Drench Resistance
Drench Resistance is caused by internal parasites developing inherited tolerance to commonly-used drenches. It results in animals with worms that don’t respond to drenching. Today, drench resistance is very common.
Theoretically, drench resistance occurs once a population of a species of worm can survive a dose of a drench that would have previously killed it. Initially resistant worms are rare in a population of worms (perhaps as few as 1 in a trillion). When a sheep is treated the resistant worms survive and, if they find a mate, can reproduce. The resultant offspring are resistant and if they survive as larvae on the pasture and infect another sheep they will make up a greater proportion of the worm population than their parents did. Over time, and with continued treatment, the overall resistance level to the treatment within the worm population increases.
Many diverse methods have been employed to slow the onset of parasite resistance to drenches, including pasture management, animal management, quarantine treatment and providing ‘refugia’ for worms by Targeted Selective Treatment.
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