Many horses are pastured in paddocks with oak trees, but cases of acorn poisoning are only anecdotally reported.
Vets in the UK recently published a report on nine cases of severe disease in horses that seem to have been definite cases of acorn poisoning. The horses were very ill, with signs of colic, and all either had diarrhoea when first seen or developed it soon afterwards.
All horses had evidence of acorns in their dung or in their gut at postmortem. All cases occurred in October or November.
Only three of the horses — those with diarrhoea that was slower in onset or milder in severity — survived. The others either died or were put down on humane grounds. All had severe inflammation of the large intestine.
Postmortem examinations produced similar results to confirmed cases of acorn poisoning in cattle, which causes severe bloody diarrhoea and can be fatal.
With this evidence on the table it is certain that acorn poisoning is a potential threat to greedy horses grazing under oak trees in the autumn, especially on sparse pasture. Owners should be aware of this risk.
Every autumn a number of free-living ponies in the New Forest die and this is put down to acorn poisoning. On Exmoor, where there are very few oak trees, there are no reported autumn deaths.
Ref: Horse & Hound; 12 February 2015