Equine grass sickness

Attention! This is a potentially life-threatening condition for your horse. Time is of the essence, contact your veterinarian immediately.Find a Vet

Equine Grass Sickness

Equine Dysautonomia

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.

FormDescriptionClinical signs
AcuteThe 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
Difficulty swallowing
Excessive dribbling of saliva
Elevated heart rate
Colic signs
SubacuteDelayed progression of symptoms that are less severe."tucked-up" abdomen
Severe weight loss
Chronic Insidious onset of symptomsSevere weight loss
Patchy sweating
Muscle tremors
Elevated heart rate
Mild colic signs
"tucked-up" abdomen
Base narrow stance
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.


Drooping of the upper eyelid(s)
Generalized muscle weakness
Base-narrow stance
Low head and neck carriage
Leaning against wall for support
Muscle atrophy
Reduced abdominal volume
Weight loss


  • History
  • Clinical signs
  • Physical exam
  • Laboratory analysis



Mildly affected horses can survive with nursing care and a wide variety of feeds.
For severe cases, veterinarians often recommend euthanasia on humane grounds, because the disorder is very painful



EGS has a case mortality rate of approximately 85%.

Scientific Research

General Overviews

Clinical Trials

Age Range

EGS affects grazing horses, and is most frequently found in horses between 2 and 7 years of age.