Scientists uncover new risks linked to pre-lamb drenching
Professor Jacqui Matthews of the Moredun Institute in the UK, said the findings are "worrying and relevant to the UK situation".
The practice of drenching ewes pre-lambing can select for resistant worms in the lambs through transfer of the drug in the ewe's milk, new research has found.
A study led by the research company AgResearch in New Zealand, assessed the risks of treating ewes with a long-acting moxidectin drench.
It has long been known that some drugs can pass to lambs through ewe milk, but, for the first time, scientists say the pre-lamb drenching of ewes can select for resistant worms in lambs.
AgResearch says this highlights the "growing and very serious problem" of parasite resistance to commonly used drenches in New Zealand.
Speaking to Farmers Weekly, Professor Jacqui Matthews of the Moredun Institute in the UK, said the findings are "worrying and relevant to the UK situation".
The research team found the drug in ewe milk almost until weaning and in lamb plasma until seven to eight weeks of age. The levels of the drug were high enough to prevent 70 per cent of the susceptible brown stomach worm larvae from establishing, but not enough to prevent resistant larvae.
"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," explained Dr Leathwick.
"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 six to eight weeks ago."
This is a concern for UK farmers too, as the brown stomach worm (Teladorsagia circumcinta) is the most common UK roundworm and resistance is already being seen.
Prof Matthews added: "This is important because a 'lambing' wormer treatment is commonly applied by UK sheep farmers around lambing because of the relaxation in immunity in ewes to this worm and thus to stop the ewes contaminating pasture.
"It is also important because moxidectin is one of our few tools for the control of sheep scab and overuse may lead to resistance selection in mites."
Farmers are advised by Prof Matthews to look at Scops guidelines and speak to their vet about balancing the need to control disease with the risk of exerting selection pressure for drug resistance.
Dr Leathwick said farmers should also consider the cost implications of using pre-lambing drenches, as previous research has shown the practice is often not cost effective. "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," he concluded.
Veterinary practices are being urged to help owners safely dispose of any unused and expired pet medicines - not just antibiotics - during November's Animal Medicines Amnesty. 
