Several cases of Potomac horse fever (PHF) have been confirmed in central Virginia, prompting the Virginia-Maryland Regional College of Veterinary Medicine (VMRCVM) to release an outbreak alert. The alert was posted on the school’s Facebook page on July 13.

PHF is caused by Neorickettsia risticii, an organism found in some flukes (a wormlike parasite) that infect aquatic snails and insects (such as caddisflies, mayflies, damselflies, and dragonflies).

"Horses become infected by inadvertently ingesting infected snails, snail slime, and/or aquatic insects through grazing and drinking," the alert stated. "Due to the abnormally rainy weather, there may be an increased number of aquatic insects and snails exposing horses to this disease."

Elsewhere, the Hagyard Equine Medical Institution reported via Twitter that PHF cases were confirmed in Kentucky in May.

While there’s no "absolute" way to prevent PHF, the VMRCVM suggested several methods to help reduce the risk or severity of infection:

  • Consider vaccinating against PHF. In the alert, the VMRCVM noted that while several of the horses that developed the disease had been vaccinated in the spring, the vaccine "may reduce the severity of illness in infected horses and may improve the outcome of these cases. For this reason it is recommended that horses receive a booster in areas where the disease has been reported."
  • Reduce horses’ risk of exposure to aquatic insects by cleaning water sources frequently and locating buckets and troughs away fro