Summer is Prime Time for Potomac Horse Fever

With summer just around the corner, now is the time to vaccinate horses against Potomac horse fever (PHF).

"Horse owners need to be aware of the disease and its seasonality," said Julia Wilson, DVM, Dipl. ACVIM, associate professor of large animal medicine at the University of Minnesota.

PHF is a potentially deadly disease that can cause mild depression, anorexia, diarrhea, and abortion in pregnant mares. Some horses also might develop severe toxemia, and up to 40% of horses with PHF will develop laminitis.

PHF is caused by aquatic insects, such as caddisflies and mayflies that are infected with the agent of disease, Neorickettsia risticii, and is traditionally seen in the summer and early fall during the insects' peak hatching times. Horses can ingest infected aquatic insects while grazing near waterways.

Caddisflies, mayflies, and other carriers also swarm around barn lights, causing infection even while horses are stabled.

"Insects like to swarm around lights," explained Frank Hurtig, DVM, MBA, director, Veterinary Services, Merial. "That's why it's so important to keep hay and water away from lights that stay on all night. Once those infected insects die, they may fall directly into the horse's food and water source."

Traditionally seen in areas surrounding freshwater streams and other bodies of water, PHF has been found in nontraditional areas, including northern Wyoming and Minnesota. PHF has been known to occur in 43 states, three Canadian provinces, and parts of South America, The Netherlands, France, and India.

"PHF boundaries are very blurry," Hurtig said. "Horse owners should consider their proximity to bodies of water, their travel plans for the summer, and most importantly, consult their veterinarian about vaccinating for PHF."

Hurtig advises horse owners to talk with their veterinarians about a vaccine that is proven safe and effective, such as Merial's Equine Potomavac. In one study, Potomavac protected 86% of horses from clinical disease. It also is proven safe for horses as young as 3 months and is available as a combination of Equine Potomavac + Imrab to help protect against both PHF and rabies.

Merial  is a world-leading, innovation-driven animal health company, providing a comprehensive range of products to enhance the health, well-being, and performance of a wide range of animals.

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