Atypical myopathy in New Zealand

Atypical myopathy, or seasonal pasture myopathy (SPM), has been reported with increasing regularity from Europe and the USA, where is it associated with grazing on land contaminated with the seeds (“samaras”) of the sycamore and box elder trees.

However, the condition is not confined to the northern hemisphere, as shown by a recent report from New Zealand.

Dr Rabecca K McKenzie described four horses aged between 5 months and 9 years, that presented with various clinical signs including recumbency, stiffness, lethargy, dehydration, depression, and myoglobinuria suggestive of acute muscle damage. Two horses were subjected to euthanasia, but two recovered.  In all cases seeds of sycamore maple or box elder were present in the area where the horse had been grazing.

Affected horses showed laboratory signs characteristic of atypical myopathy, including raised muscle enzymes and changes in the acylcarnitine profile. Hypoglycin A, the toxin responsible for atypical myopathy, was found in the serum of the affected animals, but not in unaffected ones.

Hypoglycin A was identified in seeds of Acer spp. collected on premises where atypical myopathy cases had occurred. 

The toxin was also found in 10/15 samples of seeds from sycamore maple and box elder from other areas of the country where cases had been recorded previously.

The authors conclude that sycamore and box elder trees in New Zealand are a source of hypoglycin A, the toxin associated with atypical myopathy. They advise that if pastured horses show signs of severe muscle damage then the environment should be checked for the presence of these trees. Horses should be prevented from grazing seeds from Acer spp. in the autumn.

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