Equine Grass Sickness is a poorly understood and often fatal intestinal disease that primarily affects European (especially British) horses at pasture. It affects any breed, including donkeys and mules.

Grass Sickness remains a great mystery despite extensive research. Currently the cause is unknown, however bacterial toxins are suspected to be the cause of this disease. The most likely causative agent for grass sickness is now considered to be the soil organism Clostridium botulinum. Toxin produced by this bacteria (the Botulism agent) is thought to be ingested at pasture.

The condition causes paralysis of the digestive system. The intestines stop moving and fill with fluid and gas. Fluid may back up from the paralyzed stomach and drain from the nostrils. Signs of this disease are usually impossible for horse owners to distinguish from other conditions that cause general colic signs.

There are three main forms of this disorder: acute, subacute and chronic. Acute and sub-acute cases develop rapidly and severely, with a very low survival rate and show obvious colic signs. Horses with the milder, chronic form of this disease have a higher chance of survival. The main sign of the chronic form is weight loss. Treatment of mild chronic cases is nursing care, time and the provision of palatable, soft, easily swallowed food.

Definitive diagnosis requires biopsy of the intestine. Careful colic diagnostics and a compatible history may be used to diagnose this disease.