Testing of tree samples now available for substance behind atypical myopathy

Seed pods from a sycamore tree on the ground in pasture. Ingestion of these seeds can prove toxic to horses.
Seed pods from a sycamore tree on the ground in pasture. Eating the seeds can prove toxic to horses.

Laboratory testing of tree seeds, seedlings and leaves is now available for the substance behind the dangerous horse disease atypical myopathy.

The testing is now available through a laboratory at Britain’s Royal Veterinary College.

The test is part of the push towards improved treatments and management of the disease, and to enhance the welfare of affected horses by helping horse owners to gain a better understanding of the condition.

Atypical myopathy is a severe and life threatening equine muscle disorder caused by eating sycamore tree seeds, leaves or seedlings that have typically dropped on pasture.

This plant material contains a substance called Hypoglycin A, which is converted to a toxin in the horse’s body. This toxin blocks crucial metabolic pathways, preventing energy use in the cells.

The muscles involved in breathing and posture are most commonly affected, but heart muscle can also
be involved.

Affected horses are usually at pasture or graze regularly, which makes the ingestion of seeds
and/or seedlings more likely.

Clinical signs are often insidous, with horses showing colic‐like signs and/or generalised weakness, but they can progress rapidly over 6‐12 hours to muscle tremors, low head carriage, reluctance to move or recumbancy. Horses can die with breathing problems or due to a severe heart problem.

All the risk factors for horses remain unclear. It is, for example, not currently known whether some trees in the wider Acer family are more toxic than others or whether the amount of toxin varies at certain times of the year or with certain climatic conditions.

The test, available through the college’s Comparative Neuromuscular Diseases Laboratory, follows research supported by The Horse Trust and the college’s Animal Care Trust.

Samples sent directly to the lab will be tested at a subsidised cost of £50. In addition, the lab is also offering testing of horse blood and urine samples, submitted by a vet, if they suspect atypical myopathy.

The college said the testing regime should help to establish a more rapid and accurate diagnosis and subsequent treatment than with previous tests.

There are more than 25 different tree species in the Acer family and not all of them have the toxin.

Differentiation between species is usually based on the appearance of seeds and leaves but it can be challenging.

Examples found in Britain include the field maple (Acer campestre) and the sycamore (A. pseudoplatanus). There is also the Norway maple (A. platanoides) A. palmatum and A. japonicum.

A. campestre and A. platanoides are harmless for horses as their seeds do not contain Hypoglycin A.

A. psedoplatanus (sycamore) has been shown to have variable amounts of hypoglycin A in its seeds, seedlings and leaves, although the factors that result in the variation are largely unknown.

The seeds of the Acer trees are sometimes called “helicopters” as they rotate rapidly as they fall, due to their special shape. This allows the seed to be dispersed by the wind.

Consequently, even if horse owners do not have sycamore trees on or at the boundries of their horse’s pasture, they should remain vigilant, particularly in autumn and after strong winds, as some seeds can reach pastures when trees are nearly 200 metres away.

Full details, including prices and packaging instructions are available here

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