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 from light sources that could attract insects.
  • Restrict horses’ access to streams, ponds, and other standing water sources—including low-lying pasture areas—to reduce their risk of coming in contact with snails.

In a recent study on PHF survival, researchers examining the records of 50 horses diagnosed with PHF over 15 years identified clinical signs including diarrhea in 66% of horses, fever in 48%, lack of appetite in 42%, depression in 40%, and colic in 38%. Laminitis developed in 32% of the cases; 88% of these horses were affected in all four feet.

That research team learned that 76% of all PHF cases survived to discharge but those with laminitis were less likely to survive. Additionally, they found that treating affected horses with the antibiotic oxytetracycline improved survival odds twelvefold.

"Contact your veterinarian if horses develop a fever or become depressed, as early treatment increases survivability and reduces the severity of clinical signs associated with PHF," the alert read.