Great Britain is the country most commonly associated with grass sickness although it also occurs in high frequency in Scotland, with lesser reports in Ireland, Wales as well as France, Italy, Holland, Sweden and Belgium. The condition is only seen in horses that are on pasture although even horses that are fed dry feeds in addition to pasture may develop the same condition. Donkeys, ponies and mules will also develop grass sickness and it can strike horses of any age, condition and sex.
Grass sickness is a gut paralysis that is believed to be caused by a toxin in or on the pasture grass. Many researchers now believe that the problematic toxin comes from a variety of fungus that is almost always present in pastures where reports of grass sickness occur. The fungus most likely linked to the condition is Fusarium and often a dry, cooler period that may affect the production of spores in the Fusarium is present before an outbreak. The lowest incidents of grass sickness are always during the wet and rainy seasons in August. It is interesting to note that more recently reports of grass sickness have been confirmed in Argentina and Fusarium was also found in the areas the diagnosed horses had access to.
Horses that are diagnosed with grass sickness will have symptoms that are similar to colic. They often come on very sudden and severe, with many horses with acute symptoms having to be put down in less than three days after the symptoms occur. The toxin paralyzes the gullet or esophagus and the horse cannot move the food in the digestive system. They will be in varying degrees of pain and will often kick at the stomach, lie down and roll, appear highly anxious and turn and stare at their stomach. As the condition becomes worse the abdomen will swell and a foul smelling partially digested food and mucous discharge will run from the nostrils. Constipation is often present with straining to have a bowel movement and thick, mucous coating of any feces that is passed. The horses will have fever, severe sweating and will often collapse and die within three days if not humanely put down. Sub-acute symptoms can continue on for a week before they become acute and result in death if the horse is not put down.
In chronic grass sickness the horse will have less severe symptoms that will come and go, resulting in rapid weight loss and generally poor health. Some horses, for reasons not understood, will fully recover from a bout of grass sickness and live full and healthy lives, however this is the exception to the rule.
In acute and the less obvious sub-acute levels of grass sickness there is no cure and most vets strongly recommend humanely destroying the horses to put them out of the pain and suffering they are experiencing. In some chronic cases changing the feed and taking the horse completely off grass, providing lots of high energy, high fiber foods and providing constant monitoring and attention can help the horse with chronic grass sickness. These horses are typically not in the pain that the acute and sub-acute cases are, but will need drug therapies using cisapride to stimulate the action of the digestive system. They will also need constant monitoring if on pasture to ensure they do not come down with grass sickness again in their life. Stabling during cold, dry periods and managing the pastures is essential for prevention of the condition.