All you need to know about deadly grass sickness

By Julie Harding on |


How to beat grass sickness

Grass sickness. Two words that together sound innocuous, as though your equine friend has over-indulged in his pasture and feels the worse for wear, but who will soon bounce back to better health. But actually nothing could be further from the truth, for in many cases horses who suffer from grass sickness will die.

The disease may strike indiscriminately throughout the year, but with a spike in cases between April and July, and a peak in May.

Statistics point to one in 200 equines dying in the UK every year as a result of grass sickness. In fact, the UK has the highest rate of cases in the world, with a particular concentration in the east of the country.

Horses affected usually have access to grass, although a handful of cases have been reported among those who are not turned out.

What causes equine grass sickness?

No one really knows exactly what prompts this deadly condition, but scientists believe that it may be due to a toxin ingested by horses with access to grass, as it occurs almost exclusively in those with access to grazing.

The favoured candidate is Clostridium botulinum. Other theories for a cause include mineral or vitamin deficiencies, particularly those lacking selenium, and pastures with a high clover content.

Recent research has pinpointed a greater risk in areas with a high soil nitrogen content, as well as places where the soil has been disturbed.

Cool, dry weather, when temperatures average between 7 and 11C, also seems to prompt a hike in cases.

What happens?

Because the horse’s nervous system suffers damage, his involuntary functions — those he doesn’t think about, such as breathing and digestion — are affected.

The most common symptom is paralysis of the digestive tract, which can manifest itself with colic-type symptoms.

In fact, many owners — and sometimes even vets — mistake grass sickness for colic, but if the horse is suffering from acute grass sickness the prognosis is bleak — almost all will die or will have to be put down within two days.

The symptoms

Grass sickness can vary in severity and horses tend to fall into one of three categories — acute, sub-acute and chronic, with acute cases the most serious.

In a horse with acute grass sickness, apart from signs of colic, he may sweat and display muscle tremors, be constipated and have a distended stomach with an accumulation of fluid inside. In the most severe cases, this fluid may pour out of his nose.

A horse with sub-acute grass sickness will have similar symptoms but these will be mild to moderate and the horse may lose weight. He may still be able to consume small amounts of food but the prognosis in many cases is still poor.

Horses with chronic symptoms may display a reduced appetite and signs of intermittent colic, but the most telling sign is rapid weight loss.

Most susceptible horses

For some reason, which scientists don’t yet understand, younger horses seem statistically most susceptible to grass sickness.

The disease appears to peak in those aged three to four years, and is seen commonly in those aged between two and seven. It rarely strikes young foals and there are fewer recorded cases in older horses.

This suggests that they may build up some kind of resistance over time to the cause. Vets report seeing more cases among horses who have suffered stress, as well as those who are overweight.

Other research has highlighted excess use with certain wormers as a factor.


Horses that have suffered a chronic attack can be rehabilitated through intensive management. This will involve offering food that can be easily swallowed, such as grass, chopped vegetables and high-energy concentrates soaked in molasses.

Additionally, the patient will need plenty of TLC in the form of human contact and grooming. Some will benefit from being rugged.

How can we beat grass sickness?

Despite extensive research, there is still no known cure, but some owners in areas where horses are particularly susceptible stable them during the peak spring and summer periods.

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    i had a beautiful blue and white 22months youngster he devoloped symtoms of colic on the saturday the vet came to him treated him for colic it made no difference the vet advised me over the phone to stable him over night with no food and just water, they came back to see him early monday morning and he was put to sleep 30 mins later due to grass sickness id never heard about but i do now and every time i hear a case about it brings back memories of one of the worse days of my life.

  2. DM

    I lost my beautiful mare to this awful disease. She died within two days leaving me with her one month old filly foal to bring up. Surely by now more should be known about this disease. However I have also known people who have been struck down by pathogens from soil leaving them with limited heart function. It really is time we knew more about these pathogens and how to treat human and animal.


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