Minimizing your horse’s risk of contracting Potomac horse fever (PHF) might be as simple as shutting off the lights.

Mayflies—swarms of which are common in Southeast Minnesota and adjacent areas of Wisconsin—were incriminated as a vector of PHF during a 2005 outbreak in Wisconsin and Minnesota.

Julia Wilson, DVM, Dipl. ACVIM, an associate professor of Veterinary Population Medicine at the University of Minnesota, investigated the 2005 outbreak when six horses got PHF after attending the same show. The horses’ owners reported a swarm of mayflies at the show—inches of the dead insects covered the ground and some blew into the barn and trailers.

Wilson visited the show grounds nearly a month later and found desiccated (dried out) samples of mayflies, which were found to be positive for PHF