Equine grass sickness is a disease that has been known to affect horses, ponies and donkeys ever since it was first recognised in 1907. An outbreak at an army base housing military horses in Dundee heralded the beginning of a period in which hundreds of animals died across Scotland. Nowadays, the United Kingdom records the highest number of instances of grass sickness in the world, so we spoke to our horse health expert to find out more about this mysterious disease.

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Q: So what is Equine Grass Sickness?

A: Equine dysautonomia, more commonly known as Equine Grass Sickness (EGS), is a devastating condition, and is often fatal, with a mortality rate of over 85%. It is most common during the spring and summer months. There are three classifications of EGS; acute, subacute and chronic. Animals diagnosed with acute EGS will display severe clinical signs and may not survive longer than 48 hours, while those with the chronic classification could live longer than 7 days.

Q: How exactly does it affect horses?

A: The condition causes extensive damage to neurons within the central and peripheral nervous system, particularly the neurons within the gastrointestinal tract, resulting in the complete failure of gastrointestinal function. Clinical signs include colic, difficulty swallowing, anorexia, abnormal droppings, sweating, shaking, high heart rate and depression. Chronic EGS is associated with weight loss. As it stands, the exact cause of EGS remains unknown.

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Q: Are there any recurring factors in horses suffering from EGS?

A: Almost all cases are only seen in grazing horses. Research indicates that affected horses may develop the condition after being exposed to a noxious agent while grazing. The current hypothesis is that Clostridium botulinum type C, a spore-forming bacterium, may be involved. Because of this, vaccination may eventually be a possibility and clinical trials are currently being conducted. The field trial has involved over 1,000 horses and ponies and will follow them over a two year period.

Q: What are the risk factors?

A: An important indicator could be previous cases of EGS on the same property. It mainly seems to affect horses between two and seven-years-old and, as mentioned before, grazing horses. EGS often follows periods of cool but dry weather and ground frost. Other trigger factors include horses being moved onto a new pasture, pasture disturbance, sandy soils and the arrival of new horses onto the property.

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Q: How can horse owners reduce that risk?

There are various ways that you can reduce the risk of your animals suffering from EGS, but none of them are guaranteed to work. You can begin by removing the manual droppings from the pasture and encouraging mixed grazing. Make sure they don’t rely entirely on grazing by supplementing their feeding with forage. Cut the grass regularly and minimise stress and soil disturbance. It is also sensible to limit sudden dietary changes and avoid horses being grass-kept continuously.

Without full knowledge of the causes of Equine Grass Sickness, all we can do as horse owners is exercise responsibility and restraint. Even though there are no certainties, it is sensible to reduce the risk factors in any way possible. If your horse does begin to exhibit any of the clinical signs, then you should waste no time in taking them to the vet.

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