Collaborative research between Dr. Beatrice Sponseller, clinician at Iowa State University Lloyd Veterinary Medical Center, and Dr. Stephanie Valberg, professor and director of the equine center at University of Minnesota, has led to the discovery of the cause of Seasonal Pasture Myopathy, a highly fatal muscle disease affecting horses at pasture. Clinical signs are dominated by acute muscular weakness and stiffness progressing to recumbency within 24-48 hours. Severe muscle damage of predominantly postural and respiratory muscles leads to myoglobinuria, the passage of dark brown urine that contains muscle pigment. Unfortunately, the disease is fatal in 75-90 percent of cases.
Owner surveys and pasture inspections conducted by the researchers between 2009 and 2011 revealed the uniform presence of seed laden box elder trees (Acer negundo) on affected farms. Metabolic profiling of horses with Seasonal Pasture Myopathy, including a case successfully treated at Iowa State University, showed that the disease is associated with a defect in lipid metabolism caused by inhibition of the same enzymes that are affected in human ackee fruit poisoning (Jamaican Vomiting Disease). The latter is associated with the ingestion of unripe ackee fruit that contains the toxin hypoglycin A. In 2012, this very toxin was identified in box elder seeds from affected farms.
Furthermore, the researchers were able to document the presence of metabolites of this toxin in blood and urine of affected horses, closing the link by proving ingestion of the toxin. Seasonal Pasture Myopathy usually occurs in the fall when there is an abundance of box elder seeds.
Further information on Seasonal Pasture Myopathy can be found at http://www.cvm.umn.edu/umec/SPM/home.html
Valberg SJ, Sponseller BT, Hegeman AD, Earing J, Bender JB, Martinson KL, Patterson SE, Sweetman L. Seasonal pasture myopathy/atypical myopathy in North America associated with ingestion of Hypoglycin A within seeds of the box elder tree. Equine Vet J 2012, DOI: 10.1111/j.2042-3306.2012.00684.x.
Sponseller BT, Valberg SJ, Schultz NE, Bedford H, Wong DM, Kersh K, Shelton GD. Equine multiple acyl-CoA dehydrogenase deficiency (MADD) associated with seasonal pasture myopathy in the Midwestern United States. J Vet Intern Med 2012;26:1012-1018.