The Maryland Department of Agriculture would like for Maryland horse owners to be aware of the recent alert issued by the Virginia-Maryland Regional College of Veterinary Medicine and the Marion DuPont Equine Medical Center have been contacted about several confirmed cases of Potomac Horse Fever (PHF) in central Virginia.  Notes Maryland State Vet Guy Hohenhaus:  “This is typical of what we see in wet summers, and equine vets in the region are very familiar with the disease. They are very capable and well positioned to provide good care for their clients’ horses.”

According to information provided by the vet school, PHF is a disease caused by multiple strains of the organism Neorickettsia risticii.  The organism lives in a developmental stage of a freshwater fluke. The fluke 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, and/or aquatic insects through grazing and drinking. The most likely route of infection is thought to be through the ingestion of infected adult flies, as they may travel some distance from the water source where they originated and then contaminate water or food sources on the farm. Due to the 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/or severity of infection.

  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 the 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.

Contact your veterinarian if horses develop a fever or become depressed as 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.

Why is this disease called POTOMAC Horse Fever? Is it somehow connected to the river? To Potomac, Maryland?

Alas, of all the great equine moments produced by Maryland, having a disease named after one of our most traditional equine enclaves is not one of them. Horses afflicted with this disease were first observed in 1979 in Montgomery County in the watershed to the Potomac River, but it is now found in most of the continental United States. For a background on the evolution of the understanding of PHF, please visit this article from the archives of The Horse.