What is grass sickness in horses?

Grass sickness in horses, technically known as ‘equine dysautonomia’, attracted public attention when, in 2001, it caused the death of the 2000 Dubai World Cup winner, Dubai Millenium, as a five-year-old. In response, Sheikh Mohammed bin Rashid al Maktoum, ruler of the Emirate of Dubai and owner of the Godolphin stables, established the Dubai Millennium Research Foundation (DMRF) with a view to identifying the explicitly defined cause of the disease.

The cause of grass sickness remains unknown, but the disease occurs, almost invariably, in young horses, aged between two and seven years, with access to grass. The main symptom of grass sickness is gut paralysis, caused by damage to the involuntary, or ‘autonomic’, nervous system; the nature of the damage suggests the presence of a toxin and Clostridium botulinum, a bacterium that produces lethal botulinum toxins, which block nerve functions, under low-oxygen, or ‘hypoxic’, conditions, has been strongly implicated in cases of grass sickness.

Grass sickness was first identified in Scotland in the early twentieth century, where it was responsible for the deaths of hundreds of horses, predominantly draught horses, annually throughout the Twenties. There is still no dependable, explicit treatment for grass sickness and, in the worst cases, horses die or require euthanasia within 48 hours after contracting the disease. In the United Kingdom, grass sickness is estimated to kill 0.5% of horses, of all breeds, annually.