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Grass sickness

Equine Grass Sickness (alternatively termed 'equine dysautonomia') is a rare but predominantly fatal illness in horses. Grass sickness affects all types of horses, ponies and donkeys and has affected some well known horses including the thoroughbred stallion Dubai Millennium.



The first cases of grass sickness were recorded in eastern Scotland in 1907.

Clinical signs

Grass sickness is a polyneuropathy affecting the central, peripheral and enteric nervous systems. The majority of visible clinical signs are related to paralysis within the digestive tract although nerve damage occurs throughout the body. There are three forms of grass sickness:

  • acute grass sickness (AGS) - horses display signs of colic and require euthanasia or die within 48 hours
  • subacute grass sickness (SAGS) - horses display clinical signs similar to AGS but with less severity and may survive up to 7 days
  • chronic grass sickness (CGS) - horses present with severe and rapid weight loss and a selected portion of these cases may survive.

Clinical signs common to all subsets include: depression, anorexia, colic (moderate with AGS/SAGS and mild with CGS), excess salivation, constipation, nasogastric fluid secretion, patchy sweating, muscle tremors and eyelid drooping.


Diagnosis of grass sickness in the live animal requires a thorough clinical examination including a rectal examination. Definitive diagnosis can only be made at surgery (where biopsies of the gut are taken) or at post-mortem (where biopsies from the nerves are taken). Differential diagnoses for grass sickness are varied and include: any other cause of colic and weight loss, tying-up, laminitis, botulism, choke and dental problems.


There is no treatment for grass sickness. All cases of AGS and SAGS should be euthanised upon diagnosis as the condition is fatal. A proportion of CGS cases can survive following periods of intensive nursing. Overall the mortality rate of equine grass sickness is considered to be over 95%.


The cause remains unknown, however present research suggests that toxin production from the bacterium Clostridium botulinum type C is involved.[1]

Clostridium botulinum is a soil-borne bacterium, which may be better known for producing clinical signs of botulism. Research has suggested that Clostridium botulinum may cause grass sickness when the spores of C. botulinum type C are ingested and produce their toxin locally within the intestine.

Risk factors

The main risk factor for grass sickness, as the name may suggest, is grass. The disease is almost always seen in grazing animals, although there are isolated reports of the condition occurring in stabled horses. Grass sickness is most frequently seen in young horses aged between two and seven, and is particularly prevalent during April, May and June, and later in the autumn, after a spurt of grass growth.

Research has identified a number of other risk factors which may increase the likelihood of a horse developing grass sickness including: soil disturbance, worming with Ivermectin based dewormers, movement to new pasture, stress, grazing on a field which has previously produced a case of grass sickness.


Current research in equine grass sickness includes development of a nationwide surveillance scheme for grass sickness in Great Britain. This scheme hopes to identify all cases of grass sickness which occur from 2000 onwards. This scheme is run by the Animal Health Trust, in collaboration with the Equine Grass Sickness Fund, the University of Edinburgh and the University of Liverpool, following funding from The Horse Trust. More information on this scheme can be found at the dedicated website [www.equinegrasssickness.co.uk]

The Equine Grass Sickness Fund [www.grasssickness.org.uk] are a dedicated charity raising funds to tackle the disease. Their website contains useful information regarding all aspects of grass sickness including management of chronic grass sickness cases.


  1. "Equine Dysautonomia". The Merck Veterinary Manual. 2006. http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/100805.htm. Retrieved 2007-07-08. 

External links

  • [1] World Horse Welfare (UK)
  • [2] Horsetalk (NZ)
  • [3] National Pony Society
  • [4] Newsletters


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