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 (PHF) is caused by tiny bacteria called Neorickettsia risticii.

The 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, and 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 simmer, 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 know as colitis. This can lead to fever, depression, poor appetite and in most cases, diarrhea. Some horses will founder, 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 horses, 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 sever cases may 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 that live in or travel to areas where wetlands are nearby should be vaccinated annually against Potomac Horse Fever. When the vaccine is given 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.