There are many different types of worms found in horses and they can cause slightly different problems in the horse. Low burdens of worms generally do no harm, but high burdens are more likely to cause serious problems — and in the worst cases can result in death.

Small strongyles (cyathostomins/small redworms)

  • Live in the large intestine
  • Larvae can hibernate or “encyst” in the gut wall, causing marked irritation of the intestine wall when they re-emerge
  • This re-emergence can cause diarrhoea, weight loss and colic (cyathostominosis)
  • This disease is most common in the winter, in young horses (less than six years old), and in horses who have been wormed recently
  • These worms have developed resistance to certain types of wormer
  • Encysted larvae can be killed only by certain types of wormers

Large strongyles (large redworm)

  • Worm larvae travel through the wall of the large intestine into blood vessels
  • These worms used to be a common cause of colic, but rarely cause colic these days due to the development of more effective wormers in the 1980s
  • Killed by most modern wormers


  • Live in the small intestine close to the junction between the small and large intestine
  • Increasing burdens place horses at greater risk of certain types of colic
  • These worms are killed only by certain wormers


  • Live in the small intestine of young horses (less than four years old)
  • If large burdens build up, they can cause weight loss and colic


  • Larvae live on the skin and can cause summer sores
  • Worms develop in the stomach and generally cause few problems
  • Killed by most modern wormers


  • Eggs are laid on the skin
  • Larvae develop in the stomach
  • Generally cause few problems and are killed by most wormers


  • Lives in the small colon
  • Can cause irritation resulting in tail rubbing
  • Killed by most wormers, although some wormer resistance has been reported


  • Larvae develop in the skin and can result in the development of skin lumps
  • Killed by most wormers
  • Skin lumps caused by warble larvae are now almost unheard of due to the advent of modern anthelmintics