grass sicknessGrass 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.

The Animal Health Trust launched a major nationwide blind trial of a vaccine for grass sickness in 2014. It was to involve more than 1,000 equines across a two-year period. The results of the trial have not yet been released. Researchers have also identified similarities between Alzheimer’s and equine grass sickness.

Grass sickness: which horses are at highest risk?

  • Horses between the ages of two and seven in good to fat body condition are at the highest risk, although equine of all types and ages can be affected.
  • It is particularly prevalent during April, May and June, and later in the autumn, after a spurt of grass growth, but can occur at any time.
  • Anecdotally, ground frosts and sudden weather changes are associated with disease outbreaks.
  • One study reported that the majority of cases occurred during periods of cold, dry weather.
  • Equine grass sickness almost exclusively affects grazing animals. Risks include dietary changes, recent movement to new premises or pasture and frequent administration of wormers.
  • If your horse is kept in a high-risk area, always consult your vet before choosing a worming treatment.
  • Moderate worm egg counts have been associated with decreased risk of the condition.
  • Frequent grass cutting and manual faeces removal appear to lessen the likeilhood, but pasture disturbance, such as construction work, for example replacing fencing, may increase the risk.
  • Cases are more common on previously affected premises and pastures, and at large establishments such as livery yards, riding schools and stud farms
  • Grazing with high soil nitrogen and sand or loam soil types have been found at affected locations
  • There is some evidence that premise with domesticated birds or fowl may carry a heightened risk.
  • Survival rates from chronic grass sickness are greater in Scotland (60%) than in England (45%), but the reasons for this are not yet known