Acer pseudoplatanus seeds can cause atypical myopathy, a disease that is commonly fatal. | Photo: iStock

This fall the British Equine Veterinary Association (BEVA) is warning of a perfect storm of conditions that might lead to an increased incidence of the muscle condition atypical myopathy (AM) in pastured horses in Great Britain. Atypical myopathy is a sudden onset disease that’s commonly fatal. Although veterinarians have recognized it for decades, researchers only identified the cause, hypoglycin A, in 2013.

The Risk

Hypoglycin A is most typically found in the sycamore tree (Acer pseudoplatanus), a type of maple. This is not the U.S. sycamore, Platanus occidentalis. However, a similar disease known as seasonal pasture myopathy (SPM) does occur in the United States due to consumption of Acer negundo, or box elder tree (also known as a boxelder maple or, in Canada, a Manitoba maple). This condition is most prevalent in the Midwestern United States and results from horses eating the tree’s seeds. Atypical myopathy also occurs due to seed consumption but might occur when dead wood or dead leaves are consumed, as well.

The level of the toxin varies from seed to seed, so the amount of seeds needed for the disease to occur can vary from fewer than 100 to several thousand. Symptoms can occur quickly and include weakened muscles, reluctance to walk, sudden stiffness and muscle tremors, sweating, depression, high heart rate, dark urine, collapse, and colic. The fatality rate is around 70%