Potomac Horse Fever

Jessica Michel, DVM
Annabessacook Veterinary Clinic

Neorickettsia risticii, also known as Potomac Horse Fever (PHF), equine monocytic ehrlichiosis, equine ehrlichial colitis, or Shasta River Crud, has recently caused a lot of concern among owners regarding the health of their horses and fear of possible contact with affected horses.

First recognized along the Potomac River in Maryland, PHF has now been suggested to occur in 43 states in the United States, three provinces in Canada, South America, Europe, and India. PHF is a rare occurrence here in Maine. More than 20 years of research have linked the transmission of PHF to horses that reside near water ways and are exposed to fresh water snails and aquatic insects (caddisflies, mayflies, damselflies, dragonflies, and stoneflies). The DNA of N. risticii has been found in adult trematodes, which infect the intestines of bats, birds, and amphibians. What does this mean for you, the horse owner? Horses must ingest N. risticii by either ingesting an infected snail or insect. The organisms that cause ehrlichiosis in animals only appear to be directly transmissible to other animals or to humans by blood transfusion.

Potomac Horse Fever typically is associated with severe diarrhea and colic, but can cause a variety of non specific signs including anorexia, depression, biphasic increase in body temperature. Severely affected horses can develop severe toxemia and dehydration, which result in cardiovascular compromise. Laminitis can result secondary to PHF. The severity of clinical signs varies with each horse, therefore not all horses affected with PHF may even appear ill or develop diarrhea. Case fatality rates vary from 5% to 30% and may depend on the strain involved and host response.1

Diagnosis of PHF is based on PCR or paired IFA serum titers, which can be run on a blood sample collected by your veterinarian. Early recognition and treatment can reduce severity of clinical signs and provide a better outcome. Oxytetracycline is the antibiotic of choice for treatment. Supportive care for the horses that develop diarrhea is very important as they can quickly become dehydrated. Prevention strategies for laminitis are necessary as laminitis can be a devastating sequela of affected horses.

Prevention of PHF is by limiting exposure to the infective organism. Limiting night lights in barns which can attract aquatic insects, examining hay for dead insects, keeping water buckets clean of dead insects, and limiting access to pastures where snails may be present can all help with the prevention of PHF. The efficacy of PHF vaccines in preventing clinical disease remains controversial.2 However, anecdotal reports suggest that the clinical signs of disease maybe less in a vaccinated horse. Of horses affected with PHF 37% of cases were currently vaccinated. 2

1.  Madigan J, Pusterla N. Life cycle of Potomac horse fever—implications for diagnosis, treatment, and control: a review, in Proceedings. 51st Annu Conv Am Assoc Equine Pract 2005;158-162.
2.  Wilson J, Pusterla N, Arney L. Incrimination of Mayflies as a Vector of Potomac Horse Fever in an Outbreak in Minnesota: a review, in Proceedings. 52nd Annu Conv Am Assoc Equine Pract2006 ;324-328.
3.  Center for Food Security & Public Health, Iowa State University College of Veterinary Medicine. Ehrlichiosis fact sheet. May 1, 2005.