Potomac Horse Fever

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Also Known As

Equine monocytic ehrlichiosis


Potomac horse fever (PHF) is a gastrointestinal disease that affects horses of all ages. It was first recognized as a distinct condition in 1979 in Montgomery County, Maryland, part of the Potomac region.

Potomac horse fever has appeared sporadically throughout many states in the US and has also been confirmed in Canada and Europe. A clear seasonal pattern has been identified, with cases appearing as early as May, followed by the peak season in the summer months, and ending in October or November.

The disease is associated with pastures bordering creeks and rivers, and it is believed that many cases occur when horses ingest trematodes released from fresh water snails and/or the inadvertent ingestion of aquatic insects or flies that carry the causative organism.

Horses that become ill with Potomac horse fever usually appear depressed and lose their appetite, followed by a high fever, diarrhea and dehydration.


  • Fever
  • Depression
  • Anorexia
  • Decreased gut sounds
  • Colitis
  • Diarrhea - cow-like or watery
  • Colic
  • Laminitis
  • Abortion in pregnant mares


Potomac horse fever is caused by "Neorickettsia risticii," formerly known as "Ehrlichia risticii," a Gram-negative, obligate, and intracellular bacterial pathogen that infects the small and large intestines and results in acute colitis, which is one of the principle signs of PHF.

Recent DNA analyses have revealed that "N risticii" is most closely related to "N helminthoeca," the agent of salmon poisoning in dogs, and "Ehrlichia sennetsu," a disease of humans in Japan.

"N risticii" has been identified in freshwater snails and isolated from trematodes released by the snails. It has also been detected in up to 14 species of insects and flies, including mayflies and dragonflies. One route of exposure is believed to be the inadvertent ingestion of aquatic insects that carry the causative agent in the stage of the trematode.

The causative agent is present in the feces of experimentally-infected horses, but clinically ill horses are not contagious and can be housed with healthy horses.


Several inactivated, whole-cell vaccines based on the same strain of "N risticii" are commercially available. According to the Merck Health Manual, although vaccination has been reported to protect 78 percent of experimentally-infected ponies, it has been marginally protective in the field population of horses.

The failure of the vaccine has been attributed to the fact that at least 14 different strains exist in naturally occurring cases and so far the vaccines are not able to insure against all strains of the pathogen.

Vaccine failure may also be due to a lack of antibody protection at the site of exposure because the natural route of transmission has been determined to be oral ingestion of the disease-carrying organism. Reduction of the snail population in rivers and streams may help lessen sources of infection.


Treatment of horses with PHF includes antibiotics, fluid and electrolyte replacement therapy for animals with diarrhea, and nonsteroidal anti-inflammatory drugs for the relief of pain in cases of colic and laminitis.

PHF has been successfully treated with oxytetracycline, which, if given early in the clinical course of the disease, results in a positive response, usually within 12 hours. This is associated with a drop in temperature, followed by improvement in demeanor, appetite, and an increase in regular gut sounds.

With prompt therapy, clinical signs frequently resolve by the third day of treatment and, generally, antimicrobial therapy lasts no more than five days.

In horses exhibiting signs of enterocolitis, fluids and NSAID treatment should be administered. Laminitis, if it develops, is often severe and most cases of mortality are attributed to complications of laminitis.

Administering drugs, such as antibiotics to fight infection, vasodilators to improve blood flow to the feet, anti-inflammatories, and painkillers may be recommended by the attending veterinarian.

The prognosis for survival of PHF is now reported to be over 85 percent. If horse owners and veterinarians in affected areas are proactive in making sure horses residing in or traveling to endemic areas are vaccinated and kept away from streams and rivers that may contain the causative agent, fewer cases may occur. I

t is noted that horses that do get the disease after being vaccinated usually have much milder cases than those that are not vaccinated.

Prompt attention to horses that exhibit fever and depression and live in geographic areas known for the disease can help prevent fatal development of the disease. A recently developed real-time PCR test allows the detection of "N risticii" DNA within two hours, making this a feasible test for routine diagnostic use.

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