The majority of drench products (however applied) are considered as ‘short-acting’, as they are active within the animal for a relatively short period. This may be only a few hours for oral and injectable treatments, up to 2-4 weeks or more for some pour-on macrocyclic lactone (ML) products, reflecting both the formulation (application type) and the active ingredient. For drenching recommendations, the ML pour-ons are considered as short-acting, as they remain in the animal for a far shorter period than long-acting types.
In contrast, ‘long-acting’ products can be used to improve animal productivity and growth rates, particularly when pastures have a high level of worm larval contamination. They can provide protection against incoming larvae for a period of up to 120 days, unless severe resistance is present. Long-acting products also provide long-term protection from cattle ticks.
Due to resistance in some worm species, a single long-acting treatment may not kill all of the target worms. Long-acting drenches in sheep are considered to have a significant potential to increase the level of resistance due to the worms having a longer exposure to the chemical actives. It is likely that a similar effect occurs with these products in cattle. Careful management of long-acting drenches is necessary to provide good parasite control but prevent the development of drench resistance.