Anthelmintic resistance

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

Roundworms are resistant to anthelmintics ———- cattle carry these resistant roundworms

Key points to remember:

• It is the worms that are resistant to the anthelmintic, NOT the cattle
• Resistance develops to the group, not the individual product

Resistant worms produce resistant offspring

• Leaving some animals untreated can dilute worms which survived treatment as untreated animals will be putting out susceptible worms too

• Resistance develops independently for each type of roundworm (species), therefore having resistance in one type doesn’t necessarily mean you will find resistance in all roundworms

Current status

• Resistance to the clear drenches (3-ML) is common in Cooperia
  • Study found ~50% of cooperia populations to be resistant
• Resistance to the white drenches (1-BZ)  has been identified in Ostertagia (cattle brown stomach worm)
  • Believed to still be at a relatively low level
• No information is available on the resistance status of the yellow drenches (2-LV)