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Poisonous Plants

Oak and acorns contain tannins which when ingested produce toxins which can be poisonous to horses. The risk is highest in the UK at the end of long dry summers, even so most horses are sensible enough not to eat large numbers. It seems that certain individual horses are more susceptible to acorn poisoning than others, some horses can tolerate small numbers of acorns but the toxicity of the acorns can vary from year to year. Acorns and oak contain tannic acid and other tannins which are toxic when consumed in sufficient quantities. These poisons can cause damage to the gut leading to problems such as diarrhea and colic signs, they can also cause damage to the liver and kidneys. There is no specific antedote and the prognosis is poor if kidney damage develops. In rare cases acorn poisoning can be fatal.


It is sensible to prevent access to large numbers of acorns during the autumn, particularly after strong winds when large numbers may have fallen off the trees. Acorns can be swept up or electric fencing used to keep horses away from areas around oak trees with large numbers of acorns. Some people control the acorns by using a roller to push them into the ground, but it is difficult to do practically on a regular basis. Alternatively a more novel approach is to allow pigs to graze the area since they can safely eat the acorns; this has the added advantage of training your horses to accept pigs!

For further information, see our 'latest news' article on acorn poisoning HERE.

It is well known that ragwort can cause serious damage to horses’ livers. Horses usually choose not to eat ragwort when it is growing as it is said to be unpalatable but they will sometimes eat wilted ragwort in fields or in hay as it is loses its bitterness. Never pull out plants and leave them where horses can reach them because they are more likely to be eaten.  Also if the grazing is poor and there is nothing else available then they will possibly be tempted to eat ragwort as a last resort. The more ragwort they eat, the worse the damage will be.

Unfortunately the damage caused by ragwort is irreversible. The liver is very good at coping with a certain level of damage and so many horses with mild liver damage caused by ragwort appear completely healthy. However they can deteriorate very quickly and dramatically if there is any further damage. Liver damage will cause weight loss as well as loss of appetite and other vague signs but in some severe cases the liver damage can be fatal.


We advise horse owners to remove ragwort from fields that horses graze in and to be particularly careful to make sure there is no ragwort growing in fields from which hay is cut. More information about ragwort removal can be found on the BHS website.

What is Atypical Myopathy?

The sycamore tree (Acer Pseudoplatanus) contains a toxin (poison) in its seeds, seedlings and leaves which when ingested, can cause a muscle disease called Atypical Myopathy (AM). 

It is not a new disease; as a sudden onset muscle disease it has been recognised in horses for over 60 years, but its cause, the ingestion of the toxin hypoglycin A, was only identified in 2013. 

In the UK, the most common source of this toxin is now known to be the Sycamore tree (Acer pseudoplatanus), a member of the maple tree family. Sycamore have 'helicopter shaped' seeds that help to distribute their seeds over long distances, typically several hundred metres, but in high winds can reportedly travel several kilometers.

Outbreaks tend to be seasonal with most cases occurring in the autumn where rain and high winds will bring down large numbers of seeds over a short duration of time, and then in turn in the spring when seedlings are starting to grow. So it is important to be vigilent at these times of year and take steps to prevent the disease by limiting access to areas affected.

Research undertaken by the University of Liege involving the study of 17 horses from Belgium, Germany and The Netherlands greatly advanced our understanding of this condition. There is a helpful summery of this research on the BEVA website.

What are the signs of Atypical Myopathy?

The first problem in this disease is the development of severe muscle damage. This occurs because the toxin interferes with the ability of the muscle to undergo its normal processes. Muscle damage can lead to a horse becoming weak and reluctant to move or unable to stand and may be seen to lay down a lot.

The disease is painful for the horse and will often show signs of sweatingdistress or reduced appetite and in severe cases will show signs of sever colic-like signs.

Sometimes swelling or hardening of the muscles is visible and the horse resents being touched.

If serious muscle damage occurs, toxic products released as a result of the muscle breakdown can become visible in the urine, this can lead to dark redish-brown coloured urine (myoglobinuria). These muscle break-down products can unfortunately damage the kidneys and secondary kidney failure can arise.

Horses affected by Atypical Myopathy (AM) can also have difficulty urinating and become unable to empty their bladders normally. Sometimes the respiratory or heart muscles become affected, leading to difficulties breathing or the development of heart failure. Sadly this disease is often fatal, with variable survival rates reported from different areas, ranging from 3-57%.

If your horse shows any of the signs above, we recommend contacting your vet immediately. If your horse has advanced signs of AM, then the signs will be strongly indicative of the disease. However, in the early stages, if can be difficult to say for certain whether or not it is AM. A number of other conditions including various forms of colic, equine grass sickness (EGS) or other muscle diseases and a number or neurological conditions can lead to similar signs. There is not a definitive diagnostic test for the disease and this can make early diagnosis challenging. However, a simple blood sample will allow us to see whether the horse has muscle damage.

Early detection of any of the above described signs and prompt discussion with your vet should lead to the highest likelihood of a positive outcome for you and your horse.

How can I minimise the risk of my horse developing Atypical Myopathy?

  • Check fields and tree lines adjacent to fields for the presence of sycamore trees
  • Regularly inspect the pasture for sycamore seeds, leaves or saplings and remove them if seen.  The 'helicopter' seeds, however, can be carried a long way in the wind so its worth being vigilant, even if there are no trees in direct contact with the field
  • Fence off any areas under sycamore trees in Autumn and Spring, when leaves are falling and saplings may be emerging - if possible, it makes sense to remove horses from the pasture during high risk times of year
  • We suggest that where horses are grazing near sycamores, it is imperative they have sufficient supplementary feed such as hay or haylage as this will minimise the risk of horses being tempted to eat seeds / foliage and will reduce the risk of over-grazing
  • We do not advise cutting down trees as their shade can be a vital source of shelter for the rest of the year, but please be aware there is a potential danger during the autumn and again in spring when seedlings (as seen below left) are coming up. Felling of trees laden with seeds (seen below right) can also cause a sudden and large contamination area with leaves and seeds. If felling is the only option, make sure you are aware of any tree protection orders and ensure you are certain of tree ownership
  • Monitor horses closely after they have been moved from an affected pasture as disease can occur up to four days after exposure


Unfortunately, all the above measures do not completely eliminate the risk and full time stabling is the safest way to manage at risk horses in the autumn and spring. Full time stabling brings its own risks in horses that are usually turned out, so the best compromise for each horse will depend on: the frequency with which the pasture can be checked and cleared of sycamore seeds / seedlings and the horse's ability to tolerate full time stabling

If you have any questions, please contact BELL EQUINE on 01622 813 700 and ask to speak to one of the vets.