Refugia
One of the tools in the parasite battle toolbox is the concept of "refugia". It goes against a farmer's natural instinct to kill every parasite because it means deliberately leaving 5-10% of each mob undrenched. This is done to maintain parasites susceptible to drenches because they've never been exposed. When a lamb is drenched these susceptible parasites are killed but the resistant ones live another day to spread their genes amongst survivors.If a proportion of the mob is left undrenched then these susceptible parasites will also be shed and picked up by the next animal to graze that patch.
If a susceptible parasite mates with a resistant one some degree of susceptibility to drench is passed on in the genes of the progeny. If only the resistant parasite population is left then there will soon be no susceptible parasites left - dramatically worsening the resistance problem.
Any drench, no matter how effective only kills the usually less than 5% of total worms that are in a sheep or cattle beast. The other 95+ percent is lying in wait as eggs or larvae on pasture.
These are eaten by grazing animals, develop in the gastro intestinal tract where they do their damage and mature to adults, then pass through the animals system as eggs shed onto pasture. The cycle continues.
The best five to ten percent of a mob should be left undrenched. When the lambs or young cattle come through again the undrenched ones may be picked out for drenching because of the effect of their previously undrenched worm burden. They also may not. This is the aim.
Refugia alone is only part of the many management strategies to handle drench resistance.