Understanding Equine Grass Sickness

Equine Grass Sickness is one of the most devastating diseases affecting horses. Although relatively uncommon, it is often fatal and every horse owner should have a basic understanding of the condition and its warning signs.

What Is Equine Grass Sickness?

Equine Grass Sickness (EGS) is a disease affecting horses, ponies and donkeys in which parts of the autonomic nervous system become damaged. This leads to paralysis of the digestive tract, resulting in severe gastrointestinal dysfunction.

The exact cause remains unknown, although the disease occurs almost exclusively in grazing horses.

Who Is Most at Risk?

Great Britain has the highest recorded incidence of Equine Grass Sickness in the world. Cases occur throughout England, Scotland and Wales, with some eastern counties considered higher risk.

The disease can affect horses of any age but is seen most commonly between two and seven years old, with a peak incidence at three to four years of age.

Although cases occur throughout the year, most are diagnosed between April and July, particularly during May.

Possible Risk Factors

Although the exact cause remains unknown, research has identified several factors that may increase risk:

· Access to pasture.

· Soil disturbance or high soil nitrogen levels.

· Stress, including transport, recent purchase, weaning, breaking or castration.

· Higher stocking densities.

· Mechanical droppings collection.

· Presence of domesticated birds.

· Good to overweight body condition.

Several theories have been proposed, including involvement of Clostridium botulinum, but no single cause has yet been proven. Current evidence does not support reducing routine worming programmes.

Clinical Signs

Equine Grass Sickness occurs in three recognised forms: acute, subacute and chronic. Although they overlap, all involve varying degrees of gut paralysis.

Acute

Sudden onset, severe colic, difficulty swallowing, salivation, stomach distension with foul-smelling reflux from the nostrils, constipation, muscle tremors and sweating. Acute cases are almost invariably fatal within 48 hours.

Subacute

Similar signs but generally less severe. Horses often develop weight loss, difficulty swallowing and intermittent colic. Many deteriorate within a week.

Chronic

Gradual onset with reduced appetite, progressive weight loss, varying difficulty swallowing and occasional mild colic. Some carefully selected chronic cases may survive with intensive nursing, although the prognosis remains guarded.

Diagnosis

If Equine Grass Sickness is suspected, immediate veterinary examination is essential. Diagnosis is based on the clinical signs, examination findings and, in some cases, further diagnostic testing.

Prevention

Because the cause remains unknown, there is no guaranteed method of prevention.

· Stable horses during higher-risk periods in affected areas where practical.

· Introduce new horses carefully and minimise unnecessary stress.

· Graze higher-risk fields with other livestock where appropriate.

· If a case occurs, discuss management of remaining horses with your veterinary surgeon.

Summary

· Equine Grass Sickness affects the nervous system controlling the digestive tract.

· The exact cause remains unknown.

· Most cases occur in grazing horses during spring and early summer.

· Early veterinary assessment is essential.

· Despite advances in research, prevention remains challenging.

A Final Thought

Equine Grass Sickness remains one of the most heart-breaking diseases seen in horses. Awareness of the condition cannot prevent every case, but recognising the warning signs early ensures affected horses receive prompt veterinary attention and supports ongoing research into this devastating disease.

For further information and the latest research, visit the Equine Grass Sickness Fund.

About the HØØF Knowledge Hub

The HØØF Knowledge Hub exists to provide horse owners with clear, practical and evidence-based information on hoof health, lower limb conditions and equine welfare. Every horse is an individual, and no online article can replace professional veterinary assessment.

Article Information

Author: Paul Conroy BSc (Hons) AWCF
Organisation: HØØF Equine Hoof Care
Category: Equine Health
Document Reference: HKH014
First Published: July 2026
Last Reviewed: July 2026