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Equine Grass Sickness

Equine Grass Sickness

Equine Grass Sickness (EGS) is a debilitating disease which can frequently lead to death.

Any breed of horse or pony that is grazing on pasture is potentially susceptible. Worryingly for owners, there have also been cases of horses developing EGS that have had no access to grass, with hay found to be the cause.

Signs to Look Out For

EGS can be seen in three different forms; acute (a sudden, rapid onset); sub-acute; and chronic (less severe signs and developing over a longer period of time). The signs are:

  • Excessive salivation;

  • Sweating;

  • Increased heart rate;

  • Dried inflamed nostrils;

  • Colic signs, but with no gut sounds;

  • Bloated stomach;

  • Green, foul-smelling liquid from the nose (stomach contents);

  • Drooping eyelids;

  • Muscle tremors;

  • Depression;

  • Tucked up abdomen;

  • Weight loss;

  • Difficulty swallowing;

  • Unable to eat.


The definitive cause of EGS is unfortunately still not known. What we do know is that it affects the digestive system causing paralysis of the gut. This in turn means that fluid builds in the stomach and small intestine, blockages develop in the large intestine, and dehydration occurs.

Experts believe that EGS may be caused by: a bacterium that creates dangerous toxins; a fungus found in pasture which produces a toxin; or possibly a genetic predisposition.

It is generally believed that not one of the above alone causes EGS and that other factors need to be at play for the disease to develop. The range of these other factors is vast and includes things such as soil disturbance through to the state of the immune system of the horse.

For more detail on this, visit the Moredun Foundation Equine Grass Sickness Fund by clicking here. This is the only registered charity in the UK which raises funds specifically for research into EGS. They have lots of information available on their website.


Unfortunately, most horses that develop EGS will not survive and are usually euthanised to prevent further suffering. Around 50% of the chronic cases may survive if they have intensive nursing and veterinary care, but this will take considerable time and funds.

If you suspect that your horse is showing any signs of EGS please contact your vet immediately.


As the causes are still not fully known, it is difficult to give a definitive answer as to how to

prevent EGS striking your horse. However, here are some measures which may help:

  • Avoid grazing areas known to have had cases of grass sickness or where the soil has recently been disturbed;

  • Minimise soil exposure to your horse, so move them before the grazing is too short;

  • Avoid sudden changes to diet;

  • Avoid overuse of Ivermectin-based wormers;

  • Share the pasture with ruminants such as sheep or cows;

  • Poo pick the grazing area regularly;

  • Feed forage, such as hay or haylage, alongside grass;

  • Stable your horse if the weather is cool and dry with irregular frosts.

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