Grass sickness is a disease of the intestinal system of grazing horses, seen usually from late spring to midsummer in horses aged from 2-7 years. Current research suggests that the disease is a form of botulism, caused by horses ingesting Clostridium botulinum or its toxin. Some, as yet unknown, conditions cause the organism to multiply on pasture. Cases are more likely to occur if:
- There has been some soil disturbance in the pasture
- The horses have recently changed fields
- The horses have recently been moved to new premises
- The horses have recently been wormed with an ivermectin-type wormer
- The weather is cool and dry
- There have been other cases on the premises before.
The toxin produced by this bacterium damages the nerves controlling the normal propulsive movements of the intestines, resulting in failure of the stomach to empty (it fills with fluid), no audible contractions of the intestines, and hence constipation. The disease occurs in several forms.
Acute Grass Sickness
The horse shows signs of acute colic. The stomach is tightly distended with a green fluid that can be retrieved with a stomach tube. Swallowing is difficult, and patchy sweating and muscle trembling occur. Many acute cases are assumed to be surgical colics and may be operated on, only to find that there is no correctible problem in the abdomen. There is no treatment, other than pain control. The disease has a rapid course, usually ending in the horse being humanely destroyed, although sometimes it is found dead in the field with a ruptured stomach.
Chronic Grass Sickness
In chronic grass sickness there is marked weight loss and the horse has an extremely gaunt, tucked up appearance. There is patchy sweating and tremors of the upper muscles of the fore and hind legs. There is dried sticky mucoid material at the nostrils. Very few droppings are passed, and those that are, are hard, dry and dark. The horse has a very poor appetite.
What To Do Next
If a suspect case occurs, the other horses in the group should be brought indoors and fed hay until grass sickness is confirmed.
Most chronic cases will eventually be humanely destroyed. A small proportion (probably less than 25%) may eventually recover but this may take up to a year and it requires a major commitment in time and effort by the owner. Careful nursing is required, including keeping the horse hydrated and especially maintenance of the appetite by offering palatable foods and frequent stimulation of the horse by changing its surroundings and company.