Potomac Horse Fever: Don't Ignore Fevers!

Summer and fall are key seasons for the appearance of Potomac horse fever, which is caused by the bacterium Neorickettsia risticii. Don't be fooled by the name; Potomac horse fever originally was identified in 1979 as a sporadic disease affecting horses residing in the eastern United States near the Potomac River, but since that time it has been diagnosed throughout the United States and Canada.

"Don't ignore fevers," warned Nathan Slovis, DVM, Dipl. ACVIM, of Hagyard Equine Medical Institute in Lexington, Ky. "If you have an animal in a pasture that spikes a fever (102-105°F), rule out other causes and test it for Potomac horse fever."

Slovis has seen several cases this year already in Kentucky, although he stressed this is not an outbreak, just the normal course of events in hot summer weather. One horse came from the racetrack and others were show or pleasure horses at pasture.

Research has shown that up to 80% of confirmed Potomac horse fever cases eventually die, some because of secondary laminitis.

Jackie Smith, staff epidemiologist at the Kentucky Livestock Disease Diagnostic Center, said their personnel have seen one Potomac horse fever case on necropsy and diagnosed three via PCR testing at the lab. This compares to 15 seen by this time last year. She thinks the numbers will go up as veterinarians send in more samples to be tested.

Potomac horse fever can be mild to life-threatening. According to information from Julia Wilson, DVM, Dipl. ACVIM, an associate professor of Veterinary Population Medicine at the University of Minnesota, after exposure to the organism, intestinal cells and a type of white blood cell, monocytes, are infected. (The Merck Veterinary Manual says the incubation period is 10-18 days.)

Wilson says 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. Abortion weeks to months after initial infection is also possible if a pregnant mare develops the disease, as the organism can be transmitted across the placenta.

"Prompt treatment is important and should include the antibiotic oxytetracycline as well as non-steroidal anti-inflammatory drugs to counteract endotoxin and alleviate pain," said Wilson. "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, noted Wilson. "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," she advised.

"With early initiation of treatment, the prognosis for PHF is good, providing the horse does not develop laminitis," said Wilson. "Should laminitis develop, its severity might lead to euthanasia, even after the horse has recovered from the diarrhea."

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.

According to information from merckvetmanual.com, N. risticii has been detected in 13 species of immature and adult caddisflies, mayflies, damselflies, dragonflies, and stoneflies. That source noted that "the causative organism is present in the feces of experimentally infected horses, but the biologic significance of this is unknown. Clinically ill horses are not contagious and can be housed with susceptible horses."

Wilson noted that 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.

Wilson suggested horse owners keep barn lights off at night during summer months to keep from attracting the clusters of mayflies. Wilson said she has suggested this to several other veterinarians experiencing high numbers of PHF cases, and they have reported good results.

AAEP Vaccination Guidelines

Vaccination Guidelines published by the American Association of Equine Practitioners (AAEP) notes that if Potomac horse fever has been confirmed on a farm or in a specific region, "is likely that additional cases will occur in future years. Foals appear to have a low risk of contracting the disease."

They note that the value of vaccines against Potomac horse fever have been questioned because vaccines only contain one strain of N. risticii, and there are multiple strains found in nature.

"The currently available commercial vaccines are killed, adjuvanted products," noted AAEP. "Two of these are also available combined with a rabies vaccine. None of the current vaccines carry a label claim for the prevention of abortion." (For complete Potomac horse fever guidelines see the AAEP Web site Vaccination Guidelines.)

John Madigan, DVM, MS, Dipl. ACVIM, director of the large animal hospital at the University of California, Davis, has done a tremendous amount of research on Potomac horse fever in his lab. When questioned whether he recommended vaccination, he said, "no one knows with a degree of scientific certainty" if the vaccine protects in particular regions of the country. "So the use of any of the vaccines has to be decided based on the owner choosing to use a vaccine that ‘might' help, versus choosing to use no vaccine." This should be done in consultation with your veterinarian.

Madigan said more work needs to be done on this important infection of the horse.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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