Equine grass sickness 101: How to protect your horse

Most horse owners have heard of equine grass sickness (EGS), but might not know what causes the disease, or how to decrease the risk of it affecting their horses.

Here’s everything you need to know, including details of the Moredun Foundation Equine Grass Sickness Fund (EGSF) – the UK’s only registered charity dedicated to researching the causes of EGS and improving the treatment of chronic cases.

Equine grass sickness is a complex disease that attacks a horse’s nervous system. This compromises their ability to swallow and prevents the movement of food through the digestive tract. It can result in a build-up of fluid in the stomach and small intestine, dehydration and impactaction of the large intestine.

There are three types of EGS, characterised by the speed of onset: acute, sub-acute and chronic. The overall mortality rate is 80%, making it an extremely serious illness. Acute and sub-acute EGS are almost always fatal, but around half of chronic cases survive. The majority of these horses can be ridden again once recovered.

Grass sickness occurs across Northern Europe, with the UK having the highest number of cases annually. EGS can occur in any part of the UK, but cases are more common across eastern counties.

Despite investigating for over 100 years, the definitive cause of grass sickness is still unknown. But in 2024, experts found it may be linked to a neurotoxic enzyme, produced by microorganisms in the ground. The action is similar to snake venom and causes paralysis of the gastrointestinal tract.

Experts are now trying to uncover the source of this toxin. It could be a microbe such as a bacterium or fungus which grows on fields during the cold, dry weather that often comes before increased incidences of grass sickness.

EGS can affect any horse and the UK has the highest number of reported incidences in the world. But some horses are more at risk than others, including:

• Overweight horses
• Horses aged 2–7 years old
• Horses who have recently moved to new grazing
• Horses located in the east of the country, particularly in Scotland
• Horses who have been stressed for example, when moving to a new home, mixing with different horses, travelling long distances, or being gelded

The EGSF records information for cases of EGS across the UK, with a case map of locations and seasonality. While cases occur all year round, most are seen from April to July, with a peak in May. Sometimes, a second peak occurs in autumn or winter.

The symptoms of acute cases include signs of colic. For example, rolling, pawing and looking at the flanks. Additional symptoms include:

• Drooling
• Constipation
• Bloating
• Muscle tremors
• Patchy sweating
• Difficulty swallowing
• Increased heart rate
• Foul-smelling nasal discharge
• Small, hard poop with a coating of mucus

Unfortunately, this form of grass sickness is fatal, and once diagnosed, most horses have to be euthanised within two days.

These cases show similar signs to acute grass sickness but with less severity. Accumulation of fluid in the stomach is less often seen but additional symptoms include:

• Rapid weight loss
• Difficulty swallowing
• Mild to moderate colic

Horses with sub-acute weight loss may manage to eat small amounts but are likely to die or require euthanasia within a week.

These cases develop more slowly, with some horses only showing signs of mild, intermittent colic. Horses are likely to have a reduced appetite and difficulty swallowing, but won’t drool, accumulate fluid in the stomach, or suffer from severe constipation. The most important signs to look out for include:

• Reduced appetite
• Difficulty swallowing
• Rapid, severe weight loss

Horses with chronic grass sickness need intensive nursing. Regular interactions with their owners – including grooming, frequent walks and gentle massage – are thought to help recovery.

Horses suffering from chronic EGS will also need palatable food that’s easy to swallow, with some needing hand feeding, syringe feeding or a nasogastric tube. If your horse is insured, some of the treatment costs may be covered.

Diagnosing EGS is challenging and often involves looking for patterns of clinical signs. If you suspect your horse has grass sickness, treat it as an emergency and contact your vet immediately.

As research is ongoing into what causes EGS, it’s difficult to know for certain how to prevent it. But current research suggests the best place to start is with management changes that help avoid the known risks.

• Avoid turning your horse out on fields where there have been previous cases of EGS
• Avoid grazing on fields with recently disturbed soil, such as from harrowing
• Minimise soil exposure by moving horses before the grass gets too short or fields get poached
• Avoid sudden changes to your horse’s diet
• Avoid overuse of ivermectin-based wormers
• Co-graze with ruminants (animals that chew cud) – for example, cattle, sheep, goats and deer
• Poo-pick fields regularly, but avoid mechanical methods of removal
• Feed supplementary forage alongside grazing, such as with hay or haylage
• Stable horses in cool, dry weather with irregular frosts

Not all of these suggestions will be practical for all horse owners, so it’s best to prioritise horses who are most at risk. For example, you may choose to keep younger horses stabled during cold, dry weather and turn other, older horses out.

If you live in a high-risk area or are concerned about your horse’s health, it’s always best to speak to your vet for tailored advice.

The EGSF has created the Grass Sickness Biobank to inform future research. The project works with specially trained vet ambassadors who can take samples from horses suffering from EGS, in addition to owner ambassadors who help collect case reports and botanical samples.

Kate Thomson from the EGSF explains: ‘You can take part in this exciting project by becoming an EGS detective. Please ask your vet to sign up as a vet ambassador and get in touch with us to sign up as an owner ambassador.’