Equine grass sickness (EGS), also known as equine dysautonomia, is a frequently fatal neurodegenerative disease affecting grazing horses, predominantly in northern European and South American countries. The cause of EGS has still not been identified, however it is believed that neurotoxins produced by the Clostridium botulinum Type C bacterium (the organism responsible for botulism) is responsible for the disease.

EGS presents itself in three forms--acute, subacute, and chronic. The acute and subacute forms of the disease have the highest fatality rates. Horses which develop the chronic form have a higher chance of survival.

Form	Description	Clinical signs
Acute	The most fatal form, associated with rapid onset and progression of signs. Horses usually die in less than 48 hours.	Depression Loss of appetite Muscle fasciculations Sweating Difficulty swallowing Pawing Excessive dribbling of saliva Elevated heart rate Colic signs
Subacute	Delayed progression of symptoms that are less severe.	"tucked-up" abdomen Severe weight loss
Chronic	Insidious onset of symptoms	Severe weight loss Patchy sweating Muscle tremors Elevated heart rate Mild colic signs "tucked-up" abdomen Base narrow stance Weakness

Currently, EGS can only be definitively diagnosed by histopathology of autonomic ganglia at post mortem or through conducting a biopsy of the enteric nervous system collected at exploratory laparotomy.