Is Acorn Toxicity in Horses Real?

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

About Dr. Kellon

Graduate of University of Pennsylvania Veterinary School. Owner of Equine Nutritional Solutions, www.drkellon.com, industry and private nutritional consultations, online nutritional courses. Staff Veterinary Expert at Uckele Health and Nutrition.
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5 Responses to Is Acorn Toxicity in Horses Real?

  1. Dr. Kellon thank you for providing this very important, potentially life saving information to horse owners. One of my horses is currently suffering the effects of the tannins. If I hadn’t observed them eating the acorns and become aware of the large quantities that fell this year (I have a resource that states this occurs at 2 & 5 year intervals in efforts to reproduce)), seen the development of diarrhea and checked on him at 10pm witnessing what seemed to be muscle spasms & called my vet, I’m sure he would have died and suffered. I’ve had the horses on this property since 2006 but the trees we left have grown and though I’ve noticed certain years acorn production is higher, I have never seen anything like this. By the time I started to remove them it was too late. I told my vet I was certain the colic was due to the acorns but she was hesitant, due to lack of scientific evidence to agree with me, which I understand. However. I know beyond a doubt it was the acorns. A blood test a week later showed no liver or kidney damage, normal electrolytes & protein but I don’t know if there was damage to the intestines. He lost a tremendous amount of weight very quickly. I’ve started to change his diet with the help of nutritionist at Triple Crown and think perhaps probiotics or an additional dietary support may help. He will be 25, was diagnosed with Cushings almost 3 years ago & has been on pergolide. I understand the average life span for Cushings horses is 5 years from diagnosis and I am surprised that he is doing so well considering. I am in Maine & we get brutal winters so the goal now is to see if I can succeed in putting weight on, The senior feed which I have not investigated is supposed to be for Cushings horses because I am concerned about sugars & laminits. At this point I would like to see him continue to have a good quality of life knowing the end is closer. I appreciate the article and have posted it to both personal & NP facebook sites in hopes it can help others or at least increase awareness. I am disappointed that vets don’t use databases to communicate & share information regarding colics and the causes particularly in the Fall because if they did they would probably find acorns to be a cause and could warn horse owners because the horse became ill.

    • Dr. Kellon says:

      Glad to hear your horse is doing well. That 5 year life expectancy for PPID (Cushing’s) horses is not necessarily true. They can live much, much longer than that! Consider joining my group at https://ecir.groups.io/g/main.

      • will do. fyi your editorial ‘Stewardship’ is still making an impact on everyone who comes here. Everyone interested in our work truly appreciates these animals. I did have a question regarding the intestines and potential damage by the toxins. Any advice there ? Also our website is finally going to get a professional update which will also now allow me to edit.

  2. CYNTHIA HRANICKA says:

    Does it matter if the acorns are from the red oak family vs. the white oak family? We have both here, and I do know the deer just love the white oak acorns.

    • Dr. Kellon says:

      The red and black oak have higher concentrations of tannins than the white, which would make them taste better but we don’t know for sure if that also means less toxic.

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