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Grass sickness: research on going but still no cure

Grass sickness is a devastating, widespread and usually fatal condition of horses, ponies and donkeys which has been recognised in this country since the beginning of the 1900’s. Veterinary surgeon, Jane King of the Westmorland Veterinary Group, Kendal, explains more.
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Grass sickness was first reported in North East Scotland. The first cases appeared to coincide with the movement of large numbers of horses from the remount divisions of the Scottish army.


In the first part of the 20th century, Equine Grass Sickness (EGS) was a major cause of death in hundreds of horses on the East Coast of Scotland. It decimated the Clydesdale population, which was vital to the agricultural industry at the time.


The problem became so serious that in 1918 the Highland Agricultural Society funded the first research into the disease, which was undertaken by J.F. Tocher, a chemist at Aberdeen University.


At the time there was also interest in botulism in people from improperly preserved food and Tocher noticed some similarities with EGS in horses. He found Clostridium botulinum in the spleens of some affected animals and undertook clinical trials. He went as far as producing an antiserum, which was marketed and seemed effective.


However, in the later 1920s, his work was discredited by fellow academics and was discounted for a further 80 years.


More recent work at Edinburgh, Liverpool and the Animal Health Trust has led to the belief that grass sickness is caused by a neurotoxin produced by the bacteria Clostridium botulinum type C. This is a strain of botulism associated with birds and it is possible that bacteria were brought here in guano to be used as fertiliser at the end of the 19th century.


The signs seen in EGS are caused by the neurotoxin which affects the nervous system not under voluntary control. This results in the whole of the intestinal system from the oesophagus down failing to function normally, causing a paralysis of the gut.


There are three forms of EGS - acute, sub-acute and chronic (see table).


In acute and sub-acute grass sickness, experienced vets will often recognise the clinical signs and quickly end the horse’s suffering.


Sometimes the horse may present with signs the same as those of a surgical colic or twisted gut.


These horses may be taken for surgery, where on the operating table the surgeon will see that the whole of the intestine is distended and not does have the normal motility or peristalsis.


Samples taken at surgery, sent to the pathology lab and examined under the microscope will show the characteristic changes in the nerves.


In the live horse, applying phenylephrine drops to the eye will correct the ptosis or drooping of the eyelid and can assist with diagnosis when compared to the untreated eye.


Chronic grass sickness cases can be more confusing, but the extreme weight loss, patchy sweating and muscle tremors, combined with difficulty eating and reduced droppings are big clues.


There is currently no treatment for EGS. It is a distressing condition and affected acute and sub-acute cases should be put to sleep humanely as soon as possible.


Sometimes chronic grass sickness cases will show a strong will to live, and with specialist veterinary care and dedicated nursing these horses can survive and return to a normal life.


However, nursing a chronic grass sickness case should not be undertaken lightly and requires an enormous amount of commitment for a long period of time.


The cost of treatment and special diets should not be underestimated, nor should the length of time recovery takes.

Special requirements

Chronic grass sickness cases have special nutritional requirements and will not survive on forage alone. They need well-soaked, high energy feeds little and often, as well as tempting with carrots, hand grazing, grooming and frequent individual attention to keep them interested in life.


In 2008, the Equine Grass Sickness Surveillance Scheme was launched, co-ordinated by the Animal Health Trust and with the co-operation of the Royal (Dick) School of Veterinary Studies, the University of Liverpool and the Equine Grass Sickness Fund to record cases as they occurred across the country. More than 200 veterinary practices are now signed up to the scheme.


Following a small clinical trial in 2013, a nationwide vaccine trial is being undertaken this year at veterinary practices across the UK on premises which have had previous cases of grass sickness.


For more information on grass sickness or to make a donation to the ongoing research into this devastating disease which has touched the lives of so many horse owners visit or


Risk factors

Studies over the last 100 years have identified some factors which can affect he risk of getting a case of EGS

  • Previous case on a premises
  • Age of horse most are two to seven but can be any age. Foals under four months rarely get EGS , nor do very old horses and it is thought some horses may develop immunity or in the case of foals have it passed to them from their mother in the colostrum
  • Time of year, there is a peak incidence of cases May- July and again in the autumn. There are often more cases after a period of dry cool weather of seven-11degC for more than 10 days
  • Horses kept solely at grass, stabling for even part of the day and supplementary feeding of forage is protective
  • Type of premises, more common on stud farms, riding schools or livery yards with large populations of horses
  • Recent stress, travelling, moving yards, mixing groups of horses, breaking, castration all increase the risk of EGS
  • Domestic fowl or birds kept on the premises
  • Sandy and peaty/loamy soils increase the risk, chalk soils decrease
  • High soil nitrogen  selenium status of soil/diet may play a role
  • Frequent use of ivermectin wormers
  • Recent soil disturbance, e.g . construction, pipe laying, poached gateways
  • Mechanical dropping removal increases risk, manual dropping removal from pasture decreases
  • Co-grazing with cattle and sheep is protective
  • Grazing with a field mate who has recovered from Grass Sickness is protec
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