Grass Sickness

Signalment

• Most prevalent in the UK and western Europe.
  • Common in wetter areas, e.g. the South West.
• Seen in horses out at pasture in late summer and autumn.
• Usually affects young adults.
  • 6-7 years old.

Diagnosis

Clinical signs

• • Acute oneset:
    • Muscular tremors
    • Abdominal pain
    • Does not eat
    • Constipation
    • Become severly tympanic in acute cases
    • Dull and restless
    • Avoid swallowing
    • Salivate excessively
  • Degenerative lesions are seen in the autonomic nerve ganglia, including enteric plexuses
  • May either:
    • Progress rapidly to death
    • Take a slower clinical course.
      • Eat a bit, but food drops out of mouth
      • Go on to die slowly.
  • Some horses recover
    • This is very unlikely, and the condition is usually fatal.
  • Clinically difficult to diagnose - signs are confined to the gut.
    • Easy to diagnose on post mortem

Pathology

• • Stomach and small intestine large amounts of contain watery yellow fluid.
    • There is an abrupt change in the large intestine, where no fluid is present.
      • large intestine has very dry mucoid contents.

Pathogenesis

• • Due to functional obstruction at ileocaecal valve and a degree of paralytic ileus of the small intestine.
  • The exact cause is unknown, but a type of bacterial or fungal toxin which may damage autonomic nervous system ganglia may be involved.
    • Clostridium botulinum is thought to be involved.
• A similar condition seen in hares
  • Certain yeares almost seem to have outbreaks.
• Certain pastures at certain times of year produce grass sickness quite often.
  • A definitive diagnosis must be made - if the condition is due to the grazing we need to know.
    • E.g. if on livery or stud grazing, may put people off going there.
• 'Diagnosis
  • At post mortem look for degenerative changes in coeliaco-mesenteric ganglia - need to examine histologically.
    • Ganglia are peanut sized and found in perirenal fat between adrenal gland and the aorta.

Equine grass sickness, or equine dysautonomia causes a paralysis of the gastro-intestinal tract, by disruption of the autonomic nervous system. This leads to a pooling of ingesta throughout all parts of the gastro-intestinal tract. The condition may occur acutely, or progress chronically over several weeks, but all cases will eventually die. A definitive diagnosis is obtained by taking an ileal biopsy, and inspecting the intrinsic myenteric plexus. There is no effective treatment, although in the short to medium term, horses can be successfully managed by informed and attentive owners.

References

Also known as:

Equine Dysautonomia