Grass fever, also known as equine grass sickness (EGS), is a disease of the nerves that coordinate intestinal movement and the digestive tract. The disease is nearly always fatal, but not contagious. It occurs most often in the spring and fall, when new grass growth is highest. EGS is most prevalent in Great Britain; however, it's also found in northern Europe and Ireland. As of 2011, two cases were diagnosed in North America.
Types of Grass Sickness
There are three types of EGS: acute, subacute and chronic. The disease damages the nervous system, which controls involuntary functions and results in paralysis of the gut. In sudden-onset (acute) EGS, horses will display symptoms of severe colic. Those with subacute EGS experience the same symptoms, although less severe. They typically begin losing weight gradually until it becomes so severe that they resemble a thin greyhound dog. Horses with the slowest form of the disease, or chronic EGS, generally become lethargic, depressed and have difficulty eating food.
Despite more than 100 years of research, the cause of EGS is unknown. However, scientists believe there may be a connection between selenium deficiencies and reduced levels of essential protective antioxidants within the body. Another theory under investigation is that EGS is caused by a bacteria found within the soil, called Clostridium botulinum.
Symptoms of EGS include a rapid heart rate, fatigue, prolific salivation, distension of the small intestine, muscle tremors, difficulty swallowing and intermittent sweating along the neck and flanks. The symptoms are the result of impaction of the large intestine and nerve damage.
Because not all horses exhibit every sign and symptom characteristic of the disease, and because it can be difficult to differentiate between EGS and colic, making a diagnosis can be challenging. Additionally, although blood and abdominal fluid tests can be analysed in combination with the symptoms, the only truly accurate diagnosis comes following death and the surgical removal of the small intestine.
Acute and subacute EGS are nearly always fatal despite intervention; therefore, no treatment is typically given and the horse either die or is put to sleep within two days of diagnosis. Those horses with chronic EGS who can be persuaded to eat chopped vegetables, grass and high-energy concentrated feeds soaked in molasses have a higher chance of survival. Other treatments consist of intravenous fluids to prevent dehydration, medication that increases gut activity, continuous human contact and frequent grooming to reduce sweating and prevent hypothermia. According to the Equine Grass Sickness Fund, "...those who survive more than a month following the diagnosis of a case of EGS are likely to be very resistant to the disease."
Horses that live within 6 miles of those afflicted with the disease are at an increased risk of contracting it within approximately one month of the original diagnosis. As the radius of the affected area decreases, the risk factor decreases accordingly. Grass sickness is not transferred from insect bites, toxins or from horse to horse. Young horses, particularly those 7 years old or younger, are at most risk.
Horse owners can take precautions to prevent EGS in affected areas by not stabling or allowing their horses to graze on high-risk premises (those with diagnosed cases), supplementing their horse's diet with extra forage such as hay (especially during the high-risk period of April to June), not digging up or disturbing soil in pastures, removing manure by hand, delaying worming in the months preceding spring (known to increase risk), avoiding pasture modifications during the spring (buying new horses, moving horses to different pastures) or abrupt changes in diet. Changing a horse's diet remains one of the highest risk factors for EGS.