ATYPICAL myopathy is a serious and often fatal disease affecting the muscles of horses.

It occurs in the United Kingdom and northern Europe, although a similar disease has been recognized in parts of the USA and Canada.

The first cases were recognised in Scotland in 1939, with the first major outbreak seen in Germany in 1995. The numbers of horses affected in Europe have increased significantly since 2000, which has sparked more research into the condition.

The disease leads to the rapid destruction of the muscles used to breathe, to stand and even the muscle of the heart. This breakdown of muscle leads to weakness and trembling, with many affected horses becoming recumbent.

The products of the muscle breakdown are passed out through the bloodstream and kidneys into the urine, which will be dark brown in colour. Diagnosis is made on the basis of very high levels of muscle enzymes in the blood or by the presence of myoglobin (from muscle breakdown) in the urine.

There are no specific treatments available for AM. Instead, treatment is aimed at supporting the horse by administering intravenous fluids, painkillers and lots of nursing.

Recumbent horses require round the clock attention including being turned regularly. Sadly, even with the most intensive care we can give, only around a quarter of affected horses will survive.

The disease can occur in outbreaks and is usually seen in groups of horses grazing the same pasture, often in the autumn and less commonly in the spring.

As affected horses invariably had been grazing pasture, it was always believed that the disease was a result of an environmental toxin. For many years it was unknown what caused AM, but recent research has shown the cause to be a toxin (hypoglycin A) in the seeds of the sycamore tree, found in ‘helicopters’.

In the US, the same toxin was found in the seeds of the box elder tree which is from a similar family as the sycamore. Effectively, AM is caused by sycamore seed poisoning.

Of course, sycamore trees are common in the UK, so why is this disease still quite uncommon?

The explanation lies in the fact that the level of toxin present in the seeds is variable from tree to tree and indeed highly variable even between seeds from the same tree.

This also means that the amount of seeds that a horse would have to consume to cause toxicity and lead to the development of AM will differ. For this reason, ways for us to reduce the number of horses getting AM are focused on reducing the amount of sycamore seeds that are ingested.

This can be done by reducing time spent grazing to six hours or less a day, by providing supplementary hay or haylage in the field so that horses are less inclined to ingest seeds when grazing poorer pasture and by fencing off areas of fields where many ‘helicopters’ have fallen.

Atypical myopathy is a particularly unpleasant disease of grazing horses which we still have much to learn about.

The British Equine Veterinary Association reported five times more cases in the autumn of 2014 than the previous year; now that the cause of the disease has been confirmed, its important to take steps to reduce the risk to your horses.