Potomac horse fever (PHF or equine neorickettsiosis) is caused by Neorickettsia risticii. The disease was first identified in 1979 along the Potomac River in Maryland and Virginia and has since occurred throughout the country in areas with freshwater streams and rivers. Seasonal in occurrence, Potomac horse fever appears mostly in middle to late summer into early autumn. An infected horse develops severe, pipe-stream diarrhea and colic, with about 30% mortality due to subsequent development of severe laminitis.
Over years of sleuthing, researchers identified the DNA of the infectious agent in fresh-water snails. When river water warms up, motile metacercaria (the larval stage of a snail fluke) are released from the snails. These larvae contain the infectious agent, which is picked up by aquatic insects such as caddisfly and other aquatic insect larvae that serve as natural vectors.
As insect larvae hatch and emerge from the creek, river or ditch water, they contaminate water, hay and pasture that is then ingested by a horse. Infection coincides with large hatchings of caddisflies, mayflies and stoneflies in warm months.
Even if your horse isn’t on pasture, infection is possible. At night, aquatic insects circulate around lights found alongside barns and horse facilities. Bats and adult birds eat the insects and develop infected flukes. The flukes carry the infectious agent, the bats or birds defecate into the environment, this gets into the water, then into the snail, allowing the cycle to start again.