Increasing levels of wormer resistance on all types of farms and in all areas, is leading to a growing urgency for all sheep farmers to start using the newer group 4-AD and 5-SI wormers.

That was the stark warning from the National Sheep Association (NSA), the Sustainable Control of Parasites in Sheep group (SCOPS), Moredun Research Institute and the Sheep Veterinary Society (SVS), who highlighted that despite the presence of anthelmintic resistance on 98% of farms, 85% of farmers still believe that the wormers they use, are working as well as they always had.

Speaking at an industry webinar, they also pointed out that 77% of farms found resistance to two or more wormer classes of groups, 1, 2 and 3.

In Scotland, 80% of 1 BZ resistance was found on lowland farms and 55% on upland/hill units, which compares to almost 100% on lowland farms in England and Wales and 83% of upland/hill units south of the Border. There are also increased reports of resistance to the clear or group three wormers, triple resistance and death – parasitic gastroenteritis is the biggest killer in carcase submissions for post mortem examinations, followed by enzootic abortion.

According to vet Matt Colston, ruminant technical consultant at Elanco Animal Health, this is largely down to the fact that the majority of sheep farmers do not monitor treatment efficacy or growth rates, and they are therefore not being alerted to what is essentially an invisible, yet highly consequential, problem.

"This is a dangerous strategy," he said. "As a result, many farmers are inadvertently increasing the growth of resistance to group 1, 2 or 3 wormers, while also limiting efficiency and profitability.

“Because most farmers can’t physically see the reduction in lamb performance, they’re often reluctant to adopt new management practices."

He added that sheep farmers need to know the anthelmintic groups and how they work.

"We know, swapping a dose of an older group 1, 2 or 3 wormer to a group 4-AD (orange) or group 5-SI (purple) wormer in the latter part of the grazing season, will give a visible improvement in performance in most cases, as worms left by previous treatments are removed,” said Mr Colston.

“Integrating the newer group wormers now will also slow the development of resistance to the older group 1,2 and 3 wormers. This is crucial, because once resistance has developed it’s irreversible, and we want as many available worm treatment options as possible, for as long as possible.”

SCOPS advises that one of the two newer groups to be used on all sheep farms at two points in their worm control plan. Firstly, as part of their quarantine treatments for all incoming sheep, and secondly as a one-off treatment for lambs, in the latter part of the grazing season.

“To gain the maximum benefit from the treatment for lambs, it’s important that the wormer is given towards the end of the grazing season – as a mid-late season break dose – and when a treatment is deemed necessary though a significant worm egg count. All lambs remaining on the farm should be treated,” says Mr Colston.

To avoid the risk of resistance to the newer group 4-AD and 5-SI wormers, he emphasises that it is extremely important to not dose and move lambs to cleaner grazing straight away. Lambs should be returned to the same fields for four or five days before moving them.

To help support the transition to a more sustainable worm control programme, producers are encouraged to use the Sheep Wormer Checker to inform decisions as to which wormer to use when. Find the checker here: