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Anthelmintic resistance

Anthelmintic resistance: a population of worms which survive a correctly applied anthelmintic treatment

 

Roundworms are resistant to anthelmintics (wormers) ———- horses carry these resistant roundworms

 

Key points to remember:

  • It is the worms that are resistant to the anthelmintic, NOT the horse
  • Resistance develops to the group, not the individual product
  • Resistant worms produce resistant offspring
  • Leaving some horses untreated can dilute worms which survived treatment as untreated horses will be putting out susceptible worms too

 

 

Current status

  • Resistance to all three anthelmintic (wormer) classes (1-BZ, 2-THP and 3-ML) have been reported in small red worms (cyathostomins) at varying levels. Benzimidazole (1-BZ) resistance is most commonly identified although accurate prevalence is unavailable.
  • In ascarids (Parascaris species), resistance has been reported to the macrocyclic lactone wormer group (3-ML)
  • Few reports have also been made of macrocyclic lactone (3-ML) resistance in pinworms (Oxyuris equi)