‘Efficacy’ refers to the capacity of a drench to kill worms. As drench resistance develops, drench efficacy is reduced. It is important to know which worm species are present, and choose a drench with a high efficacy against those worms. This ensures that few resistant worms remain in the cattle to produce eggs that pass out in the faeces and re-contaminate paddocks. Drenches with lower efficacy, however, will leave larger numbers of resistant worms inside the gastrointestinal tract.
Drenches often have high efficacy against one worm species but low efficacy against others. For example, the small intestinal worm (Cooperia species), have a high level of resistance to macrocyclic lactone (ML) chemical actives in some areas of Australia. A good choice of drench where small intestinal worms are the only target would be a benzimidazole (BZ) or levamisole, as these have little reported resistance. In contrast small brown stomach worm (Ostertagia ostertagii) resistance to BZ is now well established across the country but ML drenches are still effective. As cattle usually have mixed populations of worms, with several species present, drenches should have good efficacy against all of the worm species present (e.g. a combination drench).
Drenches can be tested for efficacy by conducting either a ‘before and after’ test, or faecal worm egg count reduction test (WECRT). These tests involve taking faecal samples for worm egg counts on Day 0 (day of treatment) and then again 14 days later, to calculate the estimated percentage of worms killed for each species.
A benchmark of 95% efficacy is generally used to indicate whether drench resistance is present, meaning that a reduction in worm egg count of at least 95% after treatment is required, for each species of worm. At this level, very few surviving resistant worms will be left to lay eggs and pass resistant genes on to the next generation of worms, therefore slowing the build-up of resistance.