Grass sickness is a usually fatal condition of grazing horses which causes varying degrees of paralysis of the horse’s intestines. At the present time, grass sickness is believed to only occur in the United Kingdom, but a similar condition undoubtedly occurs in other parts of the world. It now affects horses in all parts of the UK but was first recognised and tends to still be more frequently diagnosed in Scotland and the north of England. After 150 years of research, the cause of the disease remains incompletely understood, but recent developments suggest that it may be due to an environmental bacterium (Clostridium botulinum), which under conditions that remain unclear, produces a toxin in the horse’s intestine which results in its paralysis.

These are highly variable. Essentially, there are four syndromes (forms of the disease):-

The symptoms of grass sickness are caused by damage to the nerves most importantly supplying the oesophagus (‘gullet’), stomach and intestines. As manifestations are so varied in individual cases, the diagnosis can sometimes be difficult to make and in many cases with acute colic, exploratory laparotomy (surgical exploration of the abdominal organs under general anaesthesia) may be needed to rule out treatable intestinal conditions such as ‘twists’ and displacements. In some cases it is helpful to collect a small tissue sample of the wall of the small intestine for microscopic studies (ileal biopsy), during laparotomy. Definitive diagnoses are finally made on the basis of post mortem examination results where typical signs of nerve cell damage are found in specific nerve centres (sympathetic ganglia).

Most cases of grass sickness are either fatal or require euthanasia on humane grounds, and require symptomatic and supportive treatment to alleviate their suffering until a diagnosis can be reasonably made. Some horses with the chronic form of the disease have been successfully treated with long-term supportive therapy. This includes feeding the horse by stomach tube several times a day and administering medication to try to stimulate intestinal movement. This treatment needs to be continued for weeks, and sometimes months, and is only suitable for a small number of very select cases, basically those that have only slight and partial intestinal paralysis. Horses which do recover from the chronic form of the disease are permanently susceptible to choke and to colonic and rectal impactions thereafter and need careful dietary management for the rest of their lives. Horses with signs of acute and sub-acute grass sickness will not respond to treatment because their intestines are irreparably paralysed and they should be humanely destroyed as soon as the diagnosis can be reasonably made.

Historically, grass sickness cases were first diagnosed in Scotland and the north of England but cases now occur relatively commonly throughout many areas further south and have been confirmed in horses along the south coast. Although there is a seasonal (late spring and summer) increase in the number of cases diagnosed, animals can be affected at any time of the year. Horses turned out to grazing after a period of stabling appear to be particularly at risk. Grass sickness cases occasionally occur in batches, meaning that more than one animal may be affected on a particular premises or in a particular area at about the same time. This often happens following rain after a period of drought. Most however, are single isolated cases and some may be old horses who have lived on the same pasture for most of their lives.

In view of these apparent predisposing factors, horses being turned out after a period of stabling, e.g. racehorses who are resting or retiring, should not be just turned out on grass alone, but should continue to receive some hay and concentrate feed. Where a case has occurred, horses in-contact should be re-located to another paddock and should be brought into stables for a couple of hours each day to be fed hay and some concentrate food.

Grass sickness appears to be more frequently diagnosed throughout UK and apparently similar cases have now been confirmed in Ireland and continental Europe. Research is continuing in an effort to confirm the causative agent and to understand better how and why the disease occurs. Hopefully, once this research bears fruit, specific preventive measures will emerge. The latest research work has unearthed some earlier clinical work with Botulism vaccine which was done almost 70 years ago and ignored by many scientists until now.  It appears that vaccinating horses with Botulism vaccine may protect against Grass Sickness and work is now underway to develop and test a suitable vaccine for horses.  It would be great news for horse owners if a vaccine could be found for this very distressing disease.  Unfortunately we see now about 6 cases annually in the Cotswolds and very few cases recover to be useful horses in the future.