What You Need To Know About Grass Sickness in Horses

Grass sickness in horses is one of the most distressing conditions a horse owner can face. It strikes apparently healthy animals at grass, often with no warning, and in most cases it kills them. More than a century after it was first recorded, the exact cause is still not settled, although a major breakthrough in 2024 has changed what researchers believe is happening. This guide explains what the disease is, what causes it, how to recognise it early, how vets diagnose and treat it, and the practical steps that lower the risk on your own yard.

What Is Grass Sickness in Horses?

Grass sickness, also called equine dysautonomia, is a disease that damages the parts of the nervous system controlling involuntary functions such as gut movement, secretion and heart rate. The most obvious result is paralysis of the digestive tract, which stops food passing through the gut normally. The nerve damage is not confined to the gut, which is why affected horses also sweat patchily, tremble and develop drooping eyelids.

The disease affects horses, ponies and donkeys, and occurs almost exclusively in animals with access to grazing. The British Horse Society notes that a small number of cases have occurred in horses with no access to grass, where hay was found to be the source.

How the disease damages the body

When the autonomic nerves supplying the gut are destroyed, the digestive tract slows or stops. Fluid builds up in the stomach and small intestine, the large intestine becomes blocked and the horse dehydrates quickly. In the most severe cases the stomach distends so badly that foul smelling fluid runs down the nose. Because the damage is widespread rather than local, the signs extend well beyond the abdomen.

Where grass sickness occurs and how common it is

Great Britain records the highest number of cases in the world. The British Horse Society notes that cases are more frequent nearer the east coast than the west, and that Scotland is particularly affected. Cases also occur across northern Europe, and World Horse Welfare reports that the disease was first identified around 1909, having been killing grazing horses since the early 1900s. A 2025 review of dysautonomia across species in the Journal of Veterinary Internal Medicine dates that first identification to Scotland in 1907, from J. F. Tocher's account published in 1924.

In terms of scale, the Royal (Dick) School of Veterinary Studies reports that grass sickness leads to death in about 1 per cent of horses grazing on high risk premises. That figure sounds small until you consider that it applies to entire fields of otherwise healthy animals, year after year, with no reliable way to predict which horse will be affected.

What Causes Grass Sickness in Horses?

This is the question owners ask most often, and the honest answer is that the precise cause of grass sickness in horses is still not proven. What has changed is that researchers now have a strong lead, and it is not the one that dominated the field for decades.

The neurotoxic enzyme theory, the current front runner

In 2024 the Royal (Dick) School of Veterinary Studies announced that grass sickness is likely to be caused by a neurotoxic enzyme called phospholipase A2, produced by microorganisms living on pasture. Electron microscopy of muscle tissue from affected horses showed major abnormalities at the neuromuscular junctions, the sites where nerves communicate with muscles. The team believes this damage causes the paralysis of the whole gastrointestinal tract, and with it the inability to swallow, the gut distension, the colic and the muscle tremors.

The enzyme has a similar action to toxins found in snake venoms, which explains why the signs of grass sickness and the signs of snakebite have so much in common. There is no suggestion that snakes are involved. The practical hope is that drugs being developed to help people paralysed by snake venom might one day help horses recover. Work continues to pin down which microbe produces the toxin, and researchers expect it to be a bacterium or fungus growing on grass during the cold, dry weather that typically precedes outbreaks.

The study was published in the Equine Veterinary Journal on open access, and was carried out with Newcastle University, the Istituto Zooprofilattico Sperimentale delle Venezie and the University of Padova, funded by the Royal College of Veterinary Surgeons Charitable Trust and the Moredun Foundation Equine Grass Sickness Fund.

Why the botulism theory has been displaced

For many years the leading hypothesis was that grass sickness was a form of botulism caused by Clostridium botulinum type C, a soil borne bacterium. The evidence was real. The Dick Vet Equine Practice fact sheet describes a study of 30 horses with grass sickness and 77 healthy controls in which type C neurotoxin was detected in 74 per cent of acute cases and 67 per cent of subacute and chronic cases, often in large amounts, compared with 10 per cent of the control horses.

The 2024 neuromuscular junction findings directly contradict that theory, and the Dick Vet now states as much. This matters if you are reading older material: many articles, leaflets and forum posts still present grass sickness as a botulism-like disease. It is worth understanding how the two conditions differ, and our entry on equine botulism covers that disease in its own right.

Why one horse falls ill and its field companion does not

No single factor explains grass sickness on its own. The British Horse Society lists several likely contributors acting together, including heritability, weather conditions such as cool dry spells with irregular frosts, soil disturbance, pathogens and fungi associated with soil and plants, the horse's age, immune system, nutrition and gut microbes, and its stress levels. That combination model is the best explanation for the pattern owners find hardest to accept, where one horse dies and the companion sharing the same paddock and the same grass stays perfectly well.

The Three Forms of Grass Sickness

Grass sickness occurs in three forms. The signs overlap considerably, and the form largely determines whether survival is even possible.

Acute grass sickness

Acute cases appear suddenly and are the most severe. The horse shows signs of severe colic including rolling, pawing and looking at its flanks, along with difficulty swallowing and drooling of saliva. The stomach may become grossly distended with foul smelling fluid that can pour down the nose. Constipation develops further down the gut, and any dung passed tends to be small, hard pellets with a cheesy coating of mucus. Fine muscle tremors and patchy sweating are common. The Dick Vet fact sheet is unambiguous that this form is fatal and that the horse should be put down once the diagnosis is made, usually within two days of the first signs.

Subacute grass sickness

Subacute cases resemble acute ones but are less intense. Fluid may not accumulate in the stomach, and the horse may still eat small amounts, but it shows difficulty swallowing, mild to moderate colic, sweating, muscle tremors and rapid weight loss. These cases typically die or require euthanasia within a week.

Chronic grass sickness

Chronic cases develop more slowly. Colic, where present, is mild and intermittent, and salivation, gastric fluid and severe constipation are not features. The defining sign is rapid and severe weight loss that can progress to emaciation, together with a tucked up abdomen and a stance in which the horse stands with its feet drawn close together. This is the only form in which treatment is ever attempted, which is precisely why recognising it early matters so much.

Signs and Symptoms of Grass Sickness in Horses

Spotting the signs of grass sickness in horses early gives a chronic case its best chance, and gives an acute case a faster route to a humane decision. The British Horse Society lists the following signs across the three forms.

• Excessive salivation and drooling
• Difficulty swallowing, or refusing to eat
• Signs of colic with no gut sounds
• A very bloated stomach
• Foul smelling green fluid running from the nostrils
• Dried, inflamed or crusted nostrils
• Drooping eyelids
• Patchy sweating
• Fine muscle tremors
• Increased heart rate
• Depression and dullness
• Rapid, otherwise unexplained weight loss
• A tucked up abdomen, as though the horse is sucking its stomach in

Early signs worth acting on

The early picture is often vague. A horse that has gone quiet, is picking at its feed rather than eating properly, is dropping weight faster than its workload explains, and has a heart rate higher than its apparent level of discomfort would suggest, deserves a veterinary examination that day. Drooping eyelids and patchy sweating are particularly useful because they are difficult to explain away as ordinary colic.

Signs that point to the more severe forms

A grossly bloated stomach, green fluid at the nostrils and continuous severe pain point towards the acute or subacute forms and carry a hopeless prognosis. Signs of acute grass sickness can appear very rapidly, so if you see this combination, call your vet as an emergency rather than waiting to see how the horse is in the morning.

Conditions That Look Like Grass Sickness

Part of what makes grass sickness so difficult is that no single sign is unique to it. The Dick Vet fact sheet acknowledges that it can be very hard for a vet to distinguish grass sickness from other causes of colic, difficulty swallowing and weight loss. These are the conditions most often in the frame.

• Ordinary colic, which can produce similar pain and posture but not the swallowing difficulty, drooping eyelids or rapid weight loss. Our guide to spasmodic colic covers the most common intermittent form.
• gastric dilatation and rupture, where extreme distension of the stomach produces reflux and severe pain, closely mimicking the acute form.
• guttural pouch mycosis, a fungal infection that damages the cranial nerves and is a genuine cause of difficulty swallowing in horses.
• peritonitis, which produces colic signs, dullness, fever and weight loss from an abdominal source.
• Choke, where an oesophageal obstruction causes drooling and food at the nostrils without the wider nerve signs.
• Poisoning from trees and plants that are toxic to horses, which can cause colic, weakness and sudden collapse in animals at grass.

The distinguishing feature of grass sickness is the combination. A horse showing colic plus difficulty swallowing plus drooping eyelids plus patchy sweating plus muscle tremors plus rapid weight loss, with a heart rate out of step with its apparent pain, is a very different proposition from a horse with one of those signs alone.

Which Horses Are Most at Risk?

Grass sickness does not fall evenly across the horse population, and knowing where the risk concentrates helps you decide when to be vigilant.

Age, season and location

The British Horse Society reports that the disease can affect a horse of any age but is most common in young adults between two and seven years old. Cases can occur in any month, but are most common between April and June. Geographically, Great Britain carries the highest case numbers worldwide, with more cases nearer the east coast than the west and a particular concentration in Scotland. Cool, dry weather with irregular frosts is the climatic pattern most often recorded before cases appear.

Premises, management and stress

Particular properties, and sometimes particular fields within a property, produce cases repeatedly. The Dick Vet groups the recognised risk factors into horse related, premises related and management related, and advises that where resources are limited, protective changes should be prioritised towards the highest risk animals, namely young adults, new arrivals and horses kept in show condition, during the spring and early summer peak and during cool, dry spells.

Stress and change appear repeatedly in case histories. Horses recently bought or moved, transported, mixed with unfamiliar horses, broken in or castrated are over represented. Sudden changes in diet and recent soil disturbance are also linked. In practice these factors stack: moving a four year old to a new yard, turning it straight out onto a strange paddock and changing its hard feed in the same week combines several of them at once.

How Grass Sickness Is Diagnosed

There is no simple, non invasive test that confirms grass sickness in a living horse, and this is the single biggest frustration for owners and vets alike. Diagnosis rests on the horse's history and the pattern of clinical signs, supported by tests that narrow the field rather than settle it.

What your vet can do on the yard

Your vet will assess gut sounds, heart rate, hydration, swallowing and the presence of gastric reflux, and will look specifically for the eyelid droop, sweating pattern and tremors that separate grass sickness from ordinary colic. Blood tests and analysis of abdominal fluid help rule out other causes. There is also an eye drop test, in which dilute phenylephrine is applied to one eye and the eyelid angle is compared with the untreated eye, and a response supports the diagnosis without confirming it on its own.

How a definite diagnosis is reached

A definite diagnosis can only be made by microscopic examination of nerve tissue. In practice that means either surgical removal of a small piece of small intestine, which requires opening the abdomen, or examination of the autonomic nerve ganglia after death. Characteristic degenerative changes in the nerve cells confirm the disease. Because surgery on a horse that may be dying anyway is a serious decision in its own right, many cases are managed on strong clinical suspicion.

Treatment and Recovery

Treatment is attempted only in chronic cases. Acute and subacute cases, and severe chronic ones, do not recover, and euthanasia is the correct decision to prevent further suffering. There is no cure for grass sickness, and treatment is nursing and nutritional support rather than a drug that fixes the underlying damage.

Which cases can be treated

The Dick Vet is clear that selecting treatable cases correctly is essential and takes experience, because not all chronic cases are treatable. The criteria are practical and observable.

1. The horse retains some ability to swallow.
2. It still has an appetite and will eat at least a small amount.
3. It is not in continuous abdominal pain.
4. It remains interested in its surroundings.

Feeding and nursing a chronic case

Feeding is where owners most often go wrong. Chronic cases need palatable, easily swallowed, high energy food such as chopped vegetables, grass and concentrates soaked in molasses. The Dick Vet fact sheet states plainly that chronic cases fed on roughage and succulents alone will invariably die, so simply offering good hay and hoping is not a treatment plan.

Nursing is the mainstay. These horses need constant stimulation through human contact, frequent grooming to stop them becoming scurfy and sticky with sweat, and in many cases rugging, which reduces sweating and prevents hypothermia. This is labour intensive work over weeks and months, and it is worth being honest with yourself about whether your yard can provide it before starting.

What the recovery figures actually say

Prognosis is where published figures diverge, and the difference matters if you are deciding whether to commit. The British Horse Society states that around 50 per cent of chronic cases may survive if they are suitable for intensive nursing and veterinary treatment. World Horse Welfare puts it at around 40 to 50 per cent with intensive nursing. Those are the numbers to plan around.

A higher figure is often quoted. The Dick Vet Equine Hospital at Easter Bush reports a recovery rate of approximately 70 per cent, but that is a specialist referral hospital reporting on carefully selected cases, and it is not the general prognosis for a chronic case on an ordinary yard. Treat 40 to 50 per cent as your working expectation and the referral figure as a best case achieved under ideal conditions.

The outlook for survivors is genuinely encouraging, and it corrects an old belief that recovered horses were never much use afterwards. In the Dick Vet follow up, 41 per cent of recovered cases were back in work including hunting, racing and eventing, 33 per cent were being hacked, prepared for competitive work or used for breeding, and the remaining 26 per cent, which were the more recent cases, were still gaining weight and recovering when the survey was done. None of the survivors was described as being of no use.

Owners should still expect a long and expensive process with no guarantee. If your horse is insured, check the policy early. An honest conversation between owner, vet and nursing team about time, cost and likely outcome should happen before treatment starts, not three weeks into it.

How to Prevent Grass Sickness in Horses

Until the cause is confirmed there is no vaccine and no way to make a pasture safe. A pilot field vaccine trial has been carried out, but nothing is licensed and available to owners. Prevention therefore means reducing the recognised risk factors and increasing the protective ones.

Risk factors to avoid

• Minimise exposure to pastures where previous cases have occurred.
• Minimise pasture and soil disturbance. This includes harrowing a paddock, mechanical droppings removal, pipe laying and construction work.
• Minimise soil exposure, so avoid grazing fields down to bare earth and avoid poaching.
• Avoid sudden changes in diet, in both quantity and feed type.
• Follow a testing led worming programme rather than dosing routinely, to avoid unnecessary use of wormer drugs.

Protective measures worth putting in place

• Co-graze with ruminants such as sheep or cattle.
• Cut pasture grass regularly.
• Remove droppings by hand rather than mechanically.
• Feed supplementary forage such as hay or haylage to horses at grass.
• Stable high risk horses during the peak season on premises with a history of cases.

If a case occurs in a group, it is usually sensible to move the remaining horses off that field, provided the move itself does not create more stress than it removes. More general grazing and turnout guidance sits in our horse care library.

Use the weather risk alert system

One genuinely useful development that many owners still do not know about is the Equine Grass Sickness Fund weather risk alert system, run with the Moredun Foundation and supported by the British Horse Society. Owners can sign up to receive alerts for their local area when weather conditions associated with grass sickness are forecast, which gives you a reason to bring high risk horses in or increase supplementary forage at exactly the right moment.

There is also a nationwide biobank collecting samples from affected horses along with soil samples and case questionnaires, run by the Moredun Foundation and the Equine Grass Sickness Fund with British Horse Society support. If you lose a horse to grass sickness, contributing to it is one of the few constructive things available to you.

Frequently Asked Questions

Grass sickness in horses remains one of the hardest diseases in the equine world to predict, prevent or treat. The 2024 neurotoxin finding is the most significant progress in decades and may eventually produce a diagnostic test and a treatment. Until then, careful grazing management, awareness of the seasonal and weather patterns, and acting quickly at the first combination of signs remain a horse owner's strongest defence.