Horse owners are being warned sycamore seeds could be poisonous to their animals following a spate of deaths.
RSPCA Cymru issued the warning after several horses died in Pembrokeshire after eating them while grazing.
It is thought that the seeds are particularly abundant due to a warm summer followed by a spell of windy weather which has dispersed them.
Neurotoxins in the seeds can attack a horses' central nervous system, causing atypical myopathy.
RSPCA chief inspector Cathy Hyde, who heads a specialist team of equine officers, said: "It appears that cases of this debilitating and sadly often fatal condition are likely to increase significantly."
Mortality Signs to look out for are a depressed demeanour, breathing difficulties, muscle spasms and the inability to stand.
Sue Scourfield, of Pembrokeshire Riding Centre in Hundleton, lost her two-and-a-half year old Highland pony Ghillie last week.
She said: "Our vet said that he'd had to put down four other horses with atypical myopathy that weekend.
"In 50 years of keeping horses I'd never come across this before."
Local vet Graham Fowke said the disease has an 86% mortality rate and advised horse owners to check their animals twice a day and call a vet at the first sign of any illness.
Mrs Scourfield said that the only way she will feel safe now was by chopping down all the sycamore trees on her 40-acre property.