Equine Grass Sickness (EGS) is a relatively unknown, yet serious condition that affects a horse’s nervous system. First reported in Scotland in the early 1900s, the disease had been largely contained in Northern Europe. However, more recently, it was suspected in Australia and the Falkland Islands, and a mule with the symptoms was found in the U.S.
Typically affecting grazing horses, the disease is categorized by how long the horse displays signs. Acute cases are one to two days; Subacute is two to seven days; and Chronic is more than seven days. The extensive neuronal degeneration that occurs in acute and subacute cases causes intestinal dysmotility, which is usually fatal.
The exact cause of EGS is unknown, although there are certain risk factors that have been linked to the illness. Horses between two and seven years old are the most frequently stricken. Environmental changes such as moving to a new pasture, especially in spring and early summer, have been noted. Climatic conditions like cool, dry weather and irregular frosts are thought to play a part. Dietary changes and usage of ivermectin-based dewormers have also been linked to the disease. Potential causes also point to a soil-borne agent that may, in certain conditions, produce or release neurotoxins.
Horses affected by EGS show a variety of signs including: an elevated heart rate, difficulty swallowing, mucous-covered feces, weakness, abnormal salivation and sweating, gastric reflux, drooping eyelids, twitching and a “tucked up” abdomen. Testing to confirm EGS is limited to a topical application to the horse’s eye which can result in a reversal of ptosis, which suggests EGS. Tragically, acute and subacute cases are always fatal, and it’s considered merciful to euthanize the horse to avoid prolonged suffering. Chronic cases have a survival rate of about 40% with diligent nursing and offering palatable foods to the horse to survive anorexia.
