Health Alert: Potomac Horse Fever Confirmed in Virginia Mayflies are one type of insect that can cause PHF if infected and ingested.

Health Alert: Potomac Horse Fever Confirmed in Virginia


Six confirmed cases of Potomac Horse Fever reported in central Virginia have area horse owners wondering about preventative measures. PHF is a disease caused by multiple strains of the organism Neorickettsia risticii, according to the Virginia-Maryland Regional College of Veterinary Medicine.

The tiny organism lives in a developmental stage of a freshwater fluke. It infects aquatic snails and aquatic insects, such as dragon flies, damsel flies, caddis flies, stone flies and mayflies. Horses become infected by inadvertently ingesting infected snails, snail slime, or aquatic insects through grazing and drinking.

The most likely route of infection is through the ingestion of infected adult flies. They may travel some distance from the water source where they originated and then contaminate water or food sources on the farm. In abnormally rainy weather, there may be an increased number of aquatic insects and snails exposing horses to this disease. PHF is not infectious to humans or other animals, and is not transmissible from horse to horse.

Although there is no absolute way to prevent exposure to this organism, there are several preventive measures that can be taken to reduce the likelihood and severity of infection. The Virginia-Maryland Regional College of Veterinary Medicine makes the following recommendations:
1. Vaccinate horses with a Potomac Horse Fever vaccine. Several horses that have developed PHF were vaccinated in the spring. Although the PHF vaccine is not highly effective at preventing infection it may reduce the severity of illness in infected horses and may improve the outcome in these cases. For this reason, it is recommended that horses receive a booster in areas where the disease has been reported.

For more information concerning vaccination please visit the following website of the American association of Equine Practitioners:

2. Reduce horses exposure to aquatic insects. Clean water buckets and troughs frequently and remove any insects. Keep stable lights off at night to discourage insects from being attracted to the barn. In addition, keep troughs or buckets away from light sources to lower the risk of having flies fall into them.

3. Restrict horses from accessing flowing streams or ponds. Likewise, restrict access to standing water or low-lying areas in pastures in order to avoid snails.

Call your veterinarian if your horse develops a fever or becomes depressed. Vets say early treatment increases survivability and reduces the severity of the clinical signs associated with PHF. Daily monitoring of your horse’s temperature can allow for early detection of fever, should it develop.