Latest figures from the Irish Equine Centre show that up to 40 horses have now died nationwide as a result of equine atypical myopathy, or sycamore seed poisoning – a doubling of the figures in a week.

Atypical myopathy (ATM) is an often fatal muscular condition that affects grazing horses of all ages but case studies indicate that young horses are particularly at risk.

Dr Ursula Fogarty of the Irish Equine Centre said on Monday: “Cases are still emerging and the updated figures now stand at 35-40 nationwide fatalities. We expect this figure to increase as the week goes on.”

Veterinary Ireland issued a warning to horse owners over the problem on Monday while the Department of Agriculture and the Irish Thoroughbred Breeders’ Association released an alert on Friday. At that stage, there were 20 known cases nationwide.

There have now been 14 recorded cases in Co Down (eight of them fatal), several cases in Athlone veterinary laboratory catchment area and half a dozen cases in Dublin and Kildare, as well as two yearling deaths in Tipperary.

In the UK, fatality figures are now approaching the 100 mark with 12 recent cases in Hampshire alone.

Factors such as the dry weather this year have led to increased seed production and, combined with strong winds, the seeds of sycamore seeds – commonly known as helicopters – have been dispersed nationwide.

Latest US and European veterinary research has shown the seeds of sycamore trees and box elder trees – both of which belong to the maple family – are to blame.