Potomac Horse Fever: Cause and Treatment

Potomac horse fever (PHF) is a disease that affects horses during warm weather months, occasionally causing outbreaks of diarrheal illness in horses that are kept near rivers, streams, or in irrigated pastures.

The bacterium responsible for the disease, Neorickettsia risticii, has an unusual history: it has been identified in flukes (flatworms) that develop in aquatic snails. When the water warms up during the summer, infected immature flukes, called cercariae, are released from the snails into the aquatic environment. These immature flukes can be swallowed by horses drinking from rivers or streams, but, more commonly, they are picked up by aquatic insects such as caddisflies, mayflies, damselflies, and dragonflies, where they develop into their next life stage, metacercariae. Infected aquatic insects such as the caddisflies and mayflies, which can hatch in mass, also might carry the organism to horses to pick up as they graze.

This route was demonstrated in an August 2005 PHF outbreak in Minnesota in which horses at a show held more than five miles from a river were affected. A large hatch of mayflies occurred on a hot, windy weekend, presumably from the Mississippi River. They were blown toward the show grounds and were attracted to bright security lights outside the barns. The short-lived mayflies died by the thousands, littering the ground with inches of insects. At least six horses at the show developed PHF in the next two weeks. N. risticii was found in mayflies lingering in tiny cobwebs indoors. Subsequent research determined additional cases occurred on area farms in the same time frame, and lights on or near horse barns at night were associated with an increased PHF risk. This finding provides a simple tool to lower the risk of this disease: turn off night lights during times that aquatic insects are hatching.

Bats and barn swallows that feed on aquatic insects are a second link in the natural cycle of flukes that carry N. risticii. The organism has been identified in the intestinal tracts of these species. These flukes undergo one developmental stage in aquatic snails, and a second stage in aquatic insects. It's unknown if fecal material from bats and barn swallows is a major source of infection to horses.

The disease caused by N. risticii can be mild to life-threatening. After exposure to the organism, intestinal cells and a type of white blood cell, monocytes, are infected. Most horses will have an initial fever, which might go undetected. A second fever surge is accompanied by depression and poor appetite, and 80% of cases develop diarrhea. Affected horses might develop signs of laminitis, evidenced by reluctance to move, and/or limb edema. Signs of endotoxemia can be seen as well, including elevated heart rate, dark mucous membranes, sweating, and mild colic signs.

Prompt treatment is important and should include the antibiotic oxytetracycline as well as non-steroidal anti-inflammatory drugs to counteract endotoxin and alleviate pain. Potomac horse fever-induced diarrhea might cause significant dehydration, requiring intravenous fluid and electrolyte therapy. Plasma transfusion might be needed if there is significant protein loss into the lumen of the intestine. Sole support of the hoof and local therapy, such as icing the feet, might be beneficial if laminitis is suspected.

The diagnosis of PHF can be confirmed by laboratory identification of the organism in a blood or manure sample from the horse by polymerase chain reaction (PCR) tests or by a significant rise in antibody levels in the horse's bloodstream between two samples taken at least 10 days apart. As PHF might be clinically indistinguishable from salmonellosis, the affected horse should be considered contagious to other animals and humans until fecal tests for Salmonella are verified as negative. With early initiation of treatment, the prognosis for PHF is good, providing the horse does not develop laminitis. Should laminitis develop, its severity might lead to euthanasia, even after the horse has recovered from the diarrhea. Abortion weeks to months later is also possible if a pregnant mare develops the disease, as the organism can be transmitted across the placenta.

Killed vaccines can booster a horse's immunity to N. risticii. These might not prevent illness, but we hope they reduce its severity if the horse is exposed to the organism. If PHF cases are infrequent, it might make better economic sense to not vaccinate, but consult with your veterinarian to decide the best course of action, and call her if any early signs of illness are observed.

About the Author

Julia H. Wilson, DVM, Dipl. ACVIM

Julia H. Wilson, DVM, Dipl. ACVIM, is an Associate Professor of Veterinary Population Medicine and Division Head of Large Animal Medicine at the University of Minnesota.

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