Link Identified between Tree Seed and Seasonal Pasture Myopathy

New research from the University of Minnesota (UM) that was recently published in the Equine Veterinary Journal (EVJ) shows that a toxin from the box elder tree is the likely cause of seasonal pasture myopathy (SPM), the U.S. equivalent of the European disease known as atypical myopathy (AM). Preliminary comparisons of these results with cases of AM in Europe suggest that the European condition could be linked to similar trees, which could have an important bearing on the future prevention of the disease in Europe.

SPM is a highly fatal muscle disease typically identified in the midwestern United States and eastern Canada. A similar disorder, AM is becoming increasingly frequent in the U.K. and Northern Europe. Outbreaks of both diseases tend to be seasonal, with most cases occurring in the autumn. Horses that develop SPM and AM are usually kept in sparse pastures with an accumulation of dead leaves, dead wood, and trees in or around the pasture, and horses are often not fed any supplementary hay or feed.

The research identified that seeds from box elder trees were consistently present in the autumn pastures of all 12 horses enrolled in the UM study. The horses were from 11 different farms and had all presented with the clinical signs of SPM, including muscular weakness and stiffness, dark urine, periods of recumbency (inability to stand), colic-like signs, and muscle trembling. The toxic amino acid hypoglycin A, which is known to cause the human disorder acquired multiple acyl CoA dehydrogenase deficiency (MADD) was shown to be present in the box elder seeds and hypoglycin metabolites were identified in the serum or urine of all the horses. All but one of the cases proved fatal.

Study leader Stephanie Valberg, DVM, PhD, said, "The fact that another acquired form of MADD in rats and humans is caused by hypoglycin A ingestion strengthens our conclusion that hypoglycin A is the likely toxic cause of SPM in horses. We are intending to conduct further studies to determine the factors that affect the highly variable hypoglycin A concentrations in box elder seeds."

Valberg has established collaborative links with groups in Europe in order to investigate if a similar toxin is involved in European AM. Working with Vince Gerber, PhD, DVM, Dipl. ACVIM, ECEIM, FVH, and Lucia Unger, DVM, both at the University of Bern, Valberg has obtained seeds from a tree related to the Box Elder found on many pastures where AM has occurred.

Celia Marr, BVMS, MVM, PhD, Dipl. EIM, ECEIM, MRCVS, editor of EVJ, said, "This is a really important step forwards. We don't yet know for sure that the cause of (atypical myopathy) is the same as seasonal pasture myopathy in the United States. But the clinical signs and MADD aberrations are identical and both conditions have a high fatality rate. It remains to be seen whether this research will help European horses, but there is no doubt that it will allow American horse owners to prevent this devastating condition affecting their horses with immediate effect."

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.