Sheep farmers are being urged to act now to preserve the effectiveness of wormers, as growing resistance threatens the future of parasite control in UK flocks.
At a recent Banff and Buchan Monitor Farm meeting, vet Ewan Jamieson, director of Meadows Vet Centre, warned that worm resistance is becoming more widespread and severe in Aberdeenshire.
A local trial across nine farms revealed that in some cases, wormers were performing almost completely ineffectively. “One farm recorded just 12% worm control after a single drench,” said Jamieson. “It just wasn’t working.”
He explained that resistance develops with every use of a wormer — even when applied correctly. “Really, every time you use a wormer you are contributing to resistance developing, no matter how well you use them,” he said.
The on-farm trial involved collecting fresh dung samples from five lambs on each farm at the time of dosing. Using an AI-powered testing kit, Jamieson carried out faecal worm egg counts (FWECs) before and after treatment to measure efficacy.
Samples were retested a week later for yellow drenches and two weeks later for white and clear drenches, allowing comparison of worm counts pre- and post-treatment. “We could then compare the results before and after worming and calculate a percentage efficacy,” he explained.
Results showed huge variation between farms — from just 12% up to 100% efficacy. Those rotating between all three drench types achieved the most consistent results, while those relying heavily on one or two products showed faster resistance build-up.
Where serious resistance is present, Jamieson said farmers may be forced to rotate between only two drench types instead of three, which further accelerates resistance. Overuse of long-acting clear drenches, or under-dosing, also heightens the risk.
Jamieson urged producers to follow the “dose, wait, move” principle and to quarantine dose all incoming stock. He also encouraged the use of an end-of-season knock-out drench based on the two newer orange and purple wormer classes.
Regular FWEC testing remains essential, he added. “I do understand why people don’t do FWEC, as it takes planning and time. But we now have some farms struggling to control worms — in some areas, the issue has forced some farms to give up sheep altogether.”
Beyond wormers, he advised using grazing management to reduce parasite pressure, such as rotating stock onto clean reseeds, brassica or root crops, or herbal leys containing chicory, sainfoin and birdsfoot trefoil. Mixed grazing with cattle or older sheep, or cutting for hay or silage, can also help clean pasture.
This year, Jamieson worked with Monitor Farmer Bruce Irvine at Sauchentree Farm, New Aberdour, on a worming plan for 700 lambs based on FWEC results and pasture risk assessments. Fields were categorised according to ley age, grazing history and weather conditions.
Dung samples were collected for FWEC analysis before lambs received a white wormer in early May. Dry conditions initially kept worm burdens low, but cases of nematodirus began emerging across the region. “There’s no preventative treatment for nematodirus, so treatment timing is critical,” said Jamieson.
All lambs were treated again at weaning in late July with a yellow drench. Even though one group had a low worm count, all were treated because the groups were being mixed, creating higher reinfection risk.
Dry summer weather, he added, helped initially but later reduced grass growth, forcing lambs to graze closer to the soil where worms thrive. Low cobalt levels caused by the dry spell may also have weakened gut health and made lambs more susceptible.
“Bruce’s lambs were treated with Smart-Shot B12 at weaning, but boluses or mineral drenches could also have been used,” he said.
