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Veterinary
Toxicology

Atypical myopathy

02 May 2022
Clinical
14 mins read
02 July 2022
Volume 6 · Issue 3
Figure 1. The paired winged fruits (samaras) of sycamore (Acer pseudoplatanus) containing two seeds. Only the seeds contain hypoglycin A.
Figure 1. The paired winged fruits (samaras) of sycamore (Acer pseudoplatanus) containing two seeds. Only the seeds contain hypoglycin A.

Abstract

Atypical myopathy is a seasonal condition of non-exertional rhabdomyolysis seen in pastured horses. It occurs in the autumn and spring from ingestion of seeds and seedlings of sycamore (Acer pseudoplatanus) in Europe and box elder (Acer negundo) in North America. The toxins hypoglycin A and its homologue methylenecyclopropylglycine are metabolised to compounds that impair lipid metabolism, which primarily affect cardiac, respiratory and postural muscles. Initial signs are reluctance to move with muscle weakness and stiffness. Common signs are pigmenturia, hypothermia and pain of varying severity. Mortality is high and many animals succumb within 2–3 days. Treatment is intensive supportive care, including nutritional support to provide alternative energy substrates and vitamin and mineral supplementation. There is no specific antidote for atypical myopathy, so prevention is key.

Atypical myopathy (also known as seasonal pasture myopathy, equine atypical myopathy and atypical myoglobinuria) is a rhabdomyolysis of horses that is not induced by exercise, and which has a high mortality rate. It was periodically reported in the veterinary literature at least as far back as the 1930s in Wales (Bowen and Craig, 1942) and has since been reported in most of Europe (Votion et al, 2020), North America (Pope and Heslop, 1960; Valberg et al, 2013), Australia (Irwin and Pulsford, 1951) and New Zealand (McKenzie et al, 2016).

Atypical myopathy is seasonal and occurs in horses at pasture that eat the seeds and seedlings of sycamore or box elder. Various factors may be involved but most cases occur in the autumn from ingesting the winged fruits (samaras) containing the seeds (Unger et al, 2014) (Figure 1). Stormy weather may lead to heavy contamination of pasture. Sterile fruits of box elder do not contain seeds and are free of hypoglycin A (Votion et al, 2019).

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poisoning
toxicology
sycamore
atypical myopathy
hypoglycin A