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The drug may end up in the manure, and the parasites may encounter it there at a marginally-lethal dose. When the parasite progresses further in its life cycle, its offspring may be resistant when they infect the next livestock animal.

Most parasites have a life-cycle that includes time spent outside the preferred host animal, including in zoonotic species that may not have symptomatic infections. They may acquire resistance in any stage, in any place they encountered the drug.