Horses can get Potomac horse fever from ingesting the flukes tha carry the causative bacteria (released by aquatic snails) or the water-borne insects they infect. | Photo: iStock

So it was as I was sitting down to dinner one blazing hot evening last summer when my phone alerted me to an emergency. When I arrived at the farm, the mare, Molly, was in her shed with her head down, evening meal untouched. She had a fever of 103.6ºF, an increased heart rate, fluidy gut sounds, and a small amount of diarrhea behind her. I explained to her owner, Joe, that in our area the most likely diagnosis that would fit her signs was Potomac horse fever (PHF).

He was puzzled at my statement, as his horse was current on her vaccines, which included PHF. And, besides, his horses never left the farm or had contact with other horses. He was also worried about her herdmate and asked what he could do to protect the gelding.

I explained that the biggest risk factors for his horses were that they lived in an endemic region (where PHF commonly occurs) within 5 miles of a body of natural water and that we were having a hot, rainy summer. Potomac horse fever is not contagious; rather, snails and aquatic insects carry the causative agent (­Neorickettsia risticii). The bacteria live within parasites that infect the snails and insects. This Russian-doll quality has made this organism challenging for researchers to describe. Horses get sick when they ingest parasites (in natural bodies of water) or infected insects. Measures to protect the horses’ feed, hay, and water include turning off barn lights that attract insects at night, keeping stored hay covered with a tarp, and limiting grazing next to streams and creeks