25/08/2026
A Glamorgan lamb producer has revised his worming protocols after a project investigating anthelmintic resistance established which species could survive a range of treatments in his flock.
Dai John runs 500 Lleyn-cross ewes with his father and uncle, Gareth and Philip, and was among 48 farmers participating in the Farming Connect pan-Wales Welsh Anthelmintic Resistance Status (WARS) project.
This project, funded under the Farming Connect Our Farms Network, was initiated to give a clearer picture of the efficacy of the anthelmintics commonly used on sheep farms in Wales.
At Manselfold, Llanrhidian, on the Gower peninsula, the Johns had historically wormed lambs routinely but were concerned that parasites were becoming resistant to different drench groups.
As a lamb supplier to the Dunbia Group on its Taste the Difference scheme, they joined the WARS project to determine the outcomes of the SCOPS protocols they had put in place 10 years ago when they were supported through the Farming Connect Advisory Service to test resistance to the anthelmintics they were using at that time.
They had acted on the advice, reducing use of levamisole and ending use of ivermectin, switching instead to moxidectin.
Through the WARS project, they then carried out Techion UK Faecal Egg Count Reduction Tests (FECRTs) on six different wormer groups.
This test, carried out using the company’s DrenchSmart® service, shows where there is suspected lack of efficacy to wormer groups.
As a first step, faecal egg counts (FEC) were undertaken to ensure that lambs met the required worm burden threshold.
At Manselfold, those samples, taken in October 2025, before lambs were drenched, showed a worm burden of 1,861 eggs per gram (epg).
The efficacy of six anthelmintic groups were then tested on the flock - benzimidazoles, levamisole, ivermectin, moxidectin, monepantel, and derquantel/abamectin.
Faecal samples were taken from the lambs drenched with levamisole seven days after they were treated and 14 days post-treatment from lamb treated with the other wormers.
This showed that moxidectin, monepantel and derquantel/abamectin were completely effective, ivermectin had an efficacy of 95%, levamisole 91%, and benzimidazoles 80%.
In a further test, molecular technology was used to examine different worm species and how effective the wormers had been at destroying pathogenic, mildly pathogenic and non-pathogenetic strongyles.
It showed a mixed picture.
For example, in the pathogenic species, there was 37 epg of Haemonchus Contortus ‘Barber’s pole worm’ pre-treatment which reduced to 7 epg after treatment with benzimidazoles and there was zero presence of the worm in the lambs treated with levamisole.
The egp for Teladorsagia Circumcincta ‘Small brown stomach worm’ was 965 pre-tretment, 63 after treatment with benzimidazoles and 20 in the levamisole group.
Eurion Thomas, of Technion UK, says speciation testing is useful because wormers can have different levels of effectiveness against each worm species - a basic FEC can’t distinguish between different strongyle species.
“Identifying which species are present before and after treatment is important since treatment failure might only affect a particular strongyle species,” he points out.
By understanding which wormers were entirely effective, the Johns hope they can maintain that level of effectiveness by not overusing.
Eurion recommends always carrying out a FEC test after dosing to establish efficacy as the position can change.
Wormer resistance is a likely cause of reduced treatment efficacy, he suggests, although it can be down to other factors such as how wormers are administered and an animal’s health status.
Eurion says the results at Manselfold show that all the wormers could still be used to treat against nematodirus.
Wormer effectiveness against strongyles can vary through the season so treatments may perform differently at different times of year, he points out.
“The time of year the FECRT is carried out is important because the worm species present in the sheep can change throughout the year, this is important because treatment failure to one wormer may only be a problem in one species.”
For Dai, the project has been a “game changer”.
“We are now better informed and we can protect the wormers that work for us and use these strategically and at the right time of the year, when we know they will actually have an effect.”
The advice the farm had received 10 years ago on how to reduce resistance levels had demonstrated the effectiveness of this approach, he adds.
Specialist veterinary advice is available to help assess the effectiveness of anthelmintics on your farm. Registered Farming Connect businesses are eligible for funded one-to-one veterinary support, with Development Officers available to guide farmers through the application process. For more information, visit the Farming Connect website.
For the Dunbia Group, the WARS project had demonstrated the value of collaboration between farmers, industry and technical specialists, says its agriculture manager Leisia Tudor.
“It has been invaluable in helping our producers make evidence-based decisions that improve animal health and productivity.
“Healthy, efficient flocks are fundamental to maintaining a sustainable supply of high-quality lamb, and initiatives such as this help strengthen the resilience of the entire supply chain.”
PANEL
Based on the test results at Manselfold, Kate Hovers, an independent vet working with Techion UK, recommends that as benzimidazoles are effective against nematodirus but less so against other worms the Johns can use it to treat for this parasite not only as the first drench but with caution at other times, doing so informed by SCOPS and Hybu Cig Cymru local risk forecasts.
“This is because there was a percentage of non-pathogenic worms left so when you take them out the effectiveness at this one testing was better than 80%,” she says.
She advises that lambs should then be drenched as necessary with levamisole, ivermectin and moxidectin according to growth rates, egg counts and how the animals look, drench testing annually in consultation with the farm vet.
“Monepantel or derquantel/abamectin can be used as an annual break drench when lambs need treating in late summer to help maintain optimum growth rates by removing worms left behind by earlier treatments and to slow the development of resistance to older wormer groups,” she says.
If ewes are in good condition and only carrying single lambs, they may not need worming, Kate adds, recommending FEC testing before the lambing period.
“If there is a thinner group of ewes around lambing time, sample these and worm if the egg count is high, but seek advice from the farm vet on this.”
Haemonchus worms can affect adult sheep as well as lambs, typically producing a very high egg count.
Although the eggs look the same as other strongyle-type worms, a staining test can be carried out at a laboratory to specifically look for haemonchus.
The Johns graze lambs on herbal leys and this can help reduce, but not eliminate, the need for treatments.
Kate says the level of anthelmintic properties in herbal leys is debatable, although they do contain tannin which has some effect against worms.
“The stems on herbal ley swards are a bit harsher than grass so the worm larvae find it more difficult to wriggle up the stems to where they are being eaten.”
Grazing cattle and sheep on the same fields is beneficial, Kate adds, as most worms are species-specific, affecting either sheep or cattle but not both.
PANEL
In the overall study, reduced efficacy to anthelmintic treatments was detected across the 48 farms.
Benzimidazoles – white drenches - had the lowest efficacy, showing frequent failure in reducing worm counts – in some flocks egg counts increased after treatment.
“Most sheep farmers shouldn’t be using a white drench for treating normal summer worms because most have got quite a degree of resistance therefore it will be completely ineffective or not as effective, and this will affect lamb growth rates,” says Kate.
Levamisole, ivermectin and moxidectin generally provided a higher and more consistent reduction in worm burdens, although this was variable.
Monepantel and derquantel/abamectin typically achieved near or complete reductions in FECs.
Manselfold Farm Facts
61ha farmed
Swedes grown for human consumption, with fields winter-grazed post-harvesting
4ha barley and 3ha fodder beet grown
1t soya/year total purchased feed
Indoor lambing from mid-March
Ewes graze common land at Cefn Bryn after weaning in early July and lambs graze herbal leys.
Lambs supplied to Dunbia on a Taste the Difference contract
42kg average finishing liveweight to achieve an average 20kg deadweight