Grass Sickness

What is grass sickness?

Grass sickness is a disease of horses, ponies and donkeys in which parts of the nervous system stop working properly. It is thought to be caused by a toxin from Clostridium botulinum type C, a bacterium that is found in the soil. Grass sickness is not a new disease and was first identified in Scotland 100 years ago however there still is a great deal that isn’t known about it. Research into a vaccine is currently underway at the AHT (Animal Health Trust) in Newmarket. Great Britain has the highest incidence of grass sickness in the world and the disease has been reported in most areas of England, Scotland and Wales. Eastern areas of the country are at increased risk. Thankfully we don’t live in a particularly high risk area although we do see sporadic cases. It has a 95% mortality rate.

What type of horses can be affected?

Grass sickness has been reported in all breeds. A case has even been confirmed in a captive zebra. Grass sickness can basically occur in any horse, pony or donkey that has access to grass.

Factors that increase the risk for development of grass sickness are:

  • Young age (peak incidence 2-7 years) although horses outside this age range can also be affected by the disease
  • Horses at grass
  • Good to fat body condition
  • Geographic location
  • Large number of horses on the site
  • Previous history of cases on the pasture
  • Cool, dry weather in preceding 10-14 days
  • Time of year – peak in the Spring and Autumn
  • Worming or change in diet
  • Disruption of pasture
  • Movement to new pasture in the preceding 14 days
  • Mechanical removal of droppings increases the risk of grass sickness whereas manual removal of droppings reduces the risk

What are the clinical signs?

Grass Sickness is classified as acute, subacute or chronic. This refers to the severity of the clinical signs and the duration. The severity is directly linked to the amount of damage that has been done to the nerves within the gut.

ACUTE Grass Sickness

  • Most severe
  • Rapid onset of clinical signs
  • Death or euthanasia on humane grounds within 48 hours
  • Inappetence and depression
  • Excess salivation as unable to swallow
  • Colic signs (mild to severe)
  • Minimal droppings covered in mucus
  • Very high heart rate (over 60 bpm)
  • Patchy sweating
  • Reduced or no gut sounds
  • Drooping eyelids
  • Fine muscle tremors
  • Most cases euthanised on humane grounds

SUBACUTE Grass Sickness

  • Less severe clinical signs than acute cases
  • Most cases are put to sleep within 7 days
  • Loss of body condition
  • Tucked up appearance
  • High heart rate
  • Low neck carriage
  • Weight shifting
  • ‘Elephant on a tub’ stance
  • Most cases euthanased on humane grounds

CHRONIC Grass Sickness

  • Rapid weight loss (emaciated look)
  • High heart rate
  • ‘Snoring’ type noise
  • Select cases can be treated with intensive nursing. Cases are fed palatable high energy food and stimulated with regular human contact and grooming etc. Recovery takes time and requires a real commitment from owners. The good news is that most survivors recover sufficiently to return to work. Do remember that of all the cases that suffer from grass sickness the number that are suitable for treatment is small.

How is Grass Sickness diagnosed?

Most vets will have a strong suspicion of Grass Sickness based on the clinical signs. However a definitive diagnosis is only made either at surgery or post mortem examination when a biopsy is taken from the ileum (part of the small intestine). The sample will be analysed by experts in a laboratory.

Can Grass Sickness be prevented?

In yards where there have been recent confirmed cases then it is advised that young horses do not graze on the same pastures where an affected horse was kept. Good management practices are important e.g. changing diet gradually and  minimising stress. Remember in this area grass sickness is not common.

Download the "Grass Sickness" information sheet