on Aug. 15 and was subsequently euthanized due to severe deterioration of his condition on Aug. 19.

Because PHF is not a reportable disease in New Hampshire, state officials are unable to provide statistics for the number of cases and whether numbers are within the “normal” range.

“I have not seen any other confirmed cases of PHF in New Hampshire this year,” said attending veterinarian Rachel Roemer, DVM, of Great Bay Equine, in Portsmouth, New Hampshire. “I have seen other cases in the past, but I believe all of the confirmed cases I have seen personally were likely infected out of state. This horse had not left the area in many years, so he was definitely infected here, which is pretty unusual. I do have colleagues who have seen cases that were confirmed to be infected with PHF in New Hampshire and were thought to be locally infected in years past, but I don’t know of any in the past two to three-plus years.”

Neorickettsia risticii, an organism found in some wormlike parasites that infect aquatic snails and insects (such as caddisflies and mayflies), causes PHF. Horses are exposed by inadvertently ingesting aquatic insects infected with flukes carrying the bacteria and by drinking flukes directly from rivers or streams

What to Watch For

Horse health officials encourage owners—especially those with horses grazing near rivers, streams, and creeks—to watch horses closely for clinical signs, including:

  • Mild to severe fever;
  • Diarrhea;
  • Dehydration;
  • Loss of appetite;
  • Laminitis; and
  • Colic.

Potomac horse fever’s incubation period is one to three weeks, and its mortality rate is 5 to 30%; many horses with PHF respond to treatment. Vaccines against PHF aren’t always effective but could lessen disease severity. For that reason, horse owners should consult their veterinarians concerning vaccination protocols.

Equine owners are encouraged to keep horses off flooded pastures to reduce exposure potential. While horses most commonly contract PHF when they ingest infected insects from water bodies, even those residing far from water could be at risk. Barn and stall lights can attract vectors, which could end up in horses’ feed or water sources. As such, owners should keep barn and stall lights off at night.

The department encouraged owners to contact their veterinarians as soon as possible if horses exhibit clinical signs, even if the horse is vaccinated.