Equine Atypical Myopathy

• Check your fields and neighbouring fields, woodland and hedgerows for sycamore and box elder trees – seeds may spread up to several hundred metres in windy conditions or in flood water, so you need to be aware of what is growing in proximity to your fields and not just on the immediate boundaries.
• Remove sycamore and box elder trees, seeds, leaves and seedlings where possible.
• If you cannot remove the sycamore or box elder trees or seedlings, ensure any affected areas are well fenced-off.
• Using plant ID mobile apps or search engine AI technology can be helpful to identify tree species near to the horse’s grazing. These can also be used to distinguish other types of plants that may be poisonous to horses. A useful downloadable guide on identifying sycamore leaves, flowers and seeds can be found here (Renaud et al., 2019).
• Supplementary feeding in the field (such as hay, haylage or a bucket of a suitable forage replacer feed) can help to minimise the risk of horses being tempted to ingest seeds. Any supplementary feed or forage should be placed well away from any sycamore or box elder trees to reduce the risk of falling leaves or seeds causing contamination.
• Stocking density should be kept low to ensure there is plenty of grazing for all horses in the field.
• Avoid spreading manure or harrowing grazing pastures near to sycamore or box elder trees as this has also been found to increase the risk of atypical myopathy (Votion et al., 2009) – droppings should instead be manually removed.
• A common misconception is that mowing pastures or the use of herbicides on pastures with sycamore seedlings present is sufficient to reduce the content of hypoglycin A they contain. However, research has found that this is not the case, with mowing potentially even increasing the levels (Gonzalez-Medina et al., 2019) – these methods should therefore not be considered as useful preventative measures for atypical myopathy.
• Contaminated pasture should not be used for producing hay or haylage, as sycamore seeds or seedlings that have inadvertently ended up dried in bales of hay have also been found to contain detectable levels of hypoglycin A and can remain toxic even after extended periods of storage (Gonzalez-Medina et al., 2019).
• When a case is suspected, the horse’s field mates should be removed from the pasture immediately. A recent study that compared the blood test results from horses with atypical myopathy and their apparently healthy field mates found that 97% of the field mates also had hypoglycin A in their blood (Renaud et al., 2024). This study also found that there had already been changes in the breakdown of fatty acids in these horses, even though they weren’t showing any outward symptoms of atypical myopathy. This demonstrates why the management of other horses on the same grazing as a suspected case is so important in trying to reduce the impact of the disease.
• It is now also possible to send off samples of seeds, leaves and seedlings for testing to find out if they contain hypoglycin A. The Royal Veterinary College offers this service to horse owners to help assess the risk of plant material located near to where their horses are grazing

Remember that all pastures with sycamore or box elder trees in the vicinity may present a risk, even if no cases of atypical myopathy have ever occurred from those fields in the past. It has been suggested that around 80% of cases of atypical myopathy were horses grazing in pastures where there had been no previous problems (Votion et al., 2020). This could be because climatic or soil conditions may affect hypoglycin A contents at different times, and levels can also vary considerably between trees. Additionally, some horses may be more careful about what they are consuming than others, so there is no guarantee that any new horses to the field will avoid the leaves, seeds, or seedlings of affected trees. Individual horses may also have different sensitivities to the toxin, as subclinical atypical myopathy has been identified whereby fatty acid breakdown is affected but no symptoms are shown (Renaud et al., 2024). Environmental conditions that lead to poorer quality grazing may also increase the likelihood of seed, seedling, or leaf consumption. More recent research has also indicated that hypoglycin A content in sycamore seeds within the same bunch from the same tree can vary by up to around 60% between seeds, which highlights the importance of taking a representative sample for testing (González-Medina et al., 2025).