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In the Northern Hemisphere, autumn is an idyllic time for grazing—the temperature is reasonable, insect activity decreases, and pastures continue to flourish. But every rose has its thorn, and in the case of fall grazing, that thorn is atypical myopathy (AM), also referred to as seasonal pasture myopathy (SPM), a condition caused by the ingestion of the toxin hypoglycin A.

“Since the 1930s, aggressive research efforts have finally identified hypoglycin A as the cause of AM,” shared Kathleen Crandell, Ph.D., Kentucky Equine Research (KER) nutritionist.

Hypoglycin A is found in the fruit or seeds of trees belonging to the family Sapindaceae as well as other tree families, including the box elder and sycamore maple. Once ingested, the toxic amino acid is metabolized by muscle cells to a compound called methylenecyclopropyl acetic acid, or MCPA. This compound inhibits certain enzymes that produce energy in muscle, especially fibers in postural, heart, and respiratory muscle.

“This explains the severe weakness and respiratory distress observed in affected horses,” Crandell said.

The outcome of horses with AM is usually devastating, with three-quarters of cases dying within 72 hours. There is currently no treatment, leaving environmental surveillance a key way to keep horses safe on fall pastures.

According to experts in North America and Europe:

  • Most cases occur in the autumn;
  • Sporadic cases occur in North America, especially the Midwest, whereas outbreaks are usually noted throughout Europe;
  • Dry conditions, compact soil, and windy conditions can impact the concentration of hypoglycin A in the seeds and seed dispersion; and
  • Low dietary selenium may contribute to the development of AM.

“Horse owners are encouraged to keep horses away from trees thought to produce fruit and seeds with hypoglycin A during the autumn and offer supplemental feed and forage when horses are housed on sparse pastures to minimize the chance of seed ingestion,” advised Crandell.

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