Potomac Horse Fever

Potomac horse fever (PHF) is a serious illness that can be fatal and early detection is important to increase the chances of a better prognosis. Horses with the symptoms of PHF require immediate veterinary attention. PHF is not contagious to other horses, but the symptoms of PHF are very hard to distinguish from salmonella, a highly contagious condition. Therefore, horses with these symptoms must be quarantined until PHF is confirmed.

All horses are susceptible to PHF, although young horses seem to be less affected. Horses who live near bodies of water such as creeks, ponds, lakes, rivers, and streams, or whose pastures are connected with or irrigated by these bodies of water are more susceptible. However, because the aquatic insects that carry the disease fly far from water, horses that live relatively far from bodies of water are also at risk.

Severity depends on how long the infection has to develop before treatment begins, and whether complications such as laminitis occur.

Laminitis is a painful inflammation within the hooves. In severe cases, it can permanently lame the horse and lead to euthanasia.

Symptoms include:

• Increased heat in the hooves
• Increased time lying down
• Shifting weight
• Standing stiffly
• Standing in a “sawhorse” position, with the weight rocked backwards and the front limbs extended forwards
• Unwillingness to move

Pregnant horses that are infected with PHF are at risk of abortion.

Potomac horse fever is caused by the Neorickettsia risticii bacterium (formerly known as Ehrlichia risticii) infecting the intestinal tract of the horse. This bacterium begins its life cycle in a fluke parasite which lives in the body of aquatic snails. From there, the parasite carrying the bacteria is released into bodies of water including streams, ponds, lakes, and creeks. Aquatic insects such as caddisflies, mayflies, and damselflies pick up the bacterium from the surface of the water. The horse ingests the aquatic insects accidentally while it is drinking or eating. The bacteria takes up residence in the intestinal tract, causing infection.

PHF is seasonal. It occurs during the late spring, summer, and early fall in temperate climates.

Aquatic insects are most prevalent near bodies of water. They are also found on irrigated pastures, as well as in open buckets or troughs of water that are relatively far from major bodies of water.

The main symptoms of PHF are subtle and non-specific, which means that they are the symptoms of many possible conditions.

Diagnosis of Potomac horse fever requires confirmation of the bacteria, to rule out other possible causes of diarrhea. Diagnostic tools include:

• Physical examination
• Blood work
• Blood or fecal testing to detect the DNA of the Neorickettsia risticii bacterium

Treatment includes:

• IV antibiotics
• IV fluids
• Pain medication
• Anti-inflammatory medications

The prognosis for horses with Potomac horse fever depends on how quickly treatment is started. In cases where treatment begins early in the disease progression, the prognosis is fair to good. In cases where there is a delay in treatment, the prognosis is more guarded. In cases where laminitis occurs, the prognosis is poor.

Potomac horse fever is not contagious from one horse to another.

There is a vaccine available for PHF, although it does not give full immunity. Horses in areas that experience high incidence of PHF benefit from vaccination to reduce symptoms and avoid complications.

Other preventative measures aim at limiting exposure to aquatic insects, including:

• Restricting grazing near creeks, ponds, lakes, and other bodies of water during the late spring, summer, and early fall
• Keeping the horse’s food covered to prevent insect infestation
• Using repellents and other measures to decrease insect populations in or near the barn

Potomac horse fever is uncommon. It is more common near bodies of water such as lakes, creeks, rivers, and ponds.

• Intravenous antibiotics
• Intravenous fluids
• NSAIDs