Potomac Horse Fever: True or False?

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Most horse owners around the country are fully aware of the threats posed by diseases such as West Nile virus (WNV), rabies, and equine herpesvirus. But fewer might know about Potomac horse fever (PHF), its inherent risks, and how to determine whether or not their horse is at risk and should be vaccinated. Here are a few important points to keep in mind

True or False: PHF is only found near the Potomac River.

False. The name of this potentially debilitating disease is misleading. While the initial PHF outbreak in 1979 occurred near the Potomac River in Maryland, since then the disease has been identified in 43 states, three Canadian provinces, parts of South America, the Netherlands, and France. Horses grazing on a pasture near the Shasta River in the 1940s showed similar clinical signs, resulting in some referring to the disease as "Shasta River crud."

True or False: PHF is caused by bites from various insects.

False. Unlike other insect-borne diseases such as WNV and Eastern equine encephalitis, PHF is not caused by the insect actually biting a horse. Rather, it occurs when the horse ingests infected aquatic insects such as damselflies, caddis flies, and mayflies, which are commonly found in areas near creeks and rivers. This can happen while horses graze in proximity to those creeks and rivers, or when they ingest dead aquatic insects in their water buckets or hay.

True or False: The clinical signs of PHF can include fever, decreased intestinal sounds, and diarrhea.

True. The disease can be difficult to diagnose as its clinical signs are subtle and mimic other diseases, in particular salmonellosis. Fevers can range from 102 – 107°F at the disease’s onset. Within two weeks of infection, the fever could be accompanied by clinical signs of colic, mild to severe diarrhea, absent appetite, and depression. As the disease progresses, some horses suffer from toxemia and dehydration.

True or False: One of the most devastating effects of PHF is the possible development of laminitis.

True. Frequently, horses develop laminitis several days after the diarrhea starts. Approximately 40% of horses diagnosed with PHF have subsequently developed laminitis.

True or False: Horse owners can take measures to reduce the populations of insects that can be infected with the disease.

True. Good farm management practices include keeping your horse’s food covered, restricting grazing near creeks or other bodies of water during the peak season for PHF (spring, summer and early fall in temperate climates), and using repellents to decrease fly and other insect problems in your barn.

True or False: Veterinarians are the best resource for determining whether or not to vaccinate a horse to help protect against PHF.

True. While not considered a core vaccination by the American Association of Equine Practitioners, PHF is should be considered as a risk-based vaccination. Your veterinarian can help you evaluate whether or not PHF poses a significant risk and make a recommendation.


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