GRASS sickness is a devastating equine disease. Prevalent throughout the British Isles, it is both fatal and extremely distressing.

Not only does the disease occur all over Britain, it can affect any horse of any sex or breed. Donkeys are also susceptible.

According to the Equine Grass Sickness Fund, most cases are in animals between the ages of two and seven, with a peak between three and four years old. Older horses may develop a sort of natural immunity, probably through exposure to the cause, but even horses in their 20s have been affected. No horse is safe from the disease.

Grass sickness causes damage to the part of the nervous system that controls involuntary functions like breathing and digestion. The organ primarily affected is the gut, which essentially becomes paralysed. There are three forms of grass sickness, the acute form being most severe.

With the acute form, paralysis of the gut leads to severe colic.

Swallowing will be difficult and there will be excessive salivation. In many cases the stomach fills with an unpleasant liquid which may end up coming out via the nostrils.

This can be hugely distressing for horse and owner. The lack of movement in the gut leads to constipation, and if the horse does pass any droppings they will be small and hard, usually covered by mucus. The horse may also sweat in patches and exhibit muscle tremors.

The subacute form of the disease has similar symptoms but less severe. There is unlikely to be nasal discharge but the horse will lose a lot of weight rapidly.

The chronic form makes up about a third of cases. Symptoms develop more slowly and may be as subtle as mild colic. Paralysis of the gut manifests as difficulty in swallowing, leading to loss of appetite. The horse will lose a lot of weight and other signs include a crusty nose, drooping eyelids, patchy sweating and a tucked-up stance.

The good news is about half of chronic cases can be nursed back to health. However, there are no guarantees that treatment will work, and all acute and subacute grass sickness cases require euthanasia.

The Animal Health Trust has already conducted a successful pilot study into a new vaccine. Now it’s time for the next step.