New research from the USA, published in the Equine Veterinary Journal (EVJ), has shown that a toxin from the box elder tree is the likely cause of Seasonal Pasture Myopathy (SPM), the US equivalent of the European disease known as Atypical Myopathy (AM).

According to the British Equine Veterinary Association, preliminary comparisons of these results with cases of AM in Europe suggest that the European condition may be linked to similar trees which could have an important bearing on the future prevention of the disease in Europe.

Seasonal Pasture Myopathy is a highly fatal muscle disease in Midwestern USA and Eastern Canada. A similar disorder called Atypical Myopathy is becoming increasingly frequent in the UK 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 are often not fed any supplementary hay or feed.

The research¹ identified that seeds from box elder trees were consistently present in the autumn pastures of all 12 horses enrolled in the study. These horses were from 11 different farms and had all presented with the clinical signs of SPM, which include muscular weakness and stiffness, dark urine, periods of recumbency, colic-like signs and muscle trembling. The toxic amino acid hypoglycin A, which is known to cause 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.

Professor Valberg DVM PhD of the Department of Veterinary Population Medicine, College of Veterinary Medicine, University of Minnesota, who instigated the study, 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."

Professor Valberg has established collaborative links with groups in Europe in order to investigate if a similar toxin is involved in European Atypical Myopathy. Working with Professor Vince Gerber and Dr Lucia Unger at the University of Bern, Professor Valberg has obtained seeds from a tree related to the Box Elder found on many pastures where Atypical Myopathy has occurred.

Professor Celia Marr, Editor of Equine Veterinary Journal said: "This is a really important step forwards. We don't yet know for sure that the cause of European disease, Atypical Myopathy, is the same as Seasonal Pasture Myopathy in the US. 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."

¹ Seasonal Pasture Myopathy/Atypical Myopathy in North America associated with Ingestion of Hypoglycin A within Seeds of the Box Elder Tree Stephanie J Valberg DVM PhD¹, Beatrice T Sponseller Dr. med. vet², Adrian D Hegeman PhD³, Jennifer Earing PhD, Jeffery B. Bender DVM MS¹, Krishona L Martinson PhD⁴, Steven E Patterson PhD⁵, Lawrence Sweetman PhD¹

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