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Tests for Drench Resistance

As resistance to drenches by worms of cattle becomes more common, it is increasingly important to know the efficacy (effectiveness against worms) of different drenches on a property.

All current drenches had an efficacy against the major worm species of close to 100% when first introduced. However, over the years their efficacy has reduced considerably as drench resistance has developed.

The effectiveness of a drench can vary greatly between species and properties. Usually, when resistance develops, drenches have different levels of efficacy against different worm types, and may be fully effective against some but have little effect against others.

The benchmark for efficacy of cattle drenches is a 95% reduction in worm egg counts for a particular species (although this may differ from the actual reduction in number of worms in the animal). This is because most drenches when launched kill at least 95% of worms present, so a figure well below this can indicate the development of resistance (see Figure 1)

Figure 1. Graph showing an exemplar effective drench (orange) that kills on average over 95% of worms in a mob, compared to an exemplar lower-efficacy drench (blue) that kills fewer than 95% of worms.

Several methods are available to test drench products for their efficacy against roundworms. The types of test can be categorised into (click to expand):

Farm-based (live animal) tests

Farm-based tests involving faecal worm egg counts (WECs) of drenched animals provide an estimate of efficacy but can be variable as WECs do not always correlate with worm numbers.

Drench efficacy can differ between worm species. This means the same treatment, in the same cattle, could give different results if the drenched animals are infected with different species of worms. For this reason, when conducting drench tests it is important to also send a faecal sample for larval culture or DNA testing to identify the species of worms present.

Total worm counts (post-mortem)

Worms taken from the gastrointestinal contents of animals at slaughter are counted.

This measure of drench efficacy is the most accurate, but is costly, requires specialist expertise, and in practice is conducted only in research laboratories.

Worm resistance testing

Laboratory assays

Approaches for testing worms for drench resistance include assays conducted on worms of specified life cycle stages in the laboratory to determine the proportion of worms killed by varying concentrations of anthelmintic. These assays include:

  • the egg hatch assay (EHA) to detect resistance against benzimidazoles (BZ, white drenches)
  • the larval development assay (LDA) to detect resistance against benzimidazoles (BZ, white drenches) and levamisole (clear drenches).

These assays are largely used as research tools.

DNA resistance tests

DNA tests have been developed to detect resistance to the benzimidazole (BZ, white drench) anthelmintics. As the technology becomes cheaper, genomic tests will likely replace more labour-intensive laboratory assays. The mutations responsible for macrocyclic lactone (ML) and levamisole resistance are not known, so currently no DNA tests are available to assess resistance to these chemical groups.

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