There are three equally important principles to slow the development of drench resistance.
Having selected a drench, make sure the drench gun is calibrated to deliver the right dose, which will be determined by the weight of the heaviest sheep in the group. These practices are designed to avoid under and overdosing.
Underdosing is a bad practice because it will not kill all of the worms in the animal and it will select for drench resistance. Worm populations are diverse, being composed of individuals that differ in important ways including their susceptibility to drenches. Underdosing will selectively kill the most genetically susceptible worms, leaving behind the more drench resistant types to breed among themselves. Clearly this is not a good outcome for preserving the effectiveness of drenches.
Overdosing isn’t a good idea
As drench resistance worsens and drenches become less effective, overdosing will not be a reliable response. Worms that survive a drench given at the correct dose rate are equipped with a mechanism to resist the effects of the drug. Overdosing may kill some of these survivors but the increase in effectiveness is likely to be small and its magnitude hard to estimate.
Resistant survivors (worms) rely on different resistance mechanisms controlled by different sets of genes. Simply increasing the dose rate may have little effect against some of these worms (called sub-populations).
For example, recent scientific publications have demonstrated that when a resistant strain of barber’s pole worm was exposed to levamisole, there were sub-populations that required a 17-fold increase in dose rate to achieve the same effect as a normal dose rate in a susceptible strain. A similar situation was observed following exposure of a resistant strain of barber’s pole worm to monepantel. Two sub-populations were detected, one required a 7-fold dose and the other 1000-fold the dose needed to kill a susceptible strain.
Overdosing is not an effective response to drench resistance and breaches the label conditions which sets dose rates in accordance to the weight of the sheep. In addition, the level of dose required to kill the resistant populations mentioned above would easily exceed safety margins for the drenches and be likely to kill the sheep.
Take home message
To address drench resistance, overdosing should be avoided and instead, a combination of drench actives from different drench groups should be used. The basis for using a combination of drench groups is that the resistance mechanisms that allow a worm to survive one drench group provide absolutely no benefit to their survival from a second drench group and so forth with other drench groups. So the chance of a worm being resistant to all active ingredients in a combination is much lower than for each individual active on its own.
Make sure sheep are given the correct dose rate of an effective drench to avoid under and overdosing. The only way to be confident that a drench will be effective on your property is to have conducted a Drench Test within the last 2-3 years or a DrenchCheck-Day10 as a temporary alternative to provide an indication of drench resistance.