Equine grass sickness (EGS), or equine dysautonomia, is a rare and fatal disease in horses. It almost exclusively affects grazing horses kept on pasture.

EGS is characterized by the development of severe lesions on the neurons of the peripheral and central nervous systems. [2][7] Symptoms vary in severity depending on the neuronal degeneration in the horse.

EGS results in loss of normal function of the gastrointestinal tract, affecting the horse’s ability to swallow and digest food. [10][11] This disease results in a decrease in gut motility, increasing the risk of colic and causing severe weight loss.

Although the exact cause of EGS is unknown, it is believed to result from a combination of environmental factors and bacterial infection.

While there is currently no cure for this condition, preventative management practices may help protect your horse from this dangerous disease.

Equine Grass Sickness

Horses with Equine Grass Sickness are believed to develop enteric neuropathy from ingesting a neurotoxin while grazing on pasture (mycotoxicosis). [11]

The condition causes gut dysfunction, impairing the horse’s ability to digest and absorb nutrients from feed.

EGS can present in an individual horse, or in multiple horses at once as part of an outbreak.

Currently, researchers suspect that equine grass sickness may be caused by a toxin-producing bacterium called Clostridium botulinum type C. [5] This bacteria lives in soil and is linked to botulism – a neuroparalytic disease.

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Types of Equine Grass Sickness

EGS occurs in three forms – acute, subacute and chronic – reflecting the severity of nerve damage. [10]

While these categories are helpful when making a diagnosis to determine the horse’s prognosis, a horse’s symptoms can worsen and change over time. [4]

Acute EGS

Acute (“severe”) cases of EGS can be identified by the sudden onset of symptoms. Horses typically present with mild or moderate abdominal pain and nasogastric reflux, in which fluid in the intestine returns to the stomach. [8] This can lead to fluid distention.

Unfortunately, acute EGS is always fatal, and horses usually die or require euthanasia within 48 hours of symptom onset.

Subacute EGS

The symptoms of subacute (“moderate”) equine grass sickness are similar to acute symptoms, but occur gradually and are less severe.

Gastric reflux or gut distention are rare in subacute cases. The horse may be able to consume small amounts of feed, but this does not mean that the horse’s health will improve.

Horses with subacute EGS often die or require euthanasia within two to seven days, but this varies depending on the severity of symptoms.

Chronic EGS

Chronic EGS (“mild”) has a gradual onset and is not always fatal. The most common symptom of chronic EGS is weight loss or cachexia (wasting away).

Affected horses can be found standing in a tucked up posture indicative of gut discomfort. Unlike acute cases, horses with chronic EGS usually have an empty intestinal tract with no fluid distention or build-up. [1].

Horses may develop rhinitis sicca, otherwise known as a dry and crusty nose, due to alterations in the nasal mucosa caused by autonomic dysregulation.