Atypical Myopathy (AM) is a highly fatal muscle disease in horses in the UK and Northern Europe. It results in destruction of respiratory, cardiac and postural muscles and affected horses show signs of weakness, muscle trembling, lethargy and pain and may become recumbent. Even with intensive veterinary treatment, severely affected horses may die. Several horses from one location can be affectedand outbreaks occur most commonly in the autumn.
For several years, vets noticed that horses with AM were(i) often kept in sparse pastures with an accumulation of dead leaves and seeds and surrounded by trees and (ii) often not fed any supplementary hay or feed. This pattern ofpasture characteristics, and the fact that groups of grazing horses can be affected,led vets and researchers to suspect that an ingested toxin was cause of the disease.
A US study earlier this year linked a toxin called hypoglycin A inseeds of the box elder tree (Acer negundo) with Seasonal Pasture Myopathy (SPM), the US equivalent of AM. A new European study published last week has concluded that toxins from the seeds of the tree Acer pseudoplatanus(commonly known as the ‘sycamore’)are the likely cause of AM in Europe. The seeds are commonly referred to as ‘helicopters’ (Figure 1).
This new European research from the University of Liege showed high concentrations of the toxin (hypoglycin A)in the serum of horses suffering from AM in Belgium and other European countries. The pastures of 12 of the horses were visited by experienced botanists and the Acer pseudoplatanus(the sycamore tree) was found to be present in every outbreak.
The concentration of hypoglycinA in sycamore and box elder seeds varies between seeds in the same pasture and even from the same tree. It is likely that the most important factors contributing to horses being poisoned by hypoglycin A are the availability of the seed in the field combined with a lack of adequate grazing or supplementary forage.
This latest research helps us provide more specific advice to horse owners to reduce the risk of their horses contracting AM. In practical terms, horses’ ingestion of sycamore seeds in the autumn could be reduced by:
– Hoovering-up/picking up sycamore seeds
– Fencing off areas where sycamore seeds have fallen from trees
– Regularly inspecting fields to ensure seeds have not blown in from sycamore trees nearby
– Supplying extra forage (hay or haylage) especially where pasture is poor
– Reducing stocking density so there is plenty of palatable grazing for every horse
– Turning out horses for only short periods rather than extended periods of the day (ideally <6 hrs)
Figure 1: Seeds of the Sycamore tree known as ‘helicopters’ contain the toxin responsible for causing atypical myopathy: