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Roundworm (ascarids)     Parascaris equorum

Large roundworms are the largest parasite of all.  They affect foals up to 18 months of age, but after this adult horses appear to be immune to them.

Size Appearance

50cm when mature

Location In Horse

Egg laying adults are present in the small intestine, but the larvae migrate through the liver and lungs.


The liver can repair itself but the lungs do not have this capacity.  Damage caused to the lungs is permanent and the lung function is reduced.

Reduces thriftiness in foals and can cause pneumonia.
Poor growth rates
Dull coats
Fatal blockages or ruptures in the gut

Life Cycle

A single female can lay one million eggs a day that can survive for up to 10 years!!!


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

Foals should be treated from 4 weeks of age.

Preventative Measures

  • The eggs that are passed in the foals’ faeces have a sticky coat and can attach themselves to floor, walls and tools.  They remain alive for a long time and will be the source of infection to next season’s foals. Stables and equipment should be disinfected between use.  
  • 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.