Small Ruminants Practice Exam

Maintaining refugia means keeping a portion of the parasite population unexposed to the drug so resistant genes don’t sweep through the population too quickly. By treating only a subset of animals based on their parasite burden, you preserve these untreated worms and those in the environment. This preserves susceptible parasites in the population, which dilutes any resistant genes when they breed with survivors from treated animals, slowing the spread of resistance.

In practice, this is the idea behind targeted selective treatment: treat only the high-burden animals and leave others untreated. If you treated every animal on a fixed schedule, you’d remove refugia, creating strong selective pressure for resistance because nearly all parasites that survive are the resistant ones. And completely avoiding deworming isn’t a viable disease-control strategy, even though it would not impose drug pressure.