“Seasonal Pasture Myopathy” (SPM) or “Atypical Myopathy” (AM), is a devastating equine muscle disease which is fatal in over 90% of cases. Characteristic signs of disease include stiffness, difficulty walking or standing, voiding dark urine, and eventually breathing rapidly and becoming recumbent before death. It is confused with colic or founder. An ounce of prevention is worth a pound of cure with this disease, but until now, veterinarians and researchers have not known what causes the disease or how to diagnose it. We are therefore excited to report that the cause of SPM has been identified through research at the University of Minnesota

Seasonal Pasture Myopathy is most commonly seen in the fall with fewer numbers of affected horses seen in the spring and summer. It is typically not seen when snow is present. Factors that have been associated with increased risk of disease include: being pastured for over 12 hours daily; the fall season; lack of supplemental hay while horses are on pasture; sparse pasture with short grass; the presence of trees with dead wood on the ground; heavy wind or rain in the week preceding clinical signs; and introduction of a horse onto a pasture for its first season, such as a young horse or a horse that has recently moved to a farm.

A toxin in the seeds of the box elder tree (Acer negundo) has just been discovered to cause SPM in North America, making the presence of box elder seeds the final key risk factor for SPM. Ingestion of sufficient quantities of box elder seeds results in breakdown of respiratory, postural, and cardiac muscles. When one horse becomes affected, herd mates are also at risk. However, not every horse pastured near box elder seeds will develop SPM. The reason behind this is just one of the mysteries about SPM that remains to be solved. It likely has to do with the time they are exposed to seeds, the toxin level in the seeds, how many seeds blow onto the pasture and whether they received additional feed such as hay in a feeder that makes eating seeds less attractive.