The Racing Journal Logo

Avoid Mayflies to Minimize PHF Risk

July 1, 2007     E-mail this page to a friend!

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.

She sent a questionnaire to members of the Minnesota Quarter Horse Association and found five more cases had developed in horses that did not attend the show. In all but one of those cases, the horses’ owners reported seeing aquatic insects swarming around lights at night.

Wilson suggested horse owners keep barn lights off at night during summer months to keep from attracting the clusters of mayflies.

Wilson said she has suggested this to several other veterinarians experiencing high numbers of PHF cases, and they have reported good results.

Reprinted with permission from The Horse; visit for additional horse health information.