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 gains access to the horse’s gastrointestinal tract and then attacks the nerves with devastating results.

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.

Grass sickness: which horses are at highest risk?

Based on data collated from cases from 2000-2009 by The Horse Trust

  • Horses between the ages of five and nine in good to fat body condition are at the highest risk. The youngest affected animal on the database is two months old and the oldest is a reported 47 years old
  • Crossbreds account for 34% of cases. Of the 66 pure-breds recorded, thoroughbreds made up 12%, Highlands 9% and Welsh cobs 8%
  • England accounts for 58% of the cases across the UK since 2000, although this may be due to it having a larger equine population.
  • Premises where there has been a previous case of grass sickness are at a higher risk
  • Cases of grass sickness tend to occur in clusters. If one horse is diagnosed, the radius of increased risk is at least 10km for a time period of about a month
  • A sand/loam-type soil with a high nitrogen content can heighten the risk
  • There have been fewer chronic cases of grass sickness (3%) than there have fatal acute (5%) and sub-acute (21%).
  • Survival rates from chronic grass sickness are greater in Scotland (60%) than in England (45%), but the reasons for this are not yet known