We talk a lot about drench resistance in sheep with widespread triple resistance to common gastrointestinal (GIT) worms Ostertagia and Trichostrongylus across New Zealand.
Although this hasn’t changed, and in some respects has progressed, some recent data has been published about triple resistance development in cattle.
Macrocyclic lactone (ML) and benzimidazole (BZ) resistance in Cooperia is widespread and been present for more than a decade, but this is the first report of resistance to all three major drench families (including levamisole (LV)).
Unfortunately, the news does get worse and Ostertagia is also likely to have resistance to all three major drench families having previously been described as having ML and BZ resistance, plus a natural resistance to LV which creates extremely variable efficacy for this type of worm.
In the paper by the Agresearch team they outline FEC reduction tests performed in cattle over four properties (five products on each farm):
Of the 20 tests conducted, only three exceeded 90% efficacy against Cooperia spp, despite many of the tests using a combination with levamisole, historically considered necessary to control Cooperia resistant worms.
Levamisole alone achieved efficacies between 44% and 71% across the four trials
Only monepantel + abamectin (Zolvix Plus) exceeded 95% efficacy against Cooperia with 100% efficacy against all parasites.
Ostertagia spp. Showed less than 75% efficacy to all products, clearly demonstrating resistance to oxfendazole (BZ) levamisole (LV) and MLs.
In my experience with sheep, it appears that development of resistance to our ‘novel’ drenches (Zolvix Plus and Startect have been getting used for quite some time now, with a massive increase in use over the last two-three years) is occurring rapidly.
This is likely due to the mechanism for resistance to our three major drench families (ML, BZ, LV) already being present.
This is also occurring mainly in the Ostertagia spp.
Although this is a different worm present in cattle and sheep, the precedent is clear that resistance genes in a worm may allow it to be more resistant to all drench families, not just those predominantly used.
The most important aspects of drenching sheep and cattle are, to know what drenches are working at 95% or greater efficacy; and, having a well rounded worm management plan that allows implementation of cross-grazing and refugia techniques to mitigate the risk of resistant genes developing and becoming the dominant type of worm present on the farm as larvae on pasture (and within your animals).
