Equine Grass Sickness: A Fatal and Poorly Understood Disease

Equine grass sickness (EGS) is a devastating and largely fatal neurological disease affecting horses on pasture across Europe and North America. Despite decades of research, the cause remains uncertain and treatment options are limited.

Key Facts

EGS is a dysautonomia — a dysfunction of the autonomic nervous system — causing failure of gut motility, excessive salivation, difficulty swallowing, and muscle tremors.
The disease is almost exclusively seen in horses at grass; indoor-kept horses are rarely affected, pointing to a pasture-associated causative agent.
Strong epidemiological evidence implicates Clostridium botulinum type C neurotoxin as the cause, but definitive proof remains elusive.
Three forms exist: acute (death within 2 days), subacute (death within 7 days), and chronic (survival possible with intensive nursing).
Chronic EGS survival rates of 40-80% have been reported with intensive supportive care including nasogastric feeding, manual gut stimulation, and analgesics.
Scotland has the highest global incidence; the Moredun Research Institute in Edinburgh leads EGS research.
Risk factors include cool, dry spring weather, recent pasture disturbance, and high stocking density on previously affected land.
Young horses (2-7 years) on lush pasture are most at risk; previously affected premises have significantly elevated risk of recurrence.

Welfare Considerations

EGS causes profound suffering in affected horses. Acute cases involve rapidly escalating pain from ileus (gut paralysis), excessive sweating, severe distress, and depression; these horses deteriorate and die within hours to days despite treatment. Subacute cases experience prolonged pain and dysphagia (difficulty swallowing) before succumbing. Even chronic cases — the most welfare-positive outcome — require weeks to months of intensive nursing care during which horses are weak, uncomfortable, and unable to eat normally. The decision to attempt treatment versus euthanasia must be made rapidly and humanely. Pain relief (NSAIDs, opioids) is essential throughout. Horses that survive chronic EGS can make full recoveries, but the process is demanding for both animal and carer.

What You Can Do

Know the early signs of EGS: tucked-up abdomen, muscle trembling, sweating, difficulty swallowing, and patchy sweating — seek emergency veterinary care immediately.
Reduce risk by avoiding pasture disturbance in spring, maintaining appropriate stocking density, and monitoring grazing conditions on premises with EGS history.
Consider vaccination with an experimental EGS vaccine if available in your region — consult your equine vet about current trials.
Support the Equine Grass Sickness Fund (EGSF) which funds ongoing research into the cause and prevention of this devastating disease.
If a horse survives chronic EGS, follow your vet's detailed nursing protocol diligently — consistency is critical for recovery.