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Botulism and hay silage

Hay silage is often the preferred feed for horses, rather than hay. However, it is possible for soil or dead animals to get wrapped in the bale - this allows bacterium Clostridium to form a strong toxin that is very dangerous to horses. You can reduce the risk of botulism to your horse through vaccination.
Horse eating hay in a stable

What is botulism in horses?

Botulism is caused by a nerve toxin, botulinum toxin, which is extremely toxic to horses even in very small amounts. The nerve agent is formed by the bacterium Clostridium botulinum, which is found in soil, when it is allowed to grow in an oxygen-poor environment. Poisoning can occur when the horse eats contaminated hay silage.

The bacteria can, for example, occur if soil or dead animals have gotten into the wrapping of the hay layer. Then the bacteria have good opportunities to grow in the oxygen-free environment. 

The botulinum toxin affects the horse's nerves and paralyses it, which eventually affects the lungs and respiratory muscles. Among unvaccinated horses, the mortality rate is very high.

Symptoms of botulism

The most common symptoms of botulism are:

  • Fatigue and poor appetite
  • Muscle tremors or difficulty moving
  • Difficulty eating and swallowing
  • Muscle weakening, which eventually means that the horse cannot stand up or breathe and therefore dies of suffocation.

Only small amounts of the nerve agent are needed for a horse to become ill. The symptoms can appear just a few hours after the horse has ingested the bacteria.

If one horse in the stable is affected, there is a high probability that the other horses will become ill.

Horses affected by botulism can survive, but this is unfortunately rare and it can take a long time for them to fully recover.

Feeding with hay silage

A hay silage bale should not be open for more than 3 to 5 days - especially not during the warm season. After this, the preservation ends and the feed goes bad.

The hygiene and quality of the feed is important. If you find mould, carcass parts or if the bale smells strange, the entire bale should be discarded. It is also important that unopened hay silage bales are stored in a way that minimises risk or getting holes or tears in the plastic.

If there is a hole in the plastic before the bale is opened, a fermentation and rotting process starts immediately, making the feed unusable.

It is not possible to determine whether Clostridium botulinum and its toxin are present by looks or or smell.


The basic vaccination against botulism consists of three shots that must be given one month apart. The horse receives full protection two weeks after the third basic vaccination. It is good to vaccinate in good time before you start feeding with hay silage. After the basic vaccination, a revaccination is needed every year.

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