Researchers who recently discovered that a toxin from the box elder tree is the likely cause of Seasonal Pasture Myopathy in North American horses suspect that hypoglycin A is the amino acid responsible.
Preliminary comparisons indicate that the European equivalent of the disease, known as Atypical Myopathy, may be linked to similar trees.
The American research which implicated box elder trees was published recently in the Equine Veterinary Journal.
Seasonal Pasture Myopathy is a highly fatal muscle disease in the Midwestern United States and Eastern Canada.
The European equivalent is becoming increasingly frequent in Britain and Northern Europe.
Outbreaks of both diseases tend to be seasonal, with most cases occurring in the autumn.
Horses that develop them 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 research 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 Seasonal Pasture Myopathy, 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 Stephanie Valberg, of the 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 Seasonal Pasture Myopathy 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 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, 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 the 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.”
“Seasonal Pasture Myopathy/Atypical Myopathy in North America associated with Ingestion of Hypoglycin A within Seeds of the Box Elder Tree.” Stephanie J Valberg, Beatrice T Sponseller, Adrian D Hegeman, Jennifer Earing, Jeffery B. Bender, Krishona L Martinson, Steven E Patterson, Lawrence Sweetman.
The study is published by Wiley-Blackwell and the abstract can be accessed here.