More Kentucky Potomac Horse Fever Cases Confirmed

Two additional cases of Potomac horse fever (PHF) have been confirmed at Hagyard-Davidson-McGee (HDM) veterinary hospital in Lexington, Ky., bringing the clinic’s total to five confirmed cases since the end of July. Two other PHF cases previously were reported at Lexington’s Rood and Riddle Equine Hospital. Typically, Kentucky veterinarians see only one or two cases per year.

It is important to note that of the five cases confirmed at HDM, three were from Central Kentucky, one was from Cincinnati, Ohio, and one was from a boarding farm on the Indiana/Kentucky border.

Potomac horse fever’s causative agent, a bacteria named Ehrlichia risticii (recently renamed Neoriketssia ristici), has been linked to parasites of freshwater snails. The parasites are called cercariae, and they also infect the larvae of mayflies and caddis flies in fresh water. When the fly larvae mature into adult flies, they sometimes are ingested by horses which inadvertently consume the insects while grazing or eating feedstuffs. Horses kept near fresh-water streams or ponds are more likely to be at risk for getting the disease because of the close proximity of the aquatic insects. However, Nathan Slovis, DVM, Dipl. ACVIM, of HDM, said that most of the affected horses were not pastured near streams or small bodies of water.

The influx of potential PHF cases was worrisome for a few days, but at this time Slovis doesn’t anticipate bringing in epidemiologists familiar with PHF. “Things are quiet right now,” said Slovis. “We’ll see how it goes.

“If the horse has a history of fever, a low white blood cell count, and is off his feed, it raises a red flag that this might be Potomac horse fever, and that he should be tested,” explained Slovis. “Horses with these symptoms are the only ones we are testing.” He leaves it up to the veterinarian’s discretion to initiate treatment prior to receiving test results. Results from polymerase chain reaction tests, which look for the DNA of a disease-causing agent, can be returned in 24 hours.

Why does there seem to be more PHF in 2002?  No one can be sure, but Slovis offered some ideas.

Heat predisposes the snails to secrete the circarae into the water, where insect larvae are infected. Additionally, “areas that have a low level of water may have dried up now, so they’re exposing grass that a horse normally a horse might not have been able to graze,” he explained. In those areas, the grass is richer, and the horses might have an increased exposure to the aquatic insect (carrying the disease).”
He also suggests that since aquatic areas are drying up, caddis flies and mayflies might be traveling to areas that they do not typically frequent.

“Mayflies and caddis flies love light and fly everywhere,” he explained, so it might be worth the trouble to “keep the lights off in the barn during dusk and dawn hours.”

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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