Determining the cause of Atypical Myopathy
A toxin found in box elder seeds has been linked to atypical myopathy (AM), after a connection was was made between the seeds and the US equivalent of the disease – seasonal pasture myopathy (SPM).
US scientists conducted research into the cause of SPM, which has been published in the Equine Veterinary Journal (EVJ).
They found that the toxic amino acid, hypoglycin A, was present in the serum or urine of horses with clinical signs of SPM. Hypoglycin A was also found in the seeds of box elder trees, therefore, the scientists believe it is this that creates the disease's pathology.
Box elder seeds are abundant in autumn pastures, which is when the highly fatal muscle conditions SPM and AM most commonly occur.
Symptoms include muscular weakness and stiffness, dark urine, periods of recumbency, colic-like signs and muscle-trembling. The conditions tend to develop in horses that are kept in sparse pastures, with dead leaves, dead wood and trees.
The scientists enrolled 12 horses from 11 different farms for their study, all with typical clinical signs of SPM. Hypoglacin A was found in either the serum or urine of all 12 horses, and all but one case proved to be fatal.
Hypoglycin A is also known to cause acquired multiple acyl CoA dehydrogenase deficiency (MADD).
Professor Valberg, from the College of Veterinary Medicine at the University of Minnesota, who led 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 hypoclycin A concentrations in box elder seeds."
EVJ Editor Celia Marr said: “This is a really important step forwards. We don’t yet know for sure that the cause of European disease, AM, is the same as SPM 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.”
The BSAVA has opened submissions for the BSAVA Clinical Research Abstracts 2026.
