Not all maple species pose a risk to horses, researchers find

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The geographic location of the pastures where the Acer seeds, sprouts and leaves were sampled.
The geographic location of the pastures where the Acer seeds, sprouts and leaves were sampled. Images: Journal of Veterinary Internal Medicine

Not all species of maple trees are likely to be dangerous to horses, Dutch researchers report.

Scientists have in recent years shown that the serious muscular disease atypical myopathy can occur after eating the leaves, seeds and/or buds of maple trees that contain the toxic substance hypoglycin A.

Each year, hundreds of horses in Europe die due to atypical myopathy, also known as pasture myopathy.

In the past, horses with the condition were almost certain to die, but today the disease can be diagnosed and treated at a much earlier stage.

However, the mortality rate is still 70%, so prevention is important.

For horse owners with maple trees around their pastures, it is vital to know which maple species are present.

Researchers from Utrecht University’s veterinary faculty and RIKILT Wageningen UR – an organisation that specialises in detecting and identifying substances in food and animal feed and their effects – have both studied equine mortality from atypical myopathy.

As part of their work, the scientists launched a study to find out which species contained hypoglycin A.

Dr Cornélie Westermann and her colleagues called on horse owners across the Netherlands to send samples from maple trees in their vicinity.

They received 278 samples of the three most common types of maple trees in the Netherlands – the sycamore maple (Acer pseudoplatanus), the field maple or hedge maple (Acer campestre) and the Norway maple (Acer platanoides).

They then measured the concentration of hypoglycin A in all of the seeds, leaves and buds.

The study team, reporting their findings in the Journal of Veterinary Internal Medicine, found no levels of the toxin in field maple or Norway maples, but every sample of the sycamore maple contained hypoglycin A.

It seems therefore that field maples and Norway maples near pastures and paddocks did not pose a danger to horses.

However, the presence of hypoglycin A in the leaves, seeds and buds of the sycamore maple did not necessarily mean that the tree was by definition unsafe.

A, B, C: Leaves of Acer pseudoplatanus, Acer campestre and Acer platanoides.
A, B, C: Leaves of Acer pseudoplatanus, Acer campestre and Acer platanoides.

There were thousands of pastures with sycamore maples across Europe in which horses did not become ill. The concentration of hypoglycin A also does not appear to correlate to whether horses nearby will become ill, so it seems other factors play a role in the development of the disease.

The scientists said their results were consistent with a study that found hypoglycin A concentrations in seeds to be highly variable among trees on the same or different farms.

They recommend that horses with full access to a pasture surrounded by sycamore maple trees be given plentiful amounts of good quality roughage in their feed during autumn (placed on a dry spot on the ground). Horses should be provided with places to take shelter.

Owners were also advised to cut off the edges of the pasture with electric fencing to increase the distance to the trees and block access to the leaves and seeds. If necessary, owners can use a leaf blower to remove leaves and seeds from the pasture.

The researchers also recommend keeping the horses inside during storms until the branches, leaves and seeds blown from the trees could be removed. To ensure horse welfare, it was better to invest time and effort in keeping pastures leaf-free than to keep the horses in their stalls for long periods, they said.

However, it should be noted that the genus Acer has over 100 species and some species have over 1000 cultivars, but these are rarely grown in or around pastures in the Netherlands. The study therefore targeted only the three most commonly found in the Netherlands.

To date, Acer negundo (the box elder tree) and Acer pseudoplatanus (the sycamore maple) have been associated with atypical myopathy.

Discussing their findings, the researchers said: “The question arises as to why not all horses grazing pastures containing or surrounded by Acer pseudoplatanus develop atypical myopathy.

“The most likely explanation might relate to differences in the amount of seeds, sprouts, and leaves ingested, other components of the diet, and the sensitivity of the individual horse.

Acer sprouts.
Acer sprouts.

“The availability of seeds, leaves, and sprouts largely is determined by the season and weather, with changes in wind and temperature loosening the attachment of leaves and seeds.

“Indeed, inclement weather or high winds have been reported to precede clinical signs of atypical myopathy. Strong winds can disperse leaves and seeds over a long distance, perhaps depositing them in pastures that do not contain Acer trees.

Seeds not consumed by horses in autumn could sprout in the spring, they said. “All of these factors could explain why Acer trees are not always found in the paddock or pastures grazed by horses with atypical myopathy.”

The researchers said their results suggested that, per kilogram, sprouts were most dangerous to horses, followed by seeds, with leaves being potentially least harmful.

Data show that there are more cases of atypical myopathy in autumn than in spring.

“Because sprouts are more common in spring and seeds are more common in autumn, it would appear that horses eat more seeds than sprouts either because more seeds are available or because they prefer seeds to sprouts.

“However, whether horses eat seeds or sprouts is to a large extent determined by the availability of other feedstuff.

“In general, spring pasture contains more and better grass than autumn pasture, and thus horses may have less reason to eat other feedstuff.”

They said the finding that the concentration of hypoglycin A was high in sprouts suggested that horse owners should be alert to the presence of Acer sprouts in pastures.

“Owners should be advised to mow areas of pasture with maple sprouts and to remove the mown material.”

In conclusion, they said the most important recommendation is that owners should prevent horses from eating seeds, sprouts, leaves, or any combination of these from the sycamore maple, whereas the seeds of the hedge maple and Norway maple appeared to be safe for horses.

“Steps to prevent the ingestion of toxin include moving the horse to a safer pasture, decreasing the size of the pasture (away from the trees), blowing away seeds and leaves, or mowing and removing sprouts.

“Adequate roughage supplementation in sparse pastures during the high-risk season also may prevent atypical myopathy,” they wrote.

Hypoglycin A Concentrations in Maple Tree Species in theNetherlands and the Occurrence of Atypical Myopathy in Horses
C.M. Westermann, R. van Leeuwen, L.W.D. van Raamsdonk, and H.G.J. Mol
DOI: 10.1111/jvim.13927

The full study, published under a Creative Commons License, can be read here

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