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The Dangers of Equine Grass Sickness

Equine grass sickness (EGS) is a very serious, usually fatal disease that affects horses, ponies and donkeys. It’s most often presented to vets as colic, or another problem with the digestive system, but the underlying problem is damage to the nervous system. The condition is known as ‘grass sickness’ as it occurs almost exclusively in horses with access to grass. Sarah Hogg (BA VetMB MRCVS) from Paragon Equine in Cumbria, gives an insight into this unpleasant disease.

Despite the first cases of EGS being identified in Scottish army horses in 1907, we still aren’t sure what the exact cause of the disease is. However, we know that it is not contagious, and symptoms suggest that some form of toxin is involved. Clostridium botulinum, a soil-associated bacterium is thought to be the most likely culprit. This is the same family of bacteria that causes ‘botulism’ in humans and is used to produce Botox injections. Researchers continue to try and improve our understanding of how EGS affects horses and how we might be able to treat them.

Which horses are affected?
The UK sees more cases of grass sickness than any other country, and in some areas it has been calculated that 1 in 200 horses will die each year because of it. It is also reported in Northern Europe, with a few cases being reported in the United States. There is a similar condition, mal seco (dry sickness), seen in parts of South America. In the UK, Scotland and Northern and Eastern areas of England are most commonly affected.

EGS affects horses of all ages, although cases are most commonly reported in those aged 2 to 7 years, and particularly between 3 and 4 years of age. The lack of cases in very young foals has led to the belief that they are not exposed to the disease agent on grass. Researchers also think the rarity of cases in older horses suggests that they build up some resistance to the disease agent over the course of their lifetime.

EGS is mostly seen from late spring to early summer. After a drop-off of reported cases in July there is then a second, smaller reporting of cases as autumn turns to winter.

Other factors which seem to increase the risk of EGS include pastures on sandy soil, heavy use of nitrogen-rich fertilisers or other factors which increase the nitrogen content of the soil such as septic tank run-off, overgrazing, automated ‘poo picking’ systems, recent land disturbance, frequent worming with ivermectin-based products, recent stress and land-sharing with domestic fowl. However, none of these have been unshakably proven by research and need to be interpreted with caution. For example, the risk of not worming your horse when needed is much higher than the risk of your horse developing EGS. There is no suggestion that the wormers themselves contain the toxin responsible for EGS.

Grass sickness can be classified into three types: acute, sub-acute and chronic. In reality, cases are seen on a sliding scale depending on how quickly and severely symptoms arise.

Horses are usually very quiet, depressed and uninterested in their food with a mild or moderate colic. They are usually sweaty, often in visible patches on the skin, and have fast-twitching, shivery muscles. There may be a nasal discharge or crustiness around the nostrils and they may have ‘sleepy’ or drooping eyelids.

Your vet will usually find a high heart rate and no gut sounds, because damage to the nervous system causes digestive system paralysis. This, in turn, means horses stop passing droppings or they might be small, dry and covered in a layer of mucus. Because the bowels are not moving properly, stomach fluid remains in the stomach and starts to accumulate, which is very uncomfortable as horses are unable to vomit. Your vet would pass a stomach tube to relieve this, and this would usually release a large amount of fetid smelling, yellow-green fluid. Without a stomach tube, this fluid can start to drip down the nose.

As digestive system paralysis also prevents swallowing, horses may dribble saliva. If they do try and eat or drink it may look like they are ‘playing’ with it – trying to take it into the mouth and chew or swallow only for it to fall out again.

In acute grass sickness, these symptoms appear quickly and the horse will either die or require euthanasia within 48 hours. The colic seen is more severe, with pawing, rolling and flank-watching.

In subacute grass sickness, the symptoms are similar but less severe. As they may survive longer, rapid weight loss might be seen. These horses unfortunately also die, or require euthanasia within 7 days.

Although the underlying symptoms are similar in chronic grass sickness, they arise more slowly and the colic seen is often mild and intermittent. Some droppings are usually passed and stomach fluid may not accumulate, but weight loss is much more noticable.

Grass sickness is often diagnosed from a horse’s symptoms and history, which are fairly unique to the disease. Your vet may run a blood test as there are certain indicators (cortisol, catecholamines and haptoglobin) which can help point to a diagnosis. Similarly, sampling some peritoneal fluid via a needle placed through the skin can help to paint a fuller diagnostic picture.

It can only be definitively diagnosed from an intestinal biopsy, which involves a general anaesthetic and colic-like surgery. The cost, time and stress to the horse involved often make this impossible. A symptomatic diagnosis can be confirmed by post mortem.

Nearly all acute and sub-acute cases of EGS are euthanased on welfare grounds because of the severity of the disease. Unfortunately, there is no known treatment for this acute, aggressive type of grass sickness.

Some people will choose to try and treat chronic cases. With intensive nursing over weeks or months, horses can recover, although again, these horses are often euthanased out of regard for their welfare. This kind of nursing is costly and very time consuming, so is a big commitment. Whereas we used to think a full recovery was not possible, it is now a proven possibility and horses can often return to work.

It’s important your vet chooses which cases to attempt treatment with carefully as not all chronic cases will recover. They must still be able to eat and should have some visible ‘fight’ or desire to live.

If one horse in a herd is affected by grass sickness, remove the others from that field. If that isn’t possible, supplementary feed (e.g. hay) should be provided.

Preventing Grass Sickness
Unfortunately, there is no way to completely eradicate the risk of grass sickness and it is difficult to advise on preventing it without fully understanding the causes. The best advice is to stable any new horses where the disease is known to occur, and allow other stock to graze the affected pastures during spring and summer. These measures have also been suggested to reduce the risk of grass sickness:
-    Avoid pasture where grass sickness has previously been reported; it can be so localised it can just affect one field on a property
-    Keep your horse or pony lean (those in good or fat condition are most often affected)
-    Decrease reliance on nitrogen-based fertilisers
-    ‘Poo pick’ by hand
-    Stable horses for some of the day between May and July, or supplement grazing with hay or haylage
-    Avoid grazing on disturbed ground (e.g. areas for drainage) or where there are pools of stagnant water

It is also thought that co-grazing with a horse who has survived grass sickness might reduce the risk of a new horse contracting the disease.

Paragon Equine - Cumbria

Article by Sarah Hogg, BA Vet MB MRCVS, Paragon Equine.
Tel: 01768 483789
Email: Paragon Equine, Newbiggin, Cumbria
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