A bacterium called Neorickettsia risticii (formerly called Ehrlichia risticii) causes Potomac horse fever (PHF). The microscopic parasite lives on a type of flatworm called trematodes, also known as flukes, that grow and multiply in fresh water. Horses can become infected by directly ingesting flukes while grazing near or drinking from creeks, lakes or rivers. Or, some insects such as caddisflies, mayflies, damselflies, dragonflies and stoneflies, will, as aquatic larvae, feed on the flukes. When the insects mature, they fly or are carried inland by the wind. Horses then eat the fluke-infested insects – dead or alive – in their feed, pasture, water or even hay.
Some researchers have even speculated there could be a connection with bats and barn swallows, as the bacteria has been found in the intestinal tracts of both species. It is believed the bats and birds eat infected snails or insects and pass on the bacteria via their feces.
“Cases are more likely to occur on farms that are closer to the water, but may occur up to a few miles from a body of water,” said John Baird, Ontario Veterinary College professor emeritus and co-author with Dr. Luis Arroyo of Historical aspects of Potomac horse fever in Ontario (1924-2010). The disease is not contagious and large outbreaks do not necessarily occur on any one farm, he said.