Equine ehrlichial colitis (Potomac horse fever): recognition of the disease in Pennsylvania, New Jersey, New York, Ohio, Idaho, and Connecticut.

Equine ehrlichial colitis (Potomac horse fever), a newly identified colitis of the horse, was first recognized in Maryland. In this report, we document occurrence of the disease in Pennsylvania, New Jersey, New York, Ohio, Idaho, and Connecticut. Enzootic areas were recognized by a characteristic pattern. Frequently there was a seasonal pattern and high prevalence of sporadic colitis in unstressed horses. The attack rate per farm generally was low. Horses on pasture, as well as those stabled, were affected. Clinical signs varied from fever and depression to severe diarrhea and laminitis. Occasionally horses developed profound ileus and severe colic. Diagnosis was based on detection of an increase or decrease in serum antibody titers to Ehrlichia risticii, using an indirect fluorescent antibody technique.