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Equine Grass Sickness

 September 2020

Equine Grass Sickness (EGS) is a frequently fatal disease, affecting grazing horses, ponies and donkeys. However, there have been a few cases where equines with no access to grass have developed EGS and hay has been found to be the cause in those rare incidences. 

New Research Partnership

A nationwide biobank to gather vital samples for research into Equine Grass Sickness (EGS) has now been opened by The Moredun Foundation and The Equine Grass Sickness Fund (EGSF), with support from The British Horse Society. A lack of samples for clinical analysis from EGS cases has hindered research progress, so horse owners and vets can now play a crucial role in the project to understand the disease’s causes by submitting samples from affected horses, along with soil samples, to the biobank. 


Enrol now!

The University of Edinburgh and the Equine Grass Sickness Fund are launching an Equine Grass Sickness (EGS) online course (35 CPD hours), £120 for 3 months access on 1 Nov 2021. Understanding the disease process and risk factors may enable horse owners, veterinary surgeons, and other professionals to reduce the risk of EGS.

Participants will have three months access and 35 CPD hours can be claimed, and certificates issued for participation. 10% of each participant’s fee will be going back to the EGS Fund to fund future research. Together we will seek the cause and reduce the risk of EGS.



Reporting Cases of EGS

By donating samples and completing case questionnaires, horse owners and vets can help researchers understand the risk factors and protect future generations of horses and their owners from the terrible effects of this disease. 


Potential Causes

Research has been very sharply focused on a toxin produced by the bacterium Clostridium botulinum. This bacterium can be found in the soil, which would explain why EGS affects horses at pasture.

What it doesn’t explain is why only some horses get EGS, or why horses that consume food contaminated with the toxin produced by Clostridium botulinum bacteria are susceptible to botulism (a potentially fatal disease causing body weakness and sometimes paralysis) and not grass sickness. Clostridium botulinum produces a range of toxins making it a toxico-infectious form of botulism. This is where the pathogen (Clostridium botulinum) when eaten, produces a toxin, which in turn can potentially cause the illness.

However, EGS is a very complex disease and it seems that the presence of Clostridium botulinum in itself is not enough to cause grass sickness as there are a variety of other predisposing factors which need to be in place for the disease to occur. Predisposing factors to EGS could be; heritability, climatic conditions (such as cool, dry weather with irregular frosts) soil disturbance, pathogens and fungi related to soils and plants, age, immune factors, nutritional components, gut microbes, and stress levels in the horse. This would explain why one horse grazing a particular field may die from EGS while his companion can remain completely unaffected.

EGS cases can occur at any time of the year but has been found to be most prevalent between April-June. The UK has the highest number of EGS cases compared to other countries in the world, with higher occurrences nearer the east coast compared to the west, particularly in Scotland. EGS can affect any age of horse but is most commonly seen in young adult horses between 2-7 years old.


The disease causes damage to the part of the nervous system that controls involuntary functions. The organ primarily affected by the disease is the gut, as it essentially becomes paralysed, compromising the horse’s ability to swallow and transport food. This can cause fluid to build up in the stomach and small intestine, impactions in the large intestine and dehydration.

There are three forms of EGS, acute, sub-acute and chronic, with clinical signs that can overlap:

1) Acute form

Acute EGS is the most severe form and there is sadly no hope for survival. Once diagnosed by a vet, euthanasia is the only option for the horse to prevent suffering. Clinical signs of acute EGS can appear very rapidly and may include:

Salivation in a horse suffering from Grass Sickness
  • Signs of colic with no gut sounds
  • Difficulty swallowing and excessive salivation (dribbling)
  • Distended stomach filled with fluid
  • The contents of the stomach (foul-smelling liquid) starts to pour down the nose
  • Dropping eyelids
  • Patchy sweating
  • Increased heart rate
  • Muscle tremors
  • Depression

2) Sub-acute form

The sub-acute form of the disease has similar signs to the acute form but are moderately severe. There is sadly no hope for survival and once diagnosed by a vet, euthanasia is the only option for the horse to prevent suffering. Clinical signs of sub-acute EGS may include:

Rhinitis Sicca
  • A ‘tucked up’ abdomen
  • Weight loss
  • Difficulty swallowing
  • Depression
  • High heart rate
  • Colic as the disease progresses
  • The contents of the stomach (foul-smelling liquid) may start to pour down the nose as the disease progresses
  • Patchy sweating
  • Droopy eyelids
  • Muscle tremors
  • Reduced movement in the gut
  • Dried and inflamed inside of nostrils

3) Chronic form

The final type of EGS is the chronic form. Signs of chronic EGS progress more slowly, are less severe and are sometimes shown as mild colic. Signs of chronic EGS may include:

  • A severely ‘tucked up’ abdomen
  • Severe weight loss to potentially emaciation
  • Difficulty swallowing
  • Slightly high heart rate (usually no higher than 60 beats per minute)
  • ‘Base narrow stance’
  • Patchy sweating
  • Depression
  • Droopy eyelids
  • Muscle tremors
  • Mild colic
  • Reduced movement in the gut
  • Dried and inflamed inside of nostrils


The diagnosis of EGS can be complex and is normally confirmed by a clinical examination of the horse by the vet. Further tests may be used to support the diagnosis of EGS, or help rule out other conditions, such as taking blood samples or an ultrasound of the abdomen.


Due to the hopeless prognosis for survival, horses diagnosed with acute, sub-acute or severe chronic EGS usually have to be euthanised to prevent further suffering. Around 50% of chronic cases may survive if they are assessed as being suitable candidates to proceed with the intensive nursing/vet treatment.

Cases for nursing treatment must be carefully selected and is based on the severity of the clinical signs following a full clinical assessment. Suitable EGS candidates for nursing usually have some ability to swallow, some appetite and no continuous moderate/severe abdominal pain.

There is currently no cure for EGS and treatment of selected chronic cases revolves around nursing and nutritional support. There are some drugs that can be given but these are to treat signs and relieve pain.

There must be a discussion between all parties involved (for example the owner, vet and nurses) if nursing treatment is considered, to be aware of the long time-frame for treatment, potentially high financial cost and that there is no guarantee the supportive treatment will be successful. 


EGS Credit Moredun Research Institute
Credit: Moredun Research Institute


Reducing the Risk of EGS

The Moredun Foundation Equine Grass Sickness Fund have a number of current recommended management strategies to help reduce the risk of EGS. Although, it is important to note that no prospective studies have been conducted to assess the value of these strategies, with the exception of the C. botulinum vaccine trial:

  • Minimise exposure to grazing where previous cases of EGS have occurred
  • Minimise pasture/soil disturbance (for example, harrowing, removing droppings mechanically, pipe-laying/construction)
  • Minimise soil exposure (for example grazing down to the soil/poaching of fields)
  • Avoid any sudden changes in diet
  • Avoid the over-use of ivermectin-based wormers. Best practice is to consult with your vet/suitability qualified person to implement a targeted worming programme to confirm whether your horse requires worming
  • Co-graze with ruminants such as sheep
  • Remove droppings by hand, rather than mechanically
  • Supplement with additional forage such as hay


There are currently no vaccines available to help prevent EGS. However, research has suggested it maybe a feasible method of disease prevention as it has been shown that;
horses can produce antibodies against C.botulinum type C and horses with low antibodies against the bacterium are at a higher risk of developing EGS.

A full report from a 2014-2018 EGS vaccination field trial can be found on the Equine Grass Sickness Fund website.

Case Study

Ulla Balletta’s horse William was hit by grass sickness in April 2011. Read more about here story here.

Further Reading


 equine disease prevention

 horse health

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