Understanding Equine Grass Sickness: Causes, Symptoms & Prevention

As spring brings fresh grasses and warmer weather, equine grass sickness becomes a critical concern for horse owners. This rare but devastating disease primarily affects horses grazing on pasture and causes severe damage to their nervous system, leading to often fatal gut paralysis. This guide explains what equine grass sickness is, identifies risk factors, outlines symptoms to watch for, and offers practical prevention advice to help protect your horse.

What is Equine Grass Sickness?

Equine grass sickness, also known as equine dysautonomia, is a mysterious neurological illness affecting the peripheral and autonomic nervous systems of grazing horses. It mainly disrupts involuntary functions that control digestion and swallowing, causing severe gut paralysis, colic, weight loss, and often death. Although the exact cause remains unknown, research suggests a link to a neurotoxin produced by the soil-borne bacterium Clostridium botulinum type C. Horses may ingest this bacterium or its toxins while grazing on contaminated pasture, triggering nerve damage and disease.

Grass sickness mostly affects younger adult horses aged two to seven but can impact any equine, including ponies and donkeys. The disease usually appears in outbreaks during times of new grass growth, especially in spring (April to June) and autumn, when pasture conditions change.

For owners considering new horses, it's important to understand the risks of grass sickness, especially if sourcing from or moving to higher-risk areas. Selecting healthy grazing environments and responsible premises management play a vital role in prevention. For those looking to acquire a horse for sale, knowing about grass sickness risk is part of responsible ownership.

Types of Equine Grass Sickness

Equine grass sickness manifests in three clinical forms, each differing in severity and progression:

• Acute grass sickness: Rapid onset with severe gut paralysis, often fatal within 48 hours. Horses show dramatic colic symptoms, drooling, constipation, and muscle tremors.
• Subacute grass sickness: Similar signs to acute but less severe initially. The condition progresses quickly, and euthanasia is commonly recommended.
• Chronic grass sickness: Slower onset with milder symptoms like intermittent colic, weight loss, and swallowing difficulties. Some horses recover with intensive care.

Recognising Symptoms and When to Contact a Vet

Early identification is essential. Symptoms vary by type but generally include:

Acute and Subacute Symptoms

• Sudden severe colic with gut paralysis
• Difficulty swallowing and excessive drooling
• Distended stomach and foul nasal discharge
• Constipation with hard, mucus-coated droppings
• Muscle tremors and patchy sweating

Chronic Symptoms

• Gradual weight loss and poor body condition
• Mild, intermittent colic
• Swallowing problems and decreased appetite
• Tucked-up abdomen and muscle tremors

If your horse exhibits any of these signs, contact your vet immediately. Prompt diagnosis can improve management options, especially for chronic cases, and can prevent unnecessary suffering.

Risk Factors and Causes

Although the disease's cause remains unclear, multiple risk factors may increase the likelihood of grass sickness:

Horse-Related Factors

Mostly affects horses aged two to seven but can occur at any age. No specific breed predisposition has been confirmed, although older studies suggested heavy draught breeds might be at greater risk, current research disproves this.

Management-Related Factors

Equines grazing on pasture are primarily affected. Changes in diet, frequent moving between pastures, and frequent worming treatments can increase risk. Moderate worm egg counts seem to reduce susceptibility. Discuss worming programmes with your vet to optimise parasite control without raising risk.

Premises and Environmental Factors

Pastures with a history of grass sickness cases pose higher risk. Premises like livery yards, stud farms, and riding schools often have soils with high nitrogen levels and possibly disturbing activities like construction, which increase risk. Presence of poultry or other fowl on site also elevates the risk. Certain soil types including sandy or loamy soils may harbour the causative bacterium.

Seasonal and Climatic Factors

Outbreaks correlate with new grass growth in spring and autumn. Incidences spike in May. Abrupt weather changes like cold, dry conditions or ground frost also coincide with outbreaks, indicating environmental shifts play a role.

Is There a Cure?

Currently, no cure exists for equine grass sickness. Acute and subacute cases typically result in euthanasia due to poor prognosis and severe suffering. Chronic cases may survive with intensive supportive care, including nutritional support and management of swallowing difficulties, but recovery is uncertain and demanding.

Equine grass sickness is not infectious and cannot spread from horse to horse. Research continues into neurotoxins produced by Clostridium botulinum and the role of antioxidants in equine susceptibility.

Prevention: How to Reduce Risk

While prevention can be challenging, owners can implement strategies to decrease risk:

• Avoid grazing horses on pastures with previous cases of grass sickness
• Minimise disturbance of soil and grass in pastures; avoid construction or heavy maintenance during risk periods
• Prevent overgrazing and avoid poached or muddy paddocks
• Avoid sudden changes in diet or pasture
• Use chemical wormers advised by your vet, rotating products and preventing overuse
• Remove horse droppings manually rather than mechanically to reduce soil disturbance
• Regularly top up paddocks with fresh soil or seed
• Feed supplementary hay or haylage alongside pasture
• Avoid mixed grazing with sheep or cattle if possible

Summary

Equine grass sickness remains a complex and distressing disease mainly affecting grazing horses in the UK during times of new grass growth. Its unknown exact cause and lack of cure make prevention and early detection vital. Horse owners must maintain good pasture management, monitor health carefully, and seek veterinary advice promptly if symptoms appear. By understanding risk factors and how to minimise them, you can help protect your horse from this devastating condition.