Potomac horse fever is a disease of horses that was first diagnosed in horses along the Potomac River in Maryland about 20 years ago. Since that time, Dr. Jon Madigan from the University of California at Davis indicates the disease has been proven to cause disease in 14 states, including Texas, as well as other countries. Because the disease was initially believed to be transmitted by ticks it was classified as an Ehrlichia organism. Now it is known that the intermediate hosts of the disease are various species of fresh water snails and other water insects, and the name of the infectious organism was changed to neorickettsia risticii. Since fresh water snails and water insects are common around rivers and creeks, most infected horses are exposed while grazing in these areas.

Potomac horse fever causes acute onset of depression, decreased appetite, and fever in horses as well as diarrhea, decreased intestinal signs, and mild colic. As with other similar diseases in horses, dehydration can be a real problem and some of these horses can also founder. The disease is deadly in 5 to 30% of the cases and has also been shown to cause abortion. Diagnosis of the organism requires isolation of the organism from the blood or feces as serological tests for this disease are unreliable. Potomac horse fever is treated with intravenous tetracycline and this treatment has been shown to be somewhat effective. Although a vaccine is available for prevention of the disease, it has not been shown to be effective in many cases. Although this disease is not commonly seen in Texas, it has to be considered in any case of diarrhea and depression with fever.