Potomac horse fever

In many parts of the country, hot, humid summer weather means it’s time to take steps to protect horses against equine monocytic ehrlichiosis, more commonly called Potomac horse fever (PHF). The disease was named for the Potomac River Valley, where it was first recognized in 1979, but cases have been identified throughout the United States as well as Mexico and Canada.

“Equine neorickettsiosis is the better name for this disease because endemic areas occur all across the country, not just in the Potomac River area,” says Nicola Pusterla, DVM, PhD, of the University of California–Davis.

PHF occurs when horses ingest the bacterium Neorickettsia risticii. N. risticii infects a parasitic fluke (a type of flatworm), which under goes a complex life cycle in which at different stages it can be found in a number of aquatic species, including freshwater snails as well as aquatic insects, birds and bats, and it can even be free-swimming in water.

But the threat to horses comes from flying aquatic has been found in more than a dozen species, including dragonflies, damselflies and stoneflies, whose larvae consume infected free-swimming flukes then continue to carry the bacterium-containing fluke when they emerge as adults. If horses consume the bodies of infected aquatic insects while grazing or drinking, they may develop Potomac horse fever.