Potomac Horse Fever is a potentially fatal bacterial (Rickettsial) disease that is believed to be transmitted by inadvertent ingestion of aquatic insects (mayflies and caddis flies) carrying the organism. These are thought to fly into barns or feeding areas, die, and end up in feed.

The life cycle of the causative Neorickettsia risticii is complex and involves stages within a freshwater fluke and snail.

PHF occurs more frequently in wet warm climates, and particularly affects horses pastured along rivers, creeks and ditches. It is much more common in the Northeastern and Northern states, but has been diagnosed in most states within the U.S.

Initial signs of this disease are subtle and include mild colic, fever, inappetence, and diarrhea. Typically it progresses to severe colitis and diarrhea. Abortion in pregnant mares may occur due to fetal infection. Laminitis is a common consequence of infection.

Diagnosis is difficult, and often arrived at after other diseases are ruled out. The appearance of this disease is very similar to other causes of colitis and so laboratory work is necessary to definitively diagnose. In some cases, the diagnosis is presumed if a horse improves when given oxytetracycline (an antibiotic that is effective against the protozoa) Although PHF is not contagious from horse to horse, precautions should be taken immediately to prevent contact with other horses until other contagious diseases are ruled out.

Treatment is supportive nursing care (usually hospitalization) including IV fluids, plasma, colloids, and laminitis prevention measures. The only antibiotic class typically used against the organism is the tetracyclines.

In areas where the disease is common, oxytetracycline is often given at the earliest signs of disease, even when a definitive diagnosis has not been made. This likely reduces the severity of infection.