Equine Grass Sickness (EGS)

A definite reason as to what causes EGS remains unknown. It is a very complex disease that affects the digestive system causing paralysis of the gut². This causes fluid to build up in the stomach and small intestine, blockages in the large intestine, and dehydration.

There are three potential causes:

1. A bacterium that creates dangerous toxins (Clostridium botulinum)
2. A fungus found in pasture that produces a toxic chemical
3. A possible genetic link.

It appears that not one of these factors alone causes EGS, as there are a variety of other circumstances which need to be in place for the disease to occur. These include heritability³, weather conditions (such as cool, dry weather with irregular frosts), soil disturbance, pathogens and fungi related to soil and plants, age, immune system, nutrition, gut microbes, and stress levels in the horse. This would explain why one horse grazing in 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 common between April-June. Great Britain has the highest number of EGS cases compared1 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 common in young adult horses between 2-7 years old.

Sadly, the prognosis for EGS is poor. Horses diagnosed with acute, sub-acute and severe chronic forms of the disease will usually be euthanised to prevent further suffering. Around 50% of chronic cases may survive if they are suitable for intensive nursing and vet treatment. 

Suitable EGS cases for nursing usually have some ability to swallow, an appetite and no continuous abdominal pain. There is currently no cure for EGS and treatment 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 the owner, vet and nurses, if nursing is considered, to be aware of the long time-frame for treatment, potentially high financial cost (check with your insurance company) and that there is no guarantee the supportive care will be successful. 

The Moredun Foundation Equine Grass Sickness Fund gives the following recommendations:

• Minimise exposure to grazing where previous cases of EGS have occurred
• Minimise pasture/soil disturbance (for example, harrowing, pipe-laying/construction)
• Minimise soil exposure (for example grazing down to the soil/poaching of fields)
• Avoid any sudden changes in diet
• Follow a testing-led deworming programme to avoid the unnecessary use of dewormer drugs. Seek advice from your vet to discuss the frequency of testing and possible treatment that may be required for your horse
• Co-graze with ruminants such as sheep
• Where possible, remove droppings by hand, rather than mechanically
• Supplement with additional forage such as hay

There are currently no vaccines available to help prevent against EGS. A full report from a 2014-2018 EGS vaccination field trial can be found on the Equine Grass Sickness Fund website. 

"I’d ridden William on the Thursday and he was fine, but when I went out to his field on the Friday he was acting strangely. He didn’t come over to me, which was unusual, and when I brought him in he didn’t try and dive to eat any of the grass like he normally did.

"I tied him up outside his stable and he tried to pass droppings, but they were just mucus-covered pellets. He wasn’t eating or drinking and he didn’t touch his hay – and then he started showing signs of what I thought was colic. He was looking at his flanks and scraping the floor, so I called the vet out straight away."

When the vet arrived, he too initially diagnosed mild colic – but after a full examination 12 litres of green reflux was drained from William’s stomach and his heart rate found to be dangerously high. Fearing life-threatening colic, William was rushed to hospital immediately. Devastatingly for Ulla, the hospital examination revealed something much more sinister – the vets suspected grass sickness. William was now gravely ill and needed to be operated on the next morning. 

"That night, I went home and researched the condition. Nothing I read was positive. When the vets operated in the morning, they confirmed our fears – it was acute grass sickness, and nothing could be done for William. That was it. He was gone within 24 hours. 

"I’d never heard of grass sickness until it happened to William. The sheer shock is what is so scary about it because before you know it, your horse is gone. If you don’t know what grass sickness is, you think you’re dealing with colic because the symptoms are so similar. That’s why it’s so important to research grass sickness and know how to minimise the risks. You don’t have the time to do it after it strikes."

• Equine Grass Sickness Fund website
  • Coldwell Fizz’s Story
  • A research update and look to the future
  • How vets can support the Biobank work

1. Milne, E. et al (2006) Grass Sickness in Horses
2. McGorum, B. (2021). Equine grass sickness (A multiple systems neuropathy) is associated with alterations in the gastrointestinal mycobiome. Animal Microbiome.
3. Vincze, B. (2020). Family aggregation analysis shows a possible heritable background of equine grass sickness (dysautonomia) in a Hungarian stud population. Acta Veterinaria Hungarica, 68(3), 263–268.