Seasonal pasture myopathy (SPM) is a highly fatal muscle disease of horses, caused by eating box elder tree (Acer negundo) seeds. The seeds contain varying amounts of hypoglycin A, a type of nonproteogenic amino acid which is known to cause acquired multiple acyl-CoA dehydrogenase deficiency (MADD). SPM is very similar to atypical myopathy (AM), which is a fatal muscle disease that occurs in horses living in Europe, resulting from ingestion of sycamore maple (Acer pseudoplatanus) seeds.
Outbreaks of SPM occur sporadically in horses living in pastures in the Midwestern United States and eastern Canada. Most cases happen in the autumn months, although a few have occurred in the spring.
Horses with SPM develop severe acute myonecrosis which involves the respiratory and postural muscles, myoglobinuria, and in some cases the cardiac muscles. 75% of horses with SPM die within 72 hours of onset of clinical signs.