“The extreme weather over the summer may have contributed to the production of greater quantities of sycamore masts and acorns than normal,” warned British Equine Veterinary Association president David Rendle.
“High winds over the next few weeks will likely serve to increase the risk further. Ideally horses should be kept away from grazing around oaks and sycamores but if no other pasture is available supplementary feed should be provided. If horses that have access to sycamore seeds or acorns show signs of illness, veterinary attention should be sought immediately.”
Seeds (“masts” or “helicopters”) from the common sycamore tree (Acer pseudoplatanus) contain a toxin called Hypoglycin A, which can still be present in high concentrations in seedlings. When horses eat these, either by accident or because they are lacking other forage options, some individuals will develop severe and often fatal muscle damage, called atypical myopathy.
Horses with atypical myopathy show variable signs of muscle soreness, stiffness, weakness, difficulty breathing, dullness, lethargy, muscle trembling, colic-symptoms, and characteristically, brown or dark red urine. Suspected cases need immediate veterinary attention. Around three quarters of affected horses will die, often despite extensive veterinary treatment but those surviving the initial period will usually go on to make a full recovery.
Acorn “toxicity” is less common and less well understood than hypoglycin toxicity. The apparent increase in cases seen recently may be an unfortunate combination of the extreme dry summer weather and normal variation in acorn production. Just like all fruit and nut trees, oak trees can produce variable amounts of acorns from year to year, with ‘mast years’ of unusually high production.
When a group of horses are exposed to acorns from the same oak tree, only one or two horses will fall ill. This may be because individual horses are particularly susceptible, or that some trees, or even certain acorns, are particularly toxic. Protection from acorn toxicity in other species, such as pigs, comes from the production of tannin-binding salivary proteins. These proteins are not normally produced by horses, but it is possible that some individuals have them and are protected in this way.
The toxic effects of acorn ingestion can be severe and prompt veterinary attention is essential. Clinical signs include moderate to severe colic or colitis, lethargy, dehydration, and dark urine which can be a result of kidney failure. Signs may develop extremely rapidly, and death can occur within a further 12-24 hours.
Horse owners are advised to take practical steps to prevent the diseases by limiting access to sycamore seeds and acorns:
- Identify trees both around grazed fields as well as those in close proximity.
- Collect seeds or exclude horses from affected areas using electric fencing or stabling.
- Feed supplementary hay to try and prevent horses from excessive foraging for short blades of grass and inadvertent ingestion of seeds. But ensure that hay does not become contaminated by seeds.
- Don’t fell trees when laden with seeds as this can cause a sudden and massive contamination of the pasture.
- Monitor horses carefully even after they have been moved from affected pasture as disease can occur up to four days after exposure.
For further information download the Royal Veterinary College’s fact sheet on Atypical myopathy: https://www.rvc.ac.uk/Media/Default/Comparative%20Neuromuscular%20Diseases%20Laboratory/Atypical%20Myopathy%20fact%20file%20updated%202022.pdf
And see the British Horse Society’s fact sheet on Acorn Poisoning: