Equine Grass sickness is a disease of horses, ponies and donkeys in which nervous system damage to the digestive system and gut occurs. Gut paralysis is one of the major signs. It is almost always seen in horses which at grass and rarely occurs in horses with only access to hay. Some areas have a higher incidence of the disease, of the UK particularly Scotland and the East of England. The disease generally peaks between the months of April to July, and a smaller peak can occur in Autumn.
The current theory is that a bacterial toxin (Clostridium botulinum), found in pasture is responsible. The bacteria produces a toxin which is absorbed by the intestines and then affects the nervous system.
Younger horses, especially those 2-7 years old seem to be more susceptible to the disease. Mares,geldings and stallions are all equally affected, as are horses of different breeds. Horses in good to fat body condition also seem to be more commonly affected. Stress, moving premises/ field and certain weather conditions may predispose an individual horse to development of EGS.
EGS occurs in 3 main forms: acute, subacute and chronic, however there can be overlap of these forms in some cases. The main clinical signs relate to paralysis of the gastrointestinal tract from the oesophagus to the rectum.
Acute: Horses with acute EGS may be found dead or usually require to be put to sleep within 2 days of signs becoming apparent. Severe gut malfunction leads to signs of severe colic including rolling, pawing and flank-watching. Drooling of saliva and difficulty in swallowing is also seen. Spontaneous reflux, where digestive fluids build up in the stomach and then start to drain through the nostrils can be seen and imaction of the large colon (constipation) can also be seen. Many horses will have patchy sweating and muscle tremors are also common.
Subacute: Signs are similar to acute cases but are less severe. Accumulation of fluid in the stomach is rare, but weight loss and colic are often seen. Small amounts of food may be eaten; however many of these cases will die or need to be put to sleep within a week.
Chronic: The signs start more slowly and weight loss (which can be sudden and extreme) or intermittent colic are usually the signs seen. Some horses may also have difficulty in swallowing (dysphagia) and show a poor appetite. Some chronic cases can survive, although this requires intensive nursing and massive owner commitment. Some horses which survive chronic grass sickness can return to work.
Diagnosis is clear in cases with “classic” signs; however some cases need to be distinguished from forms of colic, difficulty swallowing or weight loss. The only way to make a definite diagnosis is to examine a piece of small intestine which is collected during surgery, or after death. A nerve ganglia is also commonly examined after death. Characteristic changes in these tissues give us the diagnosis. There is no way currently to definitively diagnose EGS in a horse without the need for surgery; therefore exploratory laparotomy (colic surgery) is often required to rule out other causes of colic.
Acute and sub-acute cases are generally considered incurable.
Chronic cases can respond to supportive care and symptomatic treatment (treating the symptoms we see). Nutritional support is the most important factor and specific information regarding this can be acquired from the vet. Anti-inflammatory pain killers, regular grooming as well as hand walking and veterinary care are also required. Recovery will take several months and is very variable between individuals. Cases which are likely to survive tend to gain weight in the first few weeks and do not have extreme weakness, colic or marked dullness.
Once a case is confirmed in a geographic area, it is generally advised that other horses are moved from the fields or stabled. It is not yet known how to determine when a field will be safe to use again. However some steps can be taken to try to reduce the chance of another horse being affected if horses have to stay on the premises. Avoid excessive soil exposure such as over-grazing and poached fields, soil disruption such as pipe laying/ construction work and mechanical removal of droppings from pasture.
When buying an equestrian property it is worth asking for a declaration of the history of the incidence of diseases such as Grass Sickness on the premises.
There is currently a vaccine trial being carried out in the UK by the Animal Health Trust (AHT) There are conditions for joining the research trial which include EGS having been diagnosed on your premises previously. For more information please contact the AHT .