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Potomac Horse Fever


First described in the area around the Potomac River in Maryland in 1979, Potomac Horse Fever is a serious illness that is now found around the United States and in other countries. Potomac Horse Fever is caused by a tiny bacteria Neorickettsia risticii.


The PHF organism is harbored inside flukes (trematodes) that parasitize water snails. When the water becomes warm, the flukes hatch immature forms, called cercaria, which carry the PHF organism, and pass out of the snail into the water. This water can infect horses. Currently the most important transmission route is believed to be these immature flukes, which are ingested, by a variety of aquatic insects, most commonly caddis flies and mayflies. The larval stages of these insects then molt into flying insects, carrying the immature fluke and PHF organisms into the horse’s environment. The horse then becomes infected when it eats or drinks anything contaminated with these insects. Weather conditions determine the hatch rates and timing of aquatic insects. High hatch rates may result in swarms near rivers or lowlands. For most of these insects, the swarm is adult insects that live briefly, mating, depositing eggs back in the water, then dying in 1-2 days. Aquatic insects are attracted to lights at night. Reports suggest that horses in stalls near night-lights may be at greater risk of developing clinical signs. The insects fly towards the lights and may die there in large numbers thereby getting into the horse’s feed, bedding or water. In one confirmed cluster of 4 cases in Minnesota this summer, thousands of dead mayflies were observed outside the barn, and some were found in the affected horses’ hay and stalls.


After the organism is ingested, it multiplies in the intestinal tract where it can cause inflammation also known as colitis. This can lead to fever, depression, poor appetite and in most cases, diarrhea Some horses will founder and pregnant mares can abort and some horses will also develop swelling of their lower limbs or body wall. Not all horses exposed to the PHF organism become ill. This disease can kill affected horse, but most respond well, if treated early, to oxytetracycline, an IV antibiotic. As the diarrhea can be severe, fluid therapy is often needed to address dehydration and electrolyte imbalances. Anti-inflammatory drugs such as Banamine can help reduce the effects of toxins that get into the bloodstream from the inflamed intestinal tract. Very severe cases my require intensive care, including plasma transfusions. Additional therapy may be needed if the toxins induce founder or laminitis in the horse.


It is our recommendation that horses who live in or travel in areas where wetlands are nearby should vaccinate their horses for Potomac Horse Fever annually. When the PHF vaccine is used for the first time, the horse must receive a booster in 2-4 weeks. After that, an annual vaccination is recommended. In years where high numbers of cases are seen, it is possible that we would recommend a late summer booster.

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