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Potomac Horse Fever

Updated: May 4, 2021

The initial fever often spikes undetected to 102 -107 degrees Fahrenheit

Potomac Horse Fever is caused by the bacteria Neorickettsia Risticii and can be mild to life threatening. It is an acute inflammation of the small intestine and colon that produces mild colic signs, fever, diarrhea, and sometimes abortion in pregnant mares. This illness is seen in spring, summer, early fall, and is more common around sources of warm still water. This disease is not transmitted via horse to horse contact, but rather acquired through ingesting host insects such as mayflies, caddis flies, and flatworms living near warm water.

Clinical Signs

  • The initial fever often spikes undetected to 102 - 107 degrees Fahrenheit

  • A second fever surges and then depression and poor appetite sets in

  • Within 24-48 hours of the disease, moderate to severe watery diarrhea begins

  • Abdominal discomfort

  • Depression/anxiety following the initial fever

  • Dehydration

  • Laminitis- Can display classic laminitis stance, reluctance to move, and lower limb edema.

  • Endotoxemia- elevated heart rate, dark mucous membranes, sweating and colic signs.

  • Sepsis- Chemicals released in the blood stream cause inflammation throughout the body.

  • Abortion in pregnant mares - Occurs around 190-250 days gestation when the bacteria is transferred across the placenta.


  • Determined by clinical signs and multiple tests that monitor the rise of antibody levels in the blood to definitively diagnose Potomac Horse Fever.


Catching this illness early is very important to ensure your horse has the best chance of recovery.

  • Fluids/Rest

  • NSAIDS (Banamine)

  • Antimicrobial therapy

  • Oxytetracycline effective when given early stages of the disease.

Within 12 hours, the horse should be displaying signs of improvement. The fatality rate for Potomac Horse Fever is 5-30 %.

Risk Factors

  • Live in an endemic area

  • Horses that have not been vaccinated prior

  • Horses that spend most of their time outside

  • Young horses more susceptible that older horses

  • Usually affect only one horse on a farm


  • Vaccination can boost the immunity, however does not prevent the illness completely.

  • Minimize insect ingestion by turning lights out in the barn at night

  • Ensure your horse has access to clean fresh water

  • Scrub your water buckets to keep them sanitized and clean

  • Limit your horse’s access to standing warm water such as ponds, and swamps.

Written by Ashlynn Noble

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