Potomac Horse Fever

Severe weather events that horse owners might encounter this summer, an increased possibility of Potomac horse
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Due to the severe weather events U.S. horse owners have encountered this summer, an increased possibility for Potomac horse fever (PHF) outbreaks exists. While PHF is not common in the southern United States, the uncharacteristic weather and flooding could potentially change that. The condition typically occurs in the late spring, summer, and early fall months, when temperatures are warmest.

Potomac horse fever, which first appeared in the United States more than 20 years ago, is caused by the bacterium Neorickettsia risticii. Other names for PHF include equine monocytic ehrlichiosis, equine ehrlichial colitis, and Shasta river crud.

N. risticii‘s life cycle is complicated; the bacterium has been identified in flukes (flatworms) that develop in aquatic snails and are released into bodies of water. Aquatic insects (e.g., damselflies, dragonflies, and mayflies) might then pick up infected immature flukes. Horses grazing near freshwater creeks, rivers, or on irrigated pasture can ingest the insects carrying PHF and potentially become infected. Horses can also ingest dead aquatic insects that might have inadvertently landed in their water buckets or hay/pasture. The horse does not necessarily need to be near a water source to become exposed to the disease.

The initial clinical signs of PHF can be subtle. Thus, it is important to know what is normal for your horse and respond quickly to any changes in his normal behavior. In the very first stages of the disease, horses are anorexic, febrile (feverish), and depressed. Intestinal sounds are also decreased. These signs might be followed by diarrhea, which can also cause some mild colic signs. Some horses might develop laminitis, and pregnant mares are at risk for abortion. Other complications include toxemia (toxins in the blood), ileus (decreased intestinal motility), more severe colic signs, and dehydration

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Written by:

Eleanor Lenher, DVM, is a graduate of Colorado State University, College of Veterinary Medicine and Biomedical Sciences, and now works as an associate at Deer Creek Equine Clinic in Powhatan, Va.

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