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Oak trees are common on properties where horses graze, and both acorns and leaves fall from oaks in the autumn. Acorns are not, however, as innocuous as horse owners might believe.

Acorn poisoning is frequently reported in sheep and cattle, and is being diagnosed more often in horses. Tannins in acorns and leaves bind to proteins in the lining of the horse’s digestive tract and the microflora of the gut, causing damage to cells, while toxic metabolites trigger kidney and liver damage. Interestingly, proteins in the saliva of pigs bind to the tannins, thus neutralizing the toxic effect.

Horses with acorn poisoning often present with acute onset of depression and inappetence, followed by diarrhea, sometimes hemorrhagic or blood-laden and watery. Owners sometimes confuse this with colic, and response to painkillers is poor. Some become very ill and die or are euthanized, but others can be saved with intensive medical treatment, which typically involves intravenous fluid therapy, plasma administration, and broad-spectrum antimicrobial therapy.

Environmental causes may play a role in toxicity, as oak trees may produce far more acorns in some years than in others, increasing their availability. Similarly, more horses are affected in certain years. Of interest, “the Verderers of the New Forest, who manage a herd of approximately 3,000 semiferal ponies in southern England, record the number of ponies thought to have died from acorn toxicity each year; in 2013, 69 ponies died, compared to an average of 17 ponies per year over the previous decade.”*

The disease is only seen in a small proportion of horses and ponies exposed to acorns. Scientists and veterinarians do not know how many acorns a horse must eat to be poisoned, but it would be wise to limit access to acorns in autumn. If that is not possible, provide plenty of alternate, appealing feed sources.

*Smith, S., R.J. Naylor, E.J. Knowles, T.S. Mair, S.D. Cahalan, D. Fews, and B. Dunkel. 2015. Suspected acorn toxicity in nine horses. Equine Veterinary Journal 47(5):568-572.

Warren, C.G., and S.M.Vaughan. 1985. Acorn poisoning. The Veterinary Record 116(3):82.

 

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