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Small Redworm

Small Redworm      Cyathostomes

These are currently the most important parasite of the horse for two reasons:
[1] They encyst in the horse and are undetectable at this stage.
[2] They are the only worm with proven resistance to benzimidazole wormers.

Size Appearance

Up to 2.5cm long,  thin and reddish in colour

Location In Horse

They migrate in blood vessels.  Adults are found in the large intestine and the encysted larvae are found in the gut wall.


Migrating larval stages damage the lining of the arteries particularly those in the gut.

Recurrent bouts of colic and even death
Gradual or rapid weight loss
Sudden onset of diarrhoea
Lethargic or sluggish behaviour
Loss of appetite

Typically affect horses under six or from mid teens onwards.

Life Cycle

6 weeks to 3 years


The only way to treat small strongyles is with anthelmintics (wormers) containing any of the chemicals pyrantel ivermectin, moxidectin or benzimidazole.  These chemicals will remove both the migrating larvae and adult worms.

However only the five-day Panacur Equine Guard treatment and Equest will kill the EL3, third stage encysted larvae.

It is important to kill the larvae before they emerge as that is when the severe symptoms and even death are caused.  Horses should be treated in November and the end of February.

Preventative Measures

  • Prevent pasture becoming contaminated by the regular removal of droppings.  This must be at least twice a week during summer months.
  • Regular anthelmintics treatment will reduce the number of eggs being laid.
  • Avoidance of over crowding 
  • Mixed grazing with sheep and cows