Squirrels aren’t the only ones hoovering up the Fall crop of acorns. Horses love them too. A yummy treat courtesy of Mother Nature – or are they harmful?
Many people report their horses regularly indulge in acorns in the Fall with no ill consequences but a small number of horses develop serious, even fatal, illness as a result.
A study from the UK [Smith et al 2015] reports on a series of 9 horses over a 9 year period treated for suspected acorn toxicity. Five presented with bloody diarrhea and shock, were rapidly euthanized or died. Four had colic and diarrhea. Three of those survived with medical support and the fourth was euthanized.
Post mortem examination of the 6 dead horses showed extensive intestinal and kidney damage. The authors noted toxicity seems to be more of a problem in some years than others, which has also been reported by others. It is unknown if acorns are more toxic certain years or if it is a matter of larger numbers of them being available.
The details of how acorns cause poisoning remain to be determined. Tannic acid is often blamed but in cattle, which are even more sensitive than horses, dosing with tannic acid does not reproduce the kidney damage. Gallic acid and phenolic compounds are also believed to be involved. The products of bacterial fermentation of acorns in the rumens of cattle and the colon of the horse may be the most active toxins.
The leaves of oak trees are also potentially toxic. Cattle have been poisoned by consuming oaks leaves in the spring. There have not been any recognized cases in horses but it’s possible.
Acorns pose another potential danger to horses, ponies, donkeys and mules with Equine Metabolic Syndrome [EMS]. In this condition, there are exaggerated insulin responses to simple sugars and starch which can result in laminitis. Acorns average over 40% starch which is way too much for an animal with EMS.
There is no specific treatment for acorn poisoning. Activated charcoal can help if administered immediately after the acorns are consumed but signs don’t usually appear for a few days after ingestion and at that point it’s too late for charcoal. Charcoal also can’t protect an EMS horse from acorn induced laminitis since it doesn’t have any effect on starch digestion.
After the horse becomes ill, the only treatment is supportive care such as intravenous fluids. Even with intensive treatment the fatality rate is high, as much as 67%. Not all horses become poisoned on eating acorns but that high death rate should be strong motivation to avoid exposure to acorns and oak trees.
Removing acorns as they fall is a full time job. It’s preferable to remove the trees entirely or fence off areas containing acorns in the fall and trim the trees so that leaves are not accessible. If storms bring down branches, be sure to remove them promptly.
Acorns from the mighty oak are a boon to many species but not worth the risk with a horse.
Eleanor Kellon, VMD