Recently there have been a small number of confirmed reports of reduced efficacy, including resistance, involving new sheep drenches in Australia.
The recommendations remain unchanged. By testing and monitoring, find out how well the available drenches work on your farm. Make good use of all the effective drench options you have as part of an integrated approach to worm control, as explained in WormBoss.
Drenches may be less than highly effective against some worms because it is a characteristic of that chemical from day one. For example, the mid-spectrum drench naphthalophos (e.g. Rametin® and other brands), has never been highly effective at usual dose rates against Trichostrongylus (black scour worm), Teladorsagia (brown stomach worm) and immature Haemonchus (barber’s pole worm). Eventually, however, true resistance can occur and reduce the efficacy still further.
Reduced efficacy also occurs when a previously highly effective drench becomes less effective because resistance, a genetic trait, appears in a worm population and becomes increasingly common with successive uses of the drench.
Commonly, resistance first appears a few years after a new active comes onto the market. It doesn't have to happen that quickly. We can extend the useful life of drenches by implementing the resistance management strategies outlined in WormBoss.
A small number of cases of resistance to Zolvix® (monepantel, Elanco) have recently been confirmed in sheep in Australia. So far the resistance on sheep farms has been in Haemonchus, except on the university farm at Townsville where there was also reduced efficacy against Trichostrongylus. Two commercial sheep farms in the New England region of NSW are affected, as well as sheep and goats on the James Cook University farm in Townsville, and a medium-sized flock (>1000 sheep) in the Greater Sydney region
As to Startect® (derquantel + abamectin, Zoetis), I am aware of two confirmed cases in which Startect had reduced efficacy (approx. 90% effective) against Haemonchus in sheep flocks, one in northern NSW, and one in the Greater Sydney region. But this situation is a little different.
Startect is a combination of an older drench active, abamectin, and a new active, derquantel. Abamectin belongs to the macrocyclic lactone (ML, ‘mectin’) group and, as with all older broad-spectrum groups (MLs, benzimidazoles (BZ) and levamisole (LV)), resistance is quite common, although this varies from farm to farm, even neighboring farms.
Derquantel on its own has been described as a mid-spectrum active, i.e. not highly effective against all the important sheep worms. Specifically, derquantel on its own has useful but not high activity against Teladorsagia and immature Haemonchus (4th stage larvae). However, in virtually all tests so far, Startect has been effective (>95%) to highly effective (>98%). The same applies to Zolvix.
However, as mentioned earlier, there have been a couple of cases where Startect’s efficacy against Haemonchus was found to be approximately 90%. In these cases there was severe resistance of Haemonchus to abamectin (zero efficacy) and probably a significant proportion of the Haemonchus burden at the time of drenching were 4th stage larvae. But, as with all drenches, we have to expect that in time true resistance will develop to derquantel as well.
Keep all this in perspective. As a former Prime Minister said, be alert, not alarmed.