New research highlights more risks associated with pre-lamb drenching of ewes

New research has highlighted a previously unrecognised risk from the common practice of treating ewes with long-acting anthelminitic drenches.

Work led by AgResearch principal scientist Dr Dave Leathwick and published in the journal ‘International Journal for Parasitology – Drugs and Drug Resistance’ shows the common New Zealand practice of drenching ewes pre-lambing with long-acting moxidectin drench can select for resistant worms in the lambs through the transfer of drug in the ewe’s milk. The published research follows on from a recent study on farms in the Wairarapa which indicated that the practice of pre-lamb drenching is often economically marginal.

Dr Leathwick says that it’s long been known that some drugs transfer through the ewe’s milk to the lamb – but the practical implications of this drug passing into the lambs had never been tested. His discovery that pre-lamb drenching of ewes can select for resistant worms in the lambs, even though the lambs themselves may never have been drenched, again illustrates the growing and very serious problem of parasite resistance to many commonly used drenches available here and the ways they are administered.

“We were able to measure drug in the milk of the ewe until almost weaning and young lambs had high levels of drug in their plasma. Drug was detectable in the plasma of lambs until they were 7-8 weeks old. There was enough drug in the lambs to prevent 70% of susceptible worm larvae from establishing, but not enough to stop resistant larvae, and so the farmer using these products is shifting the gene pool on his farm in favour of the worms that can survive the drug, i.e. the resistant ones,” he says.

“So, effectively you are selecting for drug resistant parasites in the lamb even though you’re not even treating it. You were simply treating its mother 6-8 weeks ago.”
Dr Leathwick says the research shows that for the first weeks of their life the lambs are getting a dose of drench every day and the dose is uncontrolled because it depends on factors such as how much milk they consume, the size of the lamb and whether they’re a single, twin or triplet.

Farmers need to be aware that there are implications to using drenches and ensure that they’re used effectively, and cost-effectively, he adds.

“These products are high risk and have variable economic benefit. Research that we undertook with farmers in the Wairarapa several years back, showed that 50% of the time when farmers used long-acting drenches in their ewes pre-lambing they lost money. So, farmers ought to consider the future when they’re using a practice that has a 50/50 chance of losing them money and is shown to select for resistant parasites on their farm.”