Autumn traditionally sees a peak in cases of the rapid-onset condition atypical myopathy (AM), now known to be linked with trees from the Acer family. This acute muscle disease, sometimes termed atypical myoglobinuria or seasonal pasture myopathy, is caused by the ingestion of some species of Acer tree seeds (such as the sycamore) that contain the toxin hypoglycin A. The consequences can prove distressing and frequently fatal — muscle groups break down, both skeletal muscles and potentially those linked to cardiac and respiratory function, and renal failure can follow.
While intensive treatment can sometimes be successful, if started sufficiently early, unfortunately the prognosis is typically bleak.
Early findings from the first UK-based AM study are helping to shed further light on this potential killer in the paddock. Owners and vets were encouraged to report confirmed cases through a surveillance website run by the University of Liège, in Belgium, and supported by the Animal Health Trust (AHT). This yielded 224 horses, with an average age of six. The breeds most commonly affected were cobs and part-bred cobs. Of the pastures they grazed on, 96% were affected with sycamore seeds and a further 68% with seedlings.
The overall survival rate was 38.6% (12% more than in a previous European study). Of notable interest, however, were potential clues about management of horses and their chances of recovery. “Those horses that had performed light to moderate exercise in the 48 hours prior to the onset of the disease, as well as having received regular supplementation with vitamins and minerals, seem to have overcome AM better,” said Dr Sonia Gonzalez-Medina, who presented the findings at the recent British Equine Veterinary Association congress. “In contrast, horses that had been grazing on pastures used all year round for equine grazing were less likely to survive.”
Certain signs — recumbency, hypothermia, a distended bladder palpable on rectal examination and an increased heart rate — also indicated less chance of survival.
Also at the same congress, a Royal Veterinary College researcher, Alex Draper, critically appraised the evidence available as to whether the removal of all Acer trees should be undertaken. H&H vet Karen Coumbe MRCVS explained: “There are many different types of Acer tree in the UK. Accurate botanical identification is vital, since Acer pseudoplatanus, the sycamore, poses the highest risk.
Ingestion of the seeds and seedlings of this type of Acer has been shown to be linked to the greatest risk for AM development in horses, across four mainland European countries, due to the high levels of toxin they contain.
“It makes sense to prevent horses grazing pastures contaminated with material from sycamores,” added Karen. “The risk of eating the leaves alone is likely to be lower than that from seed or seedling ingestion, as there has been shown to be less of the toxin present in the leaves. Having said that, the likelihood of there being leaves and no seeds
is pretty improbable.
“It is really best to keep horses away from all sycamore material, ideally with fencing and other measures.”
Take action this autumn
- Avoid pastures surrounded by Acers during autumn and early winter.
- Collect and burn any winged “helicopter” seeds, which can travel a considerable distance in windy conditions.
- Supplement horses with forage if grazing is scarce.
- Call your vet immediately if your horse displays mild to moderate colic signs, depression, reluctance to move or weakness, dark urine or inability to raise his head. The sooner treatment can be initiated, the greater the chance of recovery.
- Be vigilant for Acer seedlings next spring, removing any you find.