In order to reduce the risk of increasing worm resistance when using long acting drenches (especially pre lambing) it is recommended a “primer drench” as well as an “exit drench” be used in combination with a long acting drench. Consult your veterinarian.
Effective persistent treatments kill immature and adult worms at the time of treatment, as well as infective larvae eaten by animals (with pasture) during the period of protection of the treatment—for sheep, this is about 3 months for long-acting and 1–6 weeks for mid-length treatments (depending on the particular product).
Primer drenches clear the animal of any worms that are resistant to the long-acting treatment. A primer drench is an effective short-acting drench (preferably a combination) that does not include the same group as the long-acting product. Give a primer at the same time that a long-acting product is given.