Spring seedlings pose Atypical Myopathy risk to horses

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A classic atypical myopathy pasture in autumn with many trees and very sparse grassland. Photo: PLOS ONE

Cases of Atypical Myopathy occurring in spring in several European countries led researchers to investigate sources of poisoning with hypoglycin A (HGA) following the ingestion of the winged fruit (“samaras” or “helicopters”) of sycamore and similar trees.

Equine atypical myopathy (“sycamore poisoning”) in Europe results from poisoning with HGA. Cases were originally identified in the autumn months, but horses are also at risk if grazing contaminated pastures in the spring. It is now recognised that seedlings can be a source of the toxin.

An interim report from the International Collating Centre shows that the University of Liège recorded 73 cases of Atypical Myopathy in Belgium, France, Czech Republic, Netherlands and Britain during the month of April 2019.

A pilot study by Dominique Votion and colleagues sought to understand better the sources of HGA intoxication in the spring.

A samara or helicopter from a sycamore tree.
A samara or helicopter from a sycamore tree.

The research team collected fallen samaras and seedlings at two-week intervals during the spring of 2016 from sycamore, Norway maple and field maple trees. After a particularly wet night they collected rainwater from the seedlings. Then in mid-May they collected samaras directly from box elder, and common ash trees, and they collected clusters of flowers (inflorescences) from sycamore maples.

They found hypoglycin A in all the sycamore samples (including the rainwater), but none in samples from the Norway maple or the field maples.

From the maximum HGA concentrations present in the samples, the researchers calculated that under some conditions, about 20g of samaras, 50 seedlings, 150g of inflorescences or 2 litres of water that had been in contact with seedlings would contain the maximum tolerated daily dose for a horse.

Mowing the seedlings is not necessarily the answer. Research carried out in Britain by Sonia González-Medina and colleagues found that the toxin remains in sycamore seedlings despite mowing, herbicidal spraying or storage in hay and silage.

They recommend that pastures contaminated with sycamore material should not be used to produce hay or silage as both seedlings and seeds present in the bales still pose a risk of intoxication. They advise that mowing followed by collection of sycamore seedlings seems the current best option to avoid HGA toxicity in horses grazing contaminated pasture.

Potential new sources of hypoglycin A poisoning for equids kept at pasture in spring: a field pilot study. Votion DM, Habyarimana JA, Scippo ML, Richard EA, Marcillaud-Pitel C, Erpicum M, Gustin P. Vet Rec. (2019) https://dx.doi.org/10.1136/vr.104424

Atypical myopathy-associated hypoglycin A toxin remains in sycamore seedlings despite mowing, herbicidal spraying or storage in hay and silage. González-Medina S, Montesso F, Chang YM, Hyde C, Piercy RJ. Equine Vet J. (2019) https://dx.doi.org/10.1111/evj.13070

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