A horse starts showing stiffness and a reluctance to move. His muscles suddenly become weak to the point he can no longer remain standing. Then, as quickly as clinical signs set in, the horse dies.

Just 48 hours earlier the horse grazed happily in his pasture—an overgrazed field full of seed heads and dead leaves.

This story is typical of suspected cases of seasonal pasture myopathy (SPM), a highly fatal muscle disease described in the Midwestern United States and eastern Canada, and atypical myopathy (AM) in the United Kingdom and Europe. For decades the disease had baffled veterinarians on both continents, who struggled to pinpoint and agree on a cause.

That changed in 2011 when Stephanie Valberg, DVM, Dipl. ACVIM, and a team of researchers from University of Minnesota (UM) and Iowa State University (ISU) started investigating SPM cases and found a link to box elder trees. She presented their findings in “Identification of the Cause of Seasonal Pasture Myopathy in Horses” at the 2014 American College of Veterinary Medicine Forum, held in Seattle, Wash.

Disease Overview and Clinical Signs

Valberg started her talk with an overview of the disease. SPM and AM outbreaks vary year to year and usually occur in the fall with a few cases in taking place the following spring, she said. In North America, only a few horses are affected on a given pasture, she said, pointing out that European outbreaks include many horses on the same farm.

“Horses that develop SPM and AM are usually kept on sparse pastures with an accumulation of dead leaves, dead wood, and trees in or around the pastures,&quo