The success of drenching in controlling worms has become a double-edged sword with chemical resistance now a major problem across Australia. Resistance occurs when drenching fails to kill 95 per cent of the parasite population. The surviving worms possess a heritable trait to survive the chemical and can breed to form a resistant population. Resistance can develop to one or more drench groups and is now widespread throughout dry inland and high-rainfall areas. Resistance occurs as a result of over- or under-using drenches.
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