Grass Sickness

Grass sickness was first diagnosed in Eastern Scotland in 1909 but still a “mysterious” disease.

Thought to be related to Clostridia bacteria but also suspicion about clover species.

Colic signs of some horses that have access to pasture.

It is a disease of the Autonomic Nervous System i.e. the part of the nervous system that carries out subconscious or “automatic” actions such as gut movement, breathing etc.


Acute- Sudden onset

Sub-Acute- slightly longer in onset

Chronic- longer term but less severe signs

Clinical Signs

  1. Acute

Colic, lack of gut sounds, not eating, high heart rate, muscle twitching/tremors, irregular patchy sweating, drooping eye-lids (ptosis),

  1. Sub Acute

Slightly less severe clinical signs as acute

  1. Chronic

Weight loss, tucked-up (greyhound-like) appearance of abdomen, not eating, dry & inflamed nasal passages (rhinitis sicca)


  1. Ileal biopsy

Biopsy of the lymph nodes (“glands”) from the area of the small intestine known as the ileum, This is the only definitive disease

  1. Phenylephrine eye drops

This reduces the drooping eyelids that can be seen with this disease


  1. No treatment for this disease. Euthanasia is the best option.
  2. Sub-Acute. Realistically euthanasia is the best option.
  3. These can be nursed back to a reasonably normal life although they tend to be “poor doers”. This takes huge commitment and a lot of effort by everyone.

Risk Factors

  1. Areas or regions of the country (Scotland)
  2. Specific farms (where previous history of cases)
  3. Age of horses (peak is 3-4 years of age)
  4. Time of year- peak late summer (May) and smaller peak in Autumn
  5. Horses turned out day and night
  6. Soil types (high nitrogen content)
  7. Harrowing or automatic poo pickers (soil disturbance)
  8. Weather (2 weeks of cool & dry weather)
  9. Over-grazing
  10. New arrivals on a yard
  11. Stress- travelling, castration, breaking, new arrivals to herd
  12. Body condition- good to fat


Good pasture management

Manual poo-picking (NOT automatic poo-pickers)

Not over-grazing

Ensure adequate grass or provide additional hay/haylage

Worm control

Co-graze with cattle/sheep

Good horse health

Worm control

Good gut health


Low stress

Avoid over use of ivermectin wormers


Vaccination trials have been done recently at the Animal Health Trust with the aim to produce a commercial vaccine.

The future will hopefully yield a vaccine against Clostridia bacteria which should prevent or help to prevent Grass Sickness.