Grass sickness is a disease exclusive to horses, ponies, donkeys and hares causing damage to the nervous system controlling involuntary functions for example heart rate and gut motility. This can lead to the main symptom of gut paralysis and therefore colic. The cause is still unknown but thought to be a type of toxin. Almost all horses affected have access to grass and there are particular areas of the country in which the disease is seen more commonly, particularly south west Scotland and eastern counties.
The greatest number of cases occur in 2 to 7 year olds although all ages can be affected. Most cases are seen from April to July.
There is much research constantly ongoing to determine the causal agent which is thought to be a toxin. One theory is the involvement of Clostridium botulinum, bacteria found in the soil. Other factors that can be implicated include certain premises, stress and weather conditions.
Clinical signs appear in 3 forms (acute, subacute and chronic) and the major symptoms shown relate to paralysis of the digestive tract. Signs can include colic (ranging from sudden onset severe, to mild colic), drooling saliva, constipation, muscle tremors, patchy sweating, difficulty swallowing and rapid weight loss. The diagnosis is made based on the clinical signs. The only definite way to diagnose the disease is by taking a biopsy of the small intestine at surgery or by examination of the nerve cells at post mortem.
In some cases the horse is so sick that attempting treatment is not an option and the horse should be euthanased on humane grounds. In chronic cases then supportive treatment may be an option and is based around the provision of palatable soft high energy foods, intensive nursing and care. The recovery rates for these selected cases that are treated can be quite good and most horses that do make a full recovery go on to return to work.
For further information The Equine Grass Sickness Fund have a very useful website at www.equinegrasssickness.co.uk.
The Animal Health Trust (AHT), Newmarket has recently set up the first nationwide Equine Grass Sickness Surveillance Scheme and Three Counties Equine Hospital has enrolled on it. We will be asking permission from the owners of any horses we see with grass sickness to be contacted directly by AHT to answer a short questionnaire, or alternatively fill it in on-line at the above website. Hopefully, over time, this information will provide us with trends in the disease, allowing possible vaccine trials in the future.