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.

Researchers believe 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.

Grass sickness can be acute or chronic in nature. While some 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.

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.
  • EGS 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 EGS 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