By Vicky Rowlands Cert. EP MRCVS – Ashbrook Equine Hospital, Cheshire
It appears that atypical myopathy is caused by a toxin called hypoglycin A that is contained in sycamore seeds (‘helicopter seeds’). Although horses do not usually eat these seeds by choice, if there is scarce grazing or a lot of seeds on the pasture they may be eaten accidentally. There is usually a peak of cases in the autumn when the seeds fall. There may also be a peak in the spring if there has been an especially high number of cases during the previous autumn.
The hypoglycin A toxin appears to damage the postural, breathing and heart muscles. Cases of atypical myopathy may show mild colic symptoms, laboured breathing, weakness or stiffness initially. In some cases, these symptoms may not be seen and the horse may be found recumbent or dead for no apparent reason. If the horse is seen to urinate, the urine will usually be dark red or brown in colour. If a horse is exhibiting mild signs of colic or prolonged recumbency, it is worth examining them more carefully and requesting a veterinary visit if there is any doubt about the horse’s health. AM may affect individual horses or groups of horses.
After examining the horse, we will require a blood sample to definitively diagnose the disease. This will show a dramatic increase in muscle enzyme levels due to damaged muscle tissue. These blood results will be very similar to a horse that has ‘tied up’ during exercise, although there is obviously a very different cause and history prior to the horse showing symptoms. The blood sample may also show some level of dehydration and abnormal electrolyte levels.
Horses with severe dehydration and those that are found recumbent have a poorer chance of survival and may require euthanasia on welfare grounds. Presently, only about a quarter of horses will survive, so early and aggressive treatment is very important. These horses are treated with intravenous fluid therapy and pain relief as required. Fluids will improve the horse’s hydration, help the kidneys to eliminate the by-products of damaged muscle tissue and correct any electrolyte imbalances. Muscle relaxant drugs or vitamin supplements may be given if appropriate for that individual horse.
During the period of treatment, horses will often maintain a good or moderate appetite and a carbohydrate-rich diet is required. Some horses will not be able or willing to eat as they are unable to swallow and digest; these horses are less likely to survive. Repeated blood samples are useful because horses that show a reduction in muscle enzyme levels have a more hopeful outcome.
Although we do not seem to be able to prevent this disease, the risk may be reduced by fencing off areas of pasture that are highly contaminated by sycamore seeds. It is important to remember that seeds can be carried to other pastures by winds or flood water so vigilance after these weather conditions is vital. Ensuring the horses have access to other sources of food, such as hay, or are stabled for some proportion of the day may also be beneficial. If you need any further advice, please do not hesitate to contact your vet.