Grass sickness is an often fatal condition that typically occurs in grazing horses. It was first recognised in eastern Scotland in 1907, yet relatively little is known about the condition.
UK has the highest incidence of grass sickness in the world; it is estimated that the disease kills between one and two percent of horses in the United Kingdom annually, with cases being more common in spring.
Grass sickness can be acute or chronic in nature. While around 55% of chronic cases recover with careful nursing, acute cases are nearly always fatal. Clinical signs can be confused with colic and include problems swallowing, sweating, constipation, reflux of fluid from the nose and muscle tremors. A horse may lose weight quickly to the point of emaciation.
It has been believed that a toxin produced by the bacterium Clostridium botulinum causes damage to parts of the nervous system which control involuntary functions, producing the main symptom of gut paralysis. However, this has been questioned by recent research published in the Equine Veterinary Journal (October 2016) that identified key differences between grass sickness and botulism. The study titled Equine grass sickness, but not botulism, causes autonomic and enteric neurodegeneration and increases soluble N-ethylmaleimide-sensitive factor attachment receptor protein expression within neuronal perikarya’ suggests that grass sickness is unlikely to be caused by neurotoxins from this bacterium and concludes that further investigation of an alternative cause is needed.
The high levels of the condition in Scotland may be partly associated with the particular composition of macro and trace elements in the soil in this region, according to the study Equine grass sickness in Scotland: A case-control study of environmental geochemical risk factors, also published in the Equine Veterinary Journal (October 2016).
While the majority of horses with grass sickness can be diagnosed by vet based on clinical signs, the only way to definitively diagnose the disease in a live horse is to take a biopsy from the intestine while the horse is under a general anaesthetic. In the 2016 study Neuronal chromatolysis in the subgemmal plexus of gustatory papillae in horses with grass sickness the characteristic degeneration of nerves was identified in small biopsies collected from tongues of horses post mortem. While further validation of this technique is required, it could potentially provide a relatively non-invasive method of confirming diagnosis in a live horse.