Equine Grass Sickness

With the advent of Spring, owners are urged to be especially vigilant for Grass sickness. This devastating disease, which was first seen on an army camp in Eastern Scotland in 1907, still remains an unsolved mystery. However, with increasing research being carried out by the Equine Grass Sickness Fund, it is hoped that we are closer to establishing a cause of Grass sickness and thus a preventative vaccination.

Photo credit: Prof Bruce McGorum, University of Edinburgh

Photo credit: Prof Bruce McGorum, University of Edinburgh


Equine Grass Sickness (EGS) or Equine Dysautonomia is a predominantly fatal disease of horses, ponies and donkeys. It causes damage to the parts of the nervous system, which are involved in involuntary functions, leading to the main symptom of gut paralysis. Although the cause remains unknown, it is thought that a toxin is likely to be involved. It is estimated that approximately 1 in 200 die from the disease in some parts of the UK.


There have been a number of known risk factors, these include:

• Horses at grass, it is rarely seen in housed animals

• Age-the peak incidence is seen in young horses of 2-7 yo

• Season-it is seen in all months, but the greatest numbers of cases are seen between April and July, with the highest incidence in May.

• Premises have a high incidence of cases

• Weather-outbreaks typically happen following cool and dry weather

• Movement- the movement of new horses onto the yard

• Worming-increased cases with increased worming with certain types of wormer

• Soil type-increased incidence on sandy loam soils, those with a high nitrogen content and soil disturbance

• Geographical distribution- there is an increased prevalence in areas on the eastern side of the UK.

Over the past 100 years, there have been many postulated causes including, poisonous plants, chemicals, bacteria, viruses, insects and vitamin and mineral deficiencies. Currently the most likely cause under investigation is the possible involvement of Clostridium botulinum, a soil based bacteria.

EGS can be categorized into three main forms, acute, subacute and chronic, but there is considerable overlap in the symptoms of the three forms. The spectrum of clinical signs seen reflects the severity of nerve damage in the nervous system.

Photo credit: Animal Health Trust

Photo credit: Animal Health Trust

The acute form is the severest, with clinical signs appearing suddenly and the horse will either die or require euthanasia within 2 days. Signs are associated with paralysis of the gastrointestinal tract starting from the oesophagus and progressing downwards to the intestines. Signs include; dullness, depression, difficulty in swallowing, profuse salivation from the mouth, signs of colic including rolling and pawing at the ground, copious amounts of foul smelling fluid (spontaneous gastric reflux)may pour from the nose, patch sweating, muscle tremors, droopy eyelids due to the paralysis of the eye muscles and constipation. If any dung is passed, it will be dry, hard and often coated in a mucus like film. A major differential for such clinical signs would be a surgical colic.

In the subacute form the signs are similar but less severe and cases usually die or have to be put down in a week.

The chronic form of the disease carries a better prognosis and it is thought that 45% of cases survive. The symptoms progress more slowly and the most noticeable clinical sign, is the rapid, emaciated appearance. This has been likened to the physique of a greyhound, with a tucked up abdomen. Signs of colic if present are mild and intermittent. The salivation and fluid accumulation in the stomach seen in the acute form are not a feature. There may be accumulation of mucus casts in the nostrils (Rhinitis sicca).

The diagnosis by your vet can often be tricky as there is no simple diagnostic test in the live horse and other causes of colic, weight loss and dysphagia can present in a similar fashion. There can be clues in the history relating to the risk factors outlined above. A thorough clinical examination, including rectal examination, nasogastric intubation, blood tests can help in ruling other conditions.

Photo Credit: Moredun Foundation EGS Fund

Photo Credit: Moredun Foundation EGS Fund

The only way a definitive diagnosis can be achieved is by histopathological examination of the nerve ganglia at post mortem. In the live horse, biopsies can be taken from the intestine at surgery, however this is not 100% accurate. Another handy test can be the application of Phenylephrine eye drops, which reverses the droopiness of the eyelids, which may be seen in some cases.

Treatment should only be attempted in the chronic cases as the prognosis for the acute and subacute cases is hopeless. However, the difficulty arises when there is difficulty in differentiating between the subacute and chronic cases. The most important prognostic factor is the ability to eat. Not all the chronic cases will survive. If cases are anorexic for more than five days, euthanasia is the kindest option. Cases require intense nursing, to ensure they eat easily swallowed, highly palatable food. The human contact is essential to provide stimulation and keep them well groomed as the excessive sweating can cause them to be scurfy. Appetite stimulants can also be helpful. As long as the treated cases are carefully selected in the first place, most should make a good recovery and return to work.

Prevention is difficult as the causative agent remains aloof. However advice can be given to stable horses at higher risk times of the year, avoid grazing which has had previous cases, keep new horses stabled for eight weeks prior to turnout and cograzing with ruminants is thought to be helpful.

Currently there is a vaccination trial into the use of a Clostridium botulinum vaccine. It has been discovered that there is a high concentration of C. botulinum type C toxin in the guts of acute cases and that horses with a low levels of antibody to the bacteria and its toxin are at increased risk of the disease. Fingers crossed the results are promising and we are a step closer to preventing this devastating disease. (Enrolment of horses and ponies for the EGS vaccine trial is on-going, and your horse or pony may be eligible to participate if:

* They live on premises affected by at least one case of EGS in the previous two years

* Have a valid passport

For more information about the vaccine trial, or to request an owner information pack, please contact the Animal Health Trust Tel: 01638 555399 or Email: equinegrasssickness@aht.org.uk) 

If you have any questions about any aspects of your horse’s health, or indeed your pet, farm or smallholding animals, please do not hesitate to contact us on 01892 835456. Putlands Veterinary Surgery is based in Paddock Wood in Kent, and offers a friendly, professional and personal service for all species of animal. We have four dedicated large animal vets with a wealth of equine experience between us, and are always happy to help.



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