Potomac Horse Fever (equine intestinal ehrlichiosis) is a highly fatal enterocolitis of horses named after the Potomac River in Montgomery, Maryland, since the first outbreaks occurred on its banks. This disease came on the scene in the 1970s, and baffled researchers for some time before, the causative agent was discovered to be Ehrlichia in the mid-1980s. Since that time, a wealth of knowledge has developed about this rickettsial pathogen.
The life cycle of E. risticii is interwoven with another parasite, called a trematode. Both the trematode (a worm parasite) and E. risticii rely on snails for survival. Horses are infected by inadvertently ingesting snails or water borne insects that carry E. risticii while grazing on pastures that abut a river, like the Potomac. Outbreaks of Potomac Horse Fever have been reported in horses that live close to numerous different rivers in North America. Potomac Horse Fever is virtually always seen in the warmer months because the vectors are not active in the winter.
What to Watch For
The diagnosis of Potomac Horse Fever is primarily made on the basis of symptoms, and the fact that the horse lives adjacent to a river. The disease crops up mainly in the warmer months, presumably when snails and insects associated with river-beds are more mobile and numerous.
Infected horses develop antibodies against E. risticii and blood testing for the presence of these antibodies is an important step in making the diagnosis of Potomac Horse Fever. Newer blood tests (PCR tests) are being developed that can reveal whether actual E. risticii organisms have entered the horse's blood stream. It is important for the veterinarian to also consider other causes of diarrhea and fever, such as Salmonellosis and clostridiosis, that appear almost the same as Potomac Horse Fever.
Treatment of diarrhea associated with Potomac Horse Fever must include the intravenous administration of large volumes of fluids, the amount of which is determined by the severity of diarrhea and dehydration. Tetracycline antibiotics are effective at killing E. risticii and should be used in conjunction with other supportive treatments as needed. If recognized early in the course of the infection, the use of tetracycline antibiotics might preclude the need for IV fluids. If edema forms due to loss of body proteins, it may be necessary to give a fluid rich in protein, like plasma, or a synthetic preparation. Veterinarians who work in geographic locations for which Potomac Horse Fever is endemic become very adept at making a rapid and early diagnosis and, therefore, achieve a high success rate in treatment. If signs of laminitis are recognized, specific treatment for that important condition must also be undertaken.
Although a vaccine against Potomac Horse Fever has been marketed, this vaccine is based on only a single strain of E. risticii. There are at least six different strains of this pathogen. There is substantial doubt pertaining to the effectiveness of this vaccine and its use is not widely recommended.