Atypical myopathy is a very acute, often fatal muscle disease in members of the horse family. Over 75% of affected horses die of this disease. The disease is allegedly caused by a toxin, called Hypoglycin A, present in maple leaves, seeds or seedlings. For this reason atypical myopathy is also called ‘maple leaf poisoning’. Maple leaf poisoning predominantly occurs in autumn and spring, respectively because of falling leaves and shooting up of seedlings. Because of the severity of the disease it is crucial to prevent horses from ingesting leaves, seeds or seedlings.
Maple leaf poisoning, aka atypical myopathy, is caused by Hypoglycin A, a substance present in the seeds, leaves and seedlings of some maple-tree species. Not every species of maple-tree is toxic. Up till now most cases of maple leaf poisoning are linked to the common maple-tree or box elder tree, but much remains unclear about this condition. Hypoglycin A causes damage to the muscles in horses. Because of the muscle atrophy several substances are released in the body which are secreted via the kidneys and urine. This is extremely taxing for the kidneys and can certainly cause damage.
Maple leaf poisoning is most acute in autumn and spring. This is easy to explain when considering that leaves and seeds fall in autumn and seedlings shoot up in spring. During these periods there is also little grass in the fields so horses are much more likely to eat these leaves and seeds than when the grass is plentiful. It is known that horses younger than three and older horses are more prone to maple leaf poisoning. Also at increased risk are skinny horses. Other risk factors are fields with little grass or when horses are ground-fed in the fields with supplementary fodder. Wet fields with lots of dead leaves pose another risk of atypical myopathy.
Once the toxin has been ingested progress of the disease is fast. Occasionally horses initially display signs of colic, stiffness or are somewhat groggy, but often horses are either discovered in the field lying on their sides or they have already died by the time they are found. Once they lie down it’s difficult to get them back on their feet. Sometimes the horses are still standing, but they are very groggy, stiff, suffering from twitching muscles and heavy sweating. Very typical in horses with atypical myopathy is the dark, brown colour of the urine. Because the muscles are affected the horse’s breathing will also begin to suffer.
When there is a suspicion of maple leaf poisoning the best approach is to call the veterinarian immediately. Meanwhile the horse should be taken to the nearest stable but moving of the horse should be kept to a minimum. If possible the horse can be given small amounts of feed.
If other horses are turned out in the same field the wise thing to do is get them out of the field as soon as possible and to keep them under observation for the next few days to check for signs of atypical myopathy.
Generally the symptoms, i.e. dark urine and muscle weakness in autumn/spring, are sufficient indication for a diagnosis, but if there is any doubt a blood test can be carried out. The test will show up increased muscle- and kidney values in the blood.
Unfortunately there is no antidote for Hypoglycin A. Affected horses must be treated with painkillers, put on a drip and possibly given a drip-feed.
Horses that lie down are usually difficult to get back up on their feet and prognosis for these cases is sad. Horses that are still standing have a better chance of survival but about three-quarters of affected horses will not survive the disease. However, if they do recover they will come out of it with few after-effects. Occasionally cardiac rhythm disorders (arrhythmia) can be diagnosed as a result of atypical myopathy.
The best guideline to prevent atypical myopathy is to turn horses out in fields that have no maple trees in the close proximity. If maple trees do grow in or around the field the advice is, if in any way possible, to stable the horses up during periods of increased risk, which is from the time the leaves start falling until after the first sharp frost, and during spring. If it is not an option to remove the horses from the field at the time of increased risk, then the horses need supplementary feeds given in for example a hay rack, in order to discourage feeding from the ground and ingesting too many seeds/leaves. Because the disease progresses so fast and the mortality rate is so high, it is crucial to minimise the risks.
Source: paardenpunt Vlaanderen (equine information centre, ed.).