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A Guide to Poisoning in Horses: The Dangers of Biological Poisons

Welcome to the second part of the Complete Guide to Poisoning in Horses. In this guide we’re walking you through some of the common (and uncommon) sources of toxins and poisons that can affect our horses. This section of the guide focuses on biological poisons, such as moulds, fungi and bacteria – as well as trees and potatoes!

If you are interested in knowing how to diagnose or treat poisoning, check out part one of this guide which also contains information on heavy metal poisoning.

Part three of this guide will focus on the many plant species which are believed to be toxic to horses. Remember that if you believe that your horse has been poisoned, time is of the essence and a quick response is needed by you.

You are currently reading: The Dangers of Biological Poisons

  • Part 1 – Diagnosis, Treatment and Heavy Metal Poisoning [Read Now]
  • Part 3 – A Guide to Poisonous Plants [Read Now – Coming Soon]

Biological and Miscellaneous Poisons

Antibiotics – There are times when an equine vet may need to prescribe antibiotics for your horse, in such a case, the dose and type of antibiotic are carefully considered. Antibiotics are sometimes added to the feed of livestock however (including cattle, pigs and poultry), if a horse mistakenly consumes such a feed they are putting themselves at risk.

At low levels, antibiotics may disrupt the natural microbial population of the gut, which could lead to colic. However, if a horse consumes high levels of antibiotics such as those added to concentrate cattle feeds, serious symptoms may appear after a few days.

Bacteria – Bacteria are a major cause of poisoning in both humans and horses. If bacteria contaminate a food source, which is then eaten by a horse, the bacteria can rapidly reproduce in the intestine. Such bacteria release toxins that can cause colic and diarrhoea. The species Clostridium has been known to causes severe gas colic in horses that can prove fatal.

Other bacterial species to be aware of are Salmonella, which vermin can spread to feed via their faeces and Clostridium botulinium. C. botulinium can become a problem if horses are fed from incorrectly fermented silage – it is C. botulinium which is responsible for causing botulism.


Bark – The bark of poisonous trees such as the Yew or Laburnum can contain high levels of toxins. The bark of the False Acacia (see right) contains multiple toxic proteins that can cause gastric upset, colic and diarrhoea.

Bites and Stings – Fortunately in the UK there are very few poisonous animals that pose a risk to our horses. The European Adder is the only venomous snake in the UK and is only a threat during the summer months.

If you think your horse may have been bitten, or stung by a bee or wasp, it is the stress of the experience that causes more of a problem. Where possible, remove any debris from the bite or sting and try to calm your horse. Anti-histamines could be used to reduce the swelling around the bite or sting. It is unlikely that any further problems will occur, but if you notice any unusual behaviour (even depression) it may be worth contacting your vet for their opinion.

Fungal Toxins – Fungi can grow on other plants without causing them any harm. Ryegrass, fescue (see right) and some grasses and weeds are particularly prone to fungal infection. These fungi can release toxins when eaten by the horse however, which can cause gastric upset. Fungi infected fescue is a particular problem if grazed on by pregnant mares as it can adversely affect gestation.

Moulds – Certain moulds can produce toxins (mycotoxins) that can cause fatal poisoning in some severe cases. Lesser toxic moulds can occur on rye and other cereal grains as well as hay and straw. Dry hay and straw should be kept in a clean and dry environment to prevent the release of spores into the air. These spores can cause respiratory problems or allergies in the horse. Good ventilation is key to preventing a dusty environment.

Aflatoxin is a toxin produced by the mould species Aspergillus flavus. Known for contaminating cereal grains, this toxin can cause severe damage to the liver and is often fatal.

Nitrates – A common source of nitrates in the environment is fertiliser. If a farmer uses excessive amounts of nitrogen fertiliser of their crops, the excess can run off into streams, ground water or standing water. This leads to nitrate rich water sources and can cause some plants to take in large amounts of nitrate.

Whilst nitrates aren’t too toxic, they can be converted to nitrites by plants or by the horse. Nitrites have greater toxicity and can lead to liver damage.

Nuts – Oak acorns and Beechnuts (see right) can be toxic to horses. Acorns contain high levels of tannic acid, which can cause colic, diarrhoea or constipation. Beechnuts can cause colic or tremors, but also contain thiaminase – an enzyme that causes the breakdown of thiamine (vitamin B1). In this instance, a high quality feed supplement may be given to help restore levels of B1.

Pesticides – Pesticides are harsh chemicals used to kill vermin and pests. Rat poisons, slug pellets, herbicides and insecticides can all pose a risk if a horse is allowed to consume large amounts of them. If you use any pesticides, be sure to keep them away from you horse and their feed.

Trees – Certain species of tree are a potential hazard for horses. All parts of both the Yew and Laburnum trees are toxic to horses; Privet leaves are also toxic, but less so than the Yew or Laburnum.

Poisonous Vegetable Protein

Glycosides – Glycosides are sugar compounds, whilst not typically dangerous, some plants contain cyanogenic glycosides. These are sugar compounds with the potential to produce the highly toxic hydrogen cyanide.

Hydrogen cyanide is released when the plant is ‘attacked’ e.g. grazed, picked or cooked, as a defence mechanism. Hydrogen cyanide has the potential to cause respiratory failure.

These cyanogenic glycosides can be found in Sorghum (see above right), Cassava, lima beans (see right) and lentils. Correct preparation is required before any of the above can be given as a food source, this usually involves thorough washing and cooking in boiling water. 

Goitrins – Young horses are more susceptible to the development of goitre (swelling of the thyroid gland) following the consumption of goitrins. These compounds reduce the uptake of iodine (which leads to goitre) and can be found in vegetables such as cabbage, brussel sprouts, kale and oil-seed rape. Heat treatment can destroy the goitrin compounds and as long as the diet contains sufficient iodine, there is unlikely to be cause for concern.

Gossypol – Cotton plants and their oil contain the yellow pigment gossypol. This pigment is toxic to horses as it can cause cardiovascular failure. Cotton seed meal is sometimes added to mixed feeds, if this is the case, be sure to check that the cotton seed meal is of high quality and has been produced with a low concentration of gossypol.

Lathyrogens – Lathyrogens can cause paralysis of the larynx that can cause near suffocation. They are found in the seeds of the pea species Lathyrus, which includes; the Indian pea, the sweet pea, the ever-lasting pea (see right) and the wild winter pea.

Lectins – Lectins are a type of protein found in a number of bean and pea species. They can damage the intestine and reduce the amount of nutrients absorbed. Long cooking periods at high temperatures are required to destroy lectins, so in most cases it is simply best to avoid them. Lectins are found in; kidney, haricot, lima, soya and navy beans as well as peanuts, rice bran, cow peas and certain pulses.

Onions – If horses are fed onions in large amounts (for example waste onion crops) they can develop anaemia, loss of appetite, rapid heartbeat and jaundice, they may also stagger and blood may be present in their urine.

This is due to a volatile oil (n-propyl disulphide), which is found in onions and similar vegetables. Whilst this is not a problem in small amounts, it is unwise to feed in large amounts.

Potatoes – Horses are susceptible to poisoning that develops from an alkaloid (a nitrogen containing, organic compound that originates from a plant) found in potatoes. If a horse is fed large amounts of waste potatoes, this alkaloid can cause problems, including; diarrhoea, colic, thirst, lack of coordination, dehydration and laboured breathing.

Tannins – Tannins are weak acids that can affect the digestion of proteins and sugars. When present in the horse’s diet in high concentrations, they can cause colic. Sources rich in tannins include; brown sorghum grains and field beans.

The Complete Guide to Dealing with Poisoning in Horses

Thank you for reading part two of this guide, over the next few days, the additional sections to this guide will be released – identifying more potential poisons for horses. If you aren’t already, follow PetSci on twitter to be notified when the next instalment is released.

When finished, this guide will be available to download as an ebook in its entirety, which will be available here on PetSci.


About James Watts

BSc Bioveterinary Science. Editor of PetSci. When I'm not writing, learning, discussing, or reading about animals, you know it's the weekend! Currently developing PetSci HealthTrak, the fast and easy way to monitor your pet's weight and calorie intake. HealthTrak offers a simple way to track your pet's progress, helping them achieve a healthy weight and a long, happy life.

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  1. Thanks. This helped me a lot XD

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