Two Potomac Horse Fever Cases Confirmed in Massachusetts

The Massachusetts Department of Agricultural Resources (MDAR) has confirmed two cases of Potomac horse fever (PHF)—one in Essex County and one in Norfolk County.

The Norfolk County horse, whose age, gender, breed, and vaccination status were not reported, presented on July 16 with clinical signs including being “off feed” and uncomfortable—signs generally associated with mild colic—followed by fever and diarrhea. Veterinarians diagnosed the horse with PHF the following day. The horse is reported to be recovering. Massachusetts’ single 2018 confirmed case and a suspect case ­occurred at the same stable.

The second horse, located at an Essex County boarding stable, presented to a referral hospital with signs of colitis (inflammation of the large or small colon). The horse subsequently developed fever and diarrhea but is responding to treatment.

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

The MDAR encouraged owners—especially those with horses grazing near rivers, streams, and creeks—to watch horses closely for clinical signs of:

  • 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 are not 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.

“Please continue to monitor your horses’ health,” said MDAR Division of Animal Health director Michael Cahill in a statement. “Temperature monitoring continues to be our best early warning system.”

The department encouraged owners to contact their veterinarian as soon as possible if horses exhibit clinical signs, even if the horse is vaccinated. Potomac horse fever is a reportable disease in Massachusetts.