Pasture myopathy, also known as 'atypical myopathy' represents a severe and usually fatal form of non-exertional rhabdomyolysis in pastured horses. The disorder is a well-known cause of severe disease in horses in Europe, where it often occurs in outbreak fashion, and extensive attempts have been made in the last two decades to determine the cause. Pasture myopathy occurs sporadically in the USA and Canada, and was initially suspected to arise from ingestion of the white snakeroot plant {Ageratina altissima). Initial investigations correctly determined that pasture myopathy represents an acquired deficiency of skeletal muscle multiple acyl-CoA dehydrogenase, resulting in metabolic failure and necrosis particularly affecting type 1 (highly oxidative)muscle fibers. Hypothesized causes included Clostridium or fungal toxin ingestion, and epidemiological studies connected outbreaks with inclement weather and reduced feed availability. Finally, in 2012, it was definitively determined that pasture myopathy in the US results from ingestion of seeds from the Box Elder tree {Acer negundo). The toxic principle within the seeds is an amino acid, hypoglycin A. After ingestion, hypoglycin A is metabolized to methylenecyclopropylacetic acid-CoA (MCPA-CoA), which serves as a potent inhibitor of multiple acyl-CoA dehydrogenase, disrupting key metabolic pathways. This revelation facilitated confirmation that the same toxic principle causes disease in horses in Europe, but is associated there with ingestion of seeds of the Sycamore Maple tree (Acerpseudoplatanus).
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