What is Grass Sickness in Horses?

Equine Grass Sickness (EGS) is a rare, and in most cases, fatal disease that affects a horse's nervous system and ultimately leads to paralysis of the digestive tract. This prevents food from passing through the horse's body and causes severe complications. Due to the nature of the condition, up to 80% of horses affected will not survive.

Types of Equine Grass Sickness

There are three forms of grass sickness: acute, subacute, and chronic.
Acute and subacute cases are almost always fatal.
Chronic grass sickness may be survivable with intensive care and long-term management.

Symptoms of Grass Sickness in Horses

While symptoms vary slightly between the three forms, there are several key warning signs that owners should be aware of:

• Difficulty eating and swallowing
  

• Colic
  

• Loss of appetite
  

• Weight loss
  

• Lack of droppings
  

• Muscle tremors
  

• Excessive drooling
  

• Stomach contents pouring from the nose (often green in appearance and foul smelling)
  

• Increased heart rate
  

• Drooping eyelids
  
  
  

What Causes Grass Sickness in Horses?

The exact cause of grass sickness remains unknown. However, it is widely believed to be associated with an overgrowth of the bacterium Clostridium botulinum, which produces a neurotoxin that damages the nervous system.

While any horse can develop grass sickness, the majority of cases occur in horses between the ages of 2 and 7 years. Stress is also thought to be a contributing factor, with horses that have recently experienced a stressful event appearing more vulnerable.

Because of the complex mix of potential risk factors, it's not uncommon for one horse in a field to become sick while others grazing alongside remain entirely unaffected.

Grass sickness can occur at any time of year, but it is most commonly reported during the spring and early summer months (April to June). The UK has one of the highest rates of reported cases globally, particularly in eastern regions such as Scotland.

Diagnosing Equine Grass Sickness

If you suspect your horse has grass sickness, contact your vet immediately.

Diagnosis is typically based on clinical signs. A biopsy of the small intestine—taken while the horse is under anaesthetic—can confirm the diagnosis, but this procedure is invasive and time-consuming. Given the urgency of treatment, decisions are often made based on visible symptoms alone.

Grass Sickness Treatment Options for Horses

Treating Acute and Subacute Grass Sickness

Unfortunately, there is currently no effective treatment for acute or subacute grass sickness. These cases are nearly always fatal, and euthanasia on humane grounds is usually necessary.

Managing Chronic Grass Sickness in Horses

Some horses with chronic grass sickness can recover with intensive support. A critical part of treatment is encouraging the horse to eat. Horses that stop eating entirely are unlikely to survive.

Feeding strategies may include:

• Soaked feeds, high-energy concentrates, chopped forage, succulents, and fresh-picked grass
  
  

• Offering warmed mashes in colder conditions to improve palatability
  
  

• Hand-feeding to stimulate appetite
  
  

• Providing a variety or "buffet" of feeds so the horse can choose what it prefers
  
  

Since many horses can only manage small quantities of food, it's important to ensure caloric density. Adding a high-quality fat source like linseed oil can help. This oil is calorie-dense, highly palatable, and only needs to be fed in small amounts.

Although diets high in molasses are typically discouraged, small amounts may be added to increase feed appeal when necessary—sufficient calorie intake is vital for recovery.

Feed should be offered little and often to mimic the horse's natural grazing pattern and reduce the risk of refeeding syndrome, a dangerous condition that can occur if food is reintroduced too quickly to an emaciated horse.

Recovery After Chronic Grass Sickness Treatment

Following treatment, the horse's gut microbiome is often depleted. To support gut health, consider feeding a high-quality prebiotic supplement, such as our B-Complete, to help re-establish a healthy microbial balance.

Recovery is typically slow, and it can take months for horses to regain their original body weight. While many horses eventually return to their previous level of work, some may require a lighter workload following recovery.

Preventing Grass Sickness in Horses

While there is no guaranteed way to prevent grass sickness, there are several strategies that may reduce risk:

• Avoid grazing horses in fields with a history of EGS cases
  
  

• Minimise soil disturbance and manage pastures carefully
  
  

• Regularly remove droppings from fields
  
  

• Use testing-based worming programs to reduce resistance risk
  
  

• Feed a balanced diet and offer forage alongside grazing, especially during periods of poor grass growth
  
  

• Avoid making sudden dietary changes
  
  

• Consider co-grazing with ruminants like sheep to help control parasites and weeds
  
  

Ongoing Research into Equine Grass Sickness

There has been research into the development of a grass sickness vaccine, but results have been inconclusive to date. The Equine Grass Sickness Fund continues to fund critical research to improve understanding of this devastating condition, with the aim of finding more effective ways to prevent and manage it.

Final Thoughts on Grass Sickness in Horses

Grass sickness is a complex and often devastating condition that requires prompt recognition and action. While acute cases are usually fatal, early intervention in chronic cases can lead to recovery. By being vigilant, managing risk factors, and staying informed, horse owners can help safeguard their animals' health. For more information or to support ongoing research, visit the Equine Grass Sickness Fund.