Grass Sickness

Grass Sickness – Equine dysautonomia

Grass sickness is so called because it only affects horses and ponies at grass. The exact cause of this disease is unknown but has been linked to free living bacteria called Clostridium botulinum. The bacteria release botulism toxin once inside the horse’s intestines. The toxin damages the sympathetic nervous system that innervates the gut and paralyses it. Grass sickness is diagnosed all over the UK and invariably horses affected ultimately die or are euthanased.

Clinical signs may be very variable depending on the form of the disease that the horse is suffering but can include patchy sweating, swallowing difficulties, recurrent impaction colic, dullness, lethargy, weight loss and depression.

There are four forms of the disease:

  1. Per-acute. The horse is found dead in the field. There was no previous warning as to the fact that the horse was ill. Post mortem reveals substantial fluid accumulation within the stomach and the intestines.
  2. Acute. Principal symptom is severe, sudden onset colic. The horse may have patchy sweating, high heart rate and skin/ muscle tremors. Many horses have, in addition, a green nasal discharge that is associated with a difficulty in swallowing and regurgitation of stomach contents which is particularly noticeable since horses cannot normally vomit. Horses will normally die or be euthanased because of the uncontrollable recurrent colic bouts.
  3. Sub-acute. Symptoms include low-grade colic usually caused by mild impaction of the large colon which is induced by lack of intestinal motility. Typically these animals will have a high heart rate and patchy sweating, sometimes with some muscle tremors. Owners may notice some dribbling and inability to swallow properly. Faeces tend to be dry, small and scant in number.
  4. Chronic. The appearance of a horse with chronic grass sickness is akin to that of a greyhound. They develop a thin and tucked-up appearance from long periods of inappetance, swallowing difficulties and, therefore, weight loss.

Whether or not a horse survives Grass Sickness depends on the degree of damage suffered by the intestine and nervous system. Invariably, horses with the first three forms of Grass Sickness are euthanased or die. Rarely, horses with chronic Grass Sickness may survive with intensive supportive care. These animals tend to be forever prone to choke and impaction colic and require special attention and feeding.


Diagnosing Grass Sickness is not straightforward. The range of clinical signs that individuals can display is not consistent and varies according to the extent that the nervous system has been damaged. Frequently, severe colic cases are taken to surgery to discount other factors (such as a twisted gut or small intestinal blockage). Definitive diagnosis involves sampling a portion of small intestine (an ileal biopsy) during surgery, and examining it under a microscope. Grass sickness causes very specific damage that can only be identified microscopically.

One method of assessing sympathetic tone is to administer phenylephrine into one of the horse’s eyes. In a horse suffering Grass Sickness, the eye that received the drops will open slightly and the eyelashes will assume a different angle to that of the other eye. This test may be an interesting adjunct to other clinical signs but does not rule Grass Sickness in or out since results can be variable