Equine Grass Sickness

What equine grass sickness is, the signs to look for, why you must phone your vet immediately and the steps that may reduce the risk.

Quick Answer

Equine grass sickness is a serious, frequently fatal disease of grazing horses, ponies and donkeys, in which nerve damage stops the gut working properly.
If you suspect it, contact your vet immediately, because signs of the acute form can appear very rapidly.
Signs include colic, difficulty swallowing, drooling, muscle tremors, patchy sweating and weight loss.
It can look like colic or choke, so only a vet can tell what is wrong. Do not try home remedies or wait to see if the signs pass.
The cause is not fully understood and is still being researched, and there is currently no vaccine.

Equine grass sickness is a debilitating disease that affects grazing horses, ponies and donkeys. It damages the nerves that control the gut, so food cannot move through the digestive system as it should.

This guide explains the signs, why a vet must see the horse straight away and what is known about risk. It is written with care, because the condition is often very distressing for owners.

Dark brown horse grazing on a sunlit grass field with a pale pinto horse behind – equine grass sickness affects grazing horses

What is equine grass sickness?

The damaged nerves cause gut paralysis. As a result, fluid builds up in the stomach and small intestine, the large intestine can become blocked and the horse becomes dehydrated.

It does not appear to be contagious, and there is no human equivalent. The disease was first described in Scotland in the early 1900s.

The cause is not fully understood. World Horse Welfare says the true cause remains unknown, and research is continuing. Because of this, no one can say for certain how to prevent it.

The three forms of grass sickness

UK sources describe acute, sub-acute and chronic forms. Their signs overlap, and they reflect a scale of severity.

Acute

The signs are severe and appear suddenly. They include colic, difficulty swallowing and drooling. The stomach may swell with foul-smelling fluid that can pour down the nose.

Any droppings passed may be small and hard. Fine muscle tremors and patchy sweating are also seen. One UK source says the horse will die or need to be put down within two days of onset.

Bay horses and a foal grazing on bright green grass – equine grass sickness occurs almost only in horses with access to grass

Sub-acute

The signs are similar but less severe. They include difficulty swallowing, mild to moderate colic, sweating, muscle tremors and rapid weight loss.

A horse may still eat small amounts of food. Even so, such cases may die or need to be put down within about a week.

Chronic

Signs develop more slowly. They include a reduced appetite, varying difficulty swallowing, rapid and severe weight loss and a markedly tucked-up belly.

Some horses show mild, intermittent colic. One source also mentions standing with the feet close together.

Signs of equine grass sickness to look for

Not every horse shows every sign. UK sources list these:

Colic signs, such as rolling, pawing the ground or looking at the flanks
Difficulty swallowing, refusing food or excessive drooling
Foul-smelling green fluid or stomach contents coming down the nose
Dried, inflamed nostrils
Drooping eyelids
Muscle tremors and patchy sweating
A fast heart rate and depression
A tucked-up belly and rapid, unexplained weight loss

Many of these signs also occur in other conditions. For example, see Signs of Colic in Horses and Choke in Horses. A vet may find it difficult to tell grass sickness from other causes, so you cannot decide this yourself.

Emergency: Contact a Vet Immediately If…

A horse at grass has colic signs, especially if there are no gut sounds.
Your horse has difficulty swallowing, refuses food or is drooling.
Foul-smelling green liquid is coming down the nose.
You see muscle tremors, patchy sweating, drooping eyelids or a fast heart rate.
Your horse is losing weight rapidly, with a tucked-up belly.

Phone your vet or the nearest out-of-hours emergency vet straight away if you suspect grass sickness. Do not wait to see whether the signs pass.

When to Contact Your Vet

Immediately, if you suspect your horse is showing signs of grass sickness.
If your horse has been losing weight quickly or has trouble eating, even without colic.
If another horse on the same premises has been diagnosed. Ask your vet about your other horses.

World Horse Welfare advises contacting your vet immediately if you suspect your horse has signs of grass sickness. Do not try a home treatment first.

Who is most at risk?

These are patterns seen in cases. They do not explain the cause.

Grass access. The disease occurs almost only in horses with access to grass, but it can affect horses that have only a few minutes' access a day. Very rare cases without grass have been linked to hay.
Age. It is mostly seen in young adult horses. Young foals are very rarely affected.
Season and weather. Cases are most common in spring and early summer, and often follow cool, dry weather. However, cases can occur at any time of year.
Recent stress or change, such as a recent purchase, mixing with unfamiliar horses or long-distance travel.
Time on the premises. Horses on an affected premises for less than two months are more likely to develop it.
Good to fat condition also appears to raise the risk.

Usually one horse is affected at a time. However, several cases within a few weeks are not unusual.

How vets diagnose grass sickness

Diagnosis can be complex. Your vet will examine the horse and work to rule out other forms of colic.

Supporting tests can include blood, urine, faecal and abdominal fluid samples, an ultrasound scan and endoscopy. An eye-drop test that reverses drooping eyelids has shown potential too. However, none of these confirms the diagnosis.

A definite diagnosis comes from examining a piece of small intestine removed by surgery, or from a post-mortem. If a horse does not survive, a post-mortem is worthwhile because a definite diagnosis helps protect horses that graze alongside it and supports research.

Treatment and outlook

There is currently no cure. Treatment of suitable cases is nursing and nutritional support. Some drugs are used only to treat signs and relieve pain, and your vet decides which.

Horses with the acute, sub-acute and severe chronic forms will usually be put to sleep on welfare grounds, to prevent further suffering. This is a very hard decision, and your vet will talk it through with you. The BHS says its Friends at the End volunteers are on hand if you would like to talk to somebody independent.

Some chronic cases can be nursed. They usually still have some ability to swallow, some appetite and no continuous moderate to severe abdominal pain.

Nursing is skilled and time-consuming. It means warm, dry stabling, frequent feeding of palatable foods and as much owner contact as possible. Owners need to know about the long time-frame, the potentially high cost and that success is not guaranteed.

Reducing the risk of equine grass sickness

No vaccine is currently available. The steps below may reduce the risk, but Moredun notes no prospective studies have tested them, so they are not guarantees. Discuss them with your vet.

Limit grazing on land where previous cases have occurred.
Minimise soil and pasture disturbance, such as harrowing or building work.
Avoid grazing the field right down to bare soil.
Avoid sudden changes to the diet, and supplement with extra forage such as hay.
Co-graze with ruminants such as sheep.
Remove droppings by hand rather than mechanically.
Follow a testing-led worming programme agreed with your vet. See Worming Your Horse.

Take most care with high-risk horses, such as young adults and new arrivals, in spring and early summer. Stabling in high-risk areas at that time is one option to discuss with your vet.

The Equine Grass Sickness Fund offers weather-risk alerts for UK owners. You can also support research by reporting a case or by taking part in the biobank with your vet.

How Pet Med Direct Can Help

Suspected grass sickness is a situation for your vet first. Please phone your vet immediately, before you buy or give anything.

No vaccine is available, and none of the UK sources we read describe any product as preventing grass sickness. If your vet prescribes a medicine, how pet prescriptions work explains the process. Prescription-only medicines (POM-V) are supplied solely against a valid prescription from the horse's own vet. You can also browse our equine range.

Some pet medicines require a valid veterinary prescription. Pet Med Direct can only supply prescription medicines after the prescription has been checked. Always follow your vet's advice and the directions on the medicine label.

Frequently Asked Questions

What is grass sickness?

It is a disease in which nerve damage stops the gut working properly, so food cannot travel through the digestive system. It affects grazing horses, ponies and donkeys.

Is grass sickness contagious?

UK sources say it does not appear to be contagious, and it does not affect people.

Can grass sickness be treated?

There is no cure. Some carefully selected chronic cases can be nursed, over a long time-frame and with no guarantee of success. Horses with the acute and sub-acute forms will usually be put to sleep on welfare grounds to prevent further suffering.

When is grass sickness most common?

Spring and early summer, with the BHS giving April to June. Cases can occur at any time of year, and they often follow cool, dry weather.

Can a stabled horse get grass sickness?

It is almost always seen in horses with access to grass. Very rare cases without grass access have been linked to hay.

Does worming cause grass sickness?

One UK source says owners should not cut back on worming because of grass sickness, as not worming can be serious or fatal. Instead, follow a testing-led programme with your vet.

Can I tell grass sickness from colic myself?

No. It can look like colic or choke, and even vets can find it hard to tell them apart. Phone your vet immediately.

Related Equine Health Guides

Sources and Further Reading

This article was researched from the sources listed above. It is general information for horse owners and is not a substitute for advice from your own vet.

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This article is for informational purposes only and does not constitute veterinary advice.
If you are concerned about your horse’s health, please consult a qualified veterinarian.