Atypical Myopathy is a catastrophic type of muscle disease that is often fatal. Affected horses have signs of disintegration of the muscle fibres of the limbs, respiratory system and heart. They appear to tie up, are reluctant to move and may remain lying down. They have muscle weakness and are sometimes affected by choke.
The disease affects horses in pasture and occurs in autumn and the spring following large autumn outbreaks. Not all horses in a single group are affected. The disease has been reported in the UK and throughout Northern Europe, and is similar to a pasture associated muscle disease seen in the US, called Seasonal Pasture Myopathy. Affected horses are usually kept in pastures with trees, accumulations of dead leaves, dead wood, and not fed any supplementary forage e.g. hay or haylage. US researchers believe that a toxin called hypoglycin A, from the seeds of the box elder tree are involved.
A research team led by Dominic Votion, based at the University of Liege , studied 17 horses affected with atypical myopathy, and reported their findings in the Equine Veterinary Journal. High concentrations of the toxic metabolite hypoglycin A was found in the blood of all the horses, and further investigation by qualified botanists revealed that most of the horses were in pastures containing sycamore trees (Acer pseudoplatanus). Sycamore tree seeds contain a significant, although variable amount of hypoglycin A.
Discussions about how ingestion of sycamore or box elder seeds can cause atypical myopathy are ongoing, and researchers propose that the availability of the seeds in pasture, along with a lack of other feed, and potential high levels of the toxin due to seasonal conditions, are all involved.
Practically, owners are advised to take measures to avoid their horses eating large amounts of sycamore seeds, including clearing them from pasture, fencing off areas near sycamore trees, and offering supplementary forage especially where grass is well grazed down.
This research finding is a reminder not to trust our horses to avoid eating material that is potentially poisonous. If grass is sparse and no forage is available, horses will turn to eating other high fibre feeds, which may or may not be poisonous. Owners of overweight horses on restricted pasture should be particularly careful to offer appropriate forage because hungry horses are more likely to eat inappropriate feeds and forages.