Potomac Horse Fever Cases Popping Up in Ohio

Cases of Potomac horse fever (PHF) are starting to appear in Ohio, according to Catherine Kohn, VMD, Dipl. ACVIM, professor in the Department of Veterinary Clinical Sciences at The Ohio State University's College of Veterinary Medicine. The hospital has recently seen three cases and diagnosed another via mail-in sample. One horse was euthanatized because of laminitis.

"We're definitely starting to see cases right now," Kohn said, "indicating that the organism is around and active in our area."

The epidemiology of Neorickettsia risticii, the causative agent of PHF, is still under investigation. Ingestion by the horse of aquatic insects such as caddisfles, mayflies, damselflies, and dragonflies, which harbor the trematode host of N. risticii, is probably the most important avenue of transmission of this pathogen under natural circumstances, according to Kohn. The trematode can also infest certain species of snails. Measures to prevent infections should include minimizing exposure to aquatic insects by reducing insect populations in barns (turning out lights, screening stalls, and using insecticides. When the disease is active in your area, limiting access of horses to streams, ponds, and irrigated pastures is also advised.

The available vaccines for PHF, while not perfect, may be a useful option for horse owners in endemic areas, Kohn said.

"There may be other strains of Neorickettsia risticii floating around, that aren't represented in the vaccines and therefore the vaccines may not be optimally effective," Kohn said. "New pathogenic strains of bacteria or viruses, evolve over time, and veterinarians and researchers are on the look out for new, unique strains of N. risticii.

"In my experience, (the vaccines) may reduce the morbidity, (make the disease less severe), and therefore hopefully the mortality (reduce the number of horses that die as a result of the infection).," Kohn said. However, developing an improved vaccine would be very beneficial.

Knowing the early clinical signs of PHF can help owners and veterinarians to treat the horse while the disease is in the early stages. According to Kohn, horses with PHF often look depressed, toxic, and colicky. They might have loose manure or severe diarrhea, and will generally have a fever, which can be up to 104 to 105° F.

"If a horse has a high fever you should definitely call the vet right away," Kohn said. "I think that horses that refuse to eat, show signs of abdominal pain, and/or have diarrhea, and have a fever should be considered suspects for PHF, particularly if your barn is near a stream or river, or if you know there have been Potomac horse fever cases in the past on your premises or in your area."

Kohn cautioned that it can be difficult to distinguish a horse with PHF from one with Salmonella, another bacteria that can cause debilitating, and sometimes deadly, diarrhea. She recommends having a vet out to test suspect horses for N. risticii, and said a high IFA titer is diagnostic. Affected horses usually respond fairly well to intravenous tetracycline, Kohn said.

Laminitis is one complication associated with PHF that often strikes just as the colitis is beginning to resolve. Kohn advocated careful observation by a veterinarian during this time.

"It's important to be vigilant about the laminitis, and we recommend that anti-inflammatory drugs be given while the horse is acutely ill," Kohn said, so long as a veterinarian determines that the horse has no health issues that would make the use of non-steroidal anti-inflammtory drugs dangerous.

"These drugs should be used with the guidance of a veterinarian," Kohn said. "If used in excessively high doses or for prolonged periods, NSAIDS are potentially toxic for horses."

Renal disease can also follow the apparent resolution of the intestinal signs, Kohn said.

About the Author

Erin Ryder

Erin Ryder is a former news editor of The Horse: Your Guide To Equine Health Care. She owns a portly gray gelding named Duncan and dabbles in several equestrian disciplines, with an emphasis on dressage.

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