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Threadworms      Strongyloides Westeri

These are worms that infect foals.  The mare passes the worm to the foal in her milk.

Size Appearance

Very small, fine worm up to 1cm long

Location In Horse

They migrate through the liver and lungs.  Adults are found in the intestine


Migrating larval stages damage the lining of the arteries particularly the intestinal lining.

Loss of appetite

Life Cycle


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

The mare can be treated with ivermectin just before they foal to reduce the number of larvae passed to the foal in the milk.

Preventative Measures

  • Clean dry bedding will prevent the parasites increasing their numbers in the free living part of the life cycle.  They need damp, wet and warm conditions to do this.
  • Regular anthelmintics treatment will reduce the number of eggs being laid.