Potomac horse fever is a disease that affects horses. It is caused by the bacterium Neorickettsia risticii. It can also be caused by the bacterium Neorickettsia findlayensis.

At-risk animals

Cases of the disease in horses have been reported in Canada, the United States, South America, Europe and India. Some cases have been reported in cats and dogs.

Signs of the disease

Symptoms of the disease vary. The most common are depression, loss of appetite, fever and diarrhea. Symptoms are often moderate or go unnoticed. Abortions have been reported two to three months after the onset of symptoms, but this is uncommon.

Laminitis (founder) may develop in 20% to 25% of cases. This disease causes inflammation of the vascular system in the hoof, resulting in severe, painful and often permanent damage.

Infected horses may exhibit symptoms of colic. The reported mortality rate for Potomac horse fever ranges from 5% to 30%.

Transmission and incubation period

The bacterium that causes Potomac horse fever has a complex life cycle. It is carried by a parasitic worm that first enters a snail, is then transmitted to aquatic insects, and finally infects its definitive host (such as a bat or aquatic rodent). Horses are accidental hosts and do not shed the bacterium. Transmission occurs orally.

Horses become infected by eating grass, drinking water or accidentally ingesting aquatic insects contaminated with the bacterium. The bacterium then multiplies in the horse’s intestine and can cause colitis Read the content of the note 1

Infected horses are not contagious. The incubation period is 10 to 20 days.


Early treatment with antibiotics is generally associated with quicker recovery and lower mortality.

Non‑steroidal anti‑inflammatory drugs (NSAIDs) can be used to reduce pain, prevent damage and minimize residual effects.

Intravenous (IV) fluids can be used to correct electrolyte imbalances and treat dehydration caused by diarrhea. Rest is advised during treatment, and a hay‑based diet is recommended until diarrhea is resolved.

Protection and prevention

Keep food and water bowls clean, free of insects and at some distance from light sources. Cover food and water bowls at night and turn off insect‑attracting lights.

Hoof cryotherapy (the therapeutic use of cold) may help prevent laminitis.

Limit access to flood plains and pastures bordering water sources.

Horses can be vaccinated against Potomac horse fever from five months of age. However, this vaccine provides only partial protection.

Disease surveillance

The bacterium that causes Potomac horse fever is found in Québec. Québec’s first cases of Potomac horse fever were reported in 2010. Due to climate change, the disease is spreading further north.

The risk of infection is highest in the months when aquatic insects carrying the disease are most abundant. Most cases occur between June and October. The risk of infection is higher in stables close to bodies of water.

See your veterinarian for information about Potomac horse fever in your area and the steps you should take to control and prevent the disease.

Last update: January 8, 2024


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