Potomac Horse Fever is caused by a rickettsial parasite called Ehrlichia risticii, which is transmitted to horses by an unknown arthropod vector found most frequently in areas near rivers. The Potomac River Valley location in Maryland and Virginia has been associated with outbreaks of the disease, but evidence indicates that horses from other states and Canada have been exposed to the organism.
The organism and its mode of infection are not very well understood at this time. The disease causes similar clinical signs as horses infected with the Salmonella bacteria. Horses typically develop a fever less than a week after infection with E. risticii; owners usually do not observe this fever. Approximately two weeks after the infection, however, many horses will show significant clinical signs and will be noticeably ill.
Because many cases of PHF go undiagnosed, affected horses have died from complications such as severe dehydration. Euthanasia has been selected for horses that develop severe laminitis following PHF.
