Potomac Horse Fever or PHF is a rickettsial disease caused by the organism Neorickettsia risticii. There have been about 10 confirmed cases of the disease between Richmond and the Shenandoah Valley this spring.
The disease is often (but not always) seen in horses that live near water because part of the life cycle of the fluke organism that is the vector or carrier for N. risticii goes through freshwater snails. It is believed that larval stages of several varieties of flies ingest the fluke, and that the horse accidentally ingests the infected adult flies.
The main clinical signs include fever, lethargy, diarrhea, and laminitis. There is a test for PHF however it takes several days to get the results so your veterinarian may choose to initiate treatment before confirmation of the disease.
There is also a vaccine for the PHF, however it does not completely prevent the disease from occurring in the horse, but it may lessen the severity of clinical signs. Vaccination may occur in both the spring and fall.
Treatment for PHF includes IV oxytetracycline (an antibiotic), in addition to supportive care for other symptoms. Recommendations to help prevent the disease include maintaining barriers along bodies of water to keep aquatic insects near their place of origin, turning off outside lights around the barn to prevent insects from being attracted to the area, and cleaning water buckets and feed areas often in addition to keeping feed covered to reduce the chances of the horse ingesting infected insects.