Potomac Horse Fever
Potomac Horse Fever is a disease that may be encountered in horses in the Midwest during the warm months, often in the late summer. PHF is caused by the bacteria Neorickettsia risticii, an agent that is found in nature with a complex life cycle.
Horses become infected by consuming aquatic insects harboring the bacteria, such as mayflies, dragonflies, and caddisflies. N. risticii then infects cells in the horse’s blood and intestinal wall, especially those of the large colon, causing enterocolitis. Clinical signs may include colic, diarrhea, fever, and laminitis (founder). Diagnosis is based on clinical signs, as well as blood and fecal testing.
Treatment involves antibiotic therapy (oxytetracyline) and supportive therapy (fluid therapy, maintenance of electrolyte balance, anti-inflammatory therapy). Clinical signs of the disease are highly variable, but fatality rates are relatively low with early recognition and proper treatment of the disease.
Prevention of PHF involves vaccination, as well as management measures to limit the exposure of horses to infected aquatic insects. Horses grazing in wet areas near rivers or ponds are at increased risk. Turning off outdoor or barn lights is helpful to reduce the congregation of insects that may fall into feed and water buckets, where horses may consume them. Fencing off natural water sources may decrease horses’ exposure to insects while grazing and prevent the drinking of contaminated water.
Vaccination does not confer full protection
against the disease, however may help to decrease the incidence and
reduce severity of the disease. In the Midwest, this vaccine should
be administered in the spring to provide for immunity during the
warm months when exposure to PHF is highest. The vaccine is often
given every 6-12 months, with boosters administered more often in
highly endemic areas.
