Potomac horse fever (PHF) is caused by the intracellular bacteria Neoricketsia ristcii. Horses are typically affected during the late spring, summer, and fall months. Many cases are near bodies of water and are commonly seen in the midwest and northeast regions of the United States. Signs your horse may have contracted PHF include fever, anorexia, lethargy, abortion in pregnant mares, diarrhea, and laminitis. The generalized term for multiple diseases that have similar clinical signs to PHF is “colitis”. In order to distinguish colitis caused by PHF from other agents, your veterinarian may recommend additional testing. There are multiple tests, each with their own pros and cons. We can help decide which test will be most beneficial based upon your horses’ unique case.

How are horses infected by PHF?

The life cycle of the bacteria causing PHF is complex and involves multiple hosts (Figure 1). The most important part of this complex life cycle is the ingestion of infected mayflies and caddisflies which are previously infected. This step is essential, and prevention of infection is targeted at preventing ingestion of these vectors. Once the bacteria are ingested, it affects the mucosal cells of the large colon causing impaired water absorption. This ultimately results in diarrhea.

potomac-Horse-feverTreatment of PHF includes supportive care, such as IV fluid therapy, and targeted antimicrobial therapy specific to the N. ristcii. Due to the progressive and severe signs that may occur with PHF, it is generally recommended that horses are hospitalized for treatment. As with most things, cases caught early in the disease process typically carry a good prognosis. Potomac horse fever is not contagious, but horses living in the same location may be at an increased risk.

As a horse owner, there are things you can do to help prevent infection:

  • Turn lights off at night to reduce mayflies and caddisflies to the area their horses are kept.
  • Keeping the waterers cleaned and feed covered will reduce the risk of contamination.
  • A vaccine is available: Here are some facts about it.
    • The vaccine can be combined with Rabies and given in the spring—just prior to your horses most likely chance of exposure.
    • The vaccine may help reduce infection; however it is not labeled to prevent abortions that may occur after infection with PHF.

Dr. Sierra Philipp DVMIf you have questions or concerns regarding PHF please contact us at Abraham’s Equine Clinic at 319-366-6441.

Sierra Philipp, DVM