A project in Wales has detected resistance to one or more wormer groups on 98% of participating farms.
The initiative, funded by Welsh government, used a flock faecal egg count reduction test (FECRT) to establish whether resistance exists to anthelmintic drenches.
Last year, 49 sheep farms used the service, with testing carried out between June and November.
Sampling packs were provided by parasite management firm Techion and faecal egg counts (FECs) were carried out on pooled dung samples.
Readings of around 500 eggs per gram (EPG) were needed to start the process.
FECs were taken from at least 90 lambs that had not been wormed for at least four weeks.
Once this baseline was set lambs were split into four treatment groups with 20 in each group.
They were dosed under strict protocols with either a white drench (benzimidazole), yellow drench (levamisole), clear drench (3ML) or moxidectin.
A sample was taken from each lamb by a trained technician – this was repeated at between 7 and 14 days later, depending on the type of drench used.
The samples were then sent to the Techion lab to assess the efficacy of that wormer group.
James Hadwin, who delivers the service for the Welsh government's Farming Connect scheme, said that at first sight, the results 'seem worrying'.
"But because the testing is relatively sensitive and accurate, it means that on many farms we are identifying resistance in the early stages," he added.
“That’s great news for the farmers involved because it means if they are careful and take advice, they can maintain good levels of worm control."
Mr Hadwin, of JH Agri Consultancy (AgriPlan Cymru), also said the results were 'concerning'.
But he said if farmers took positive action to reduce their reliance on wormers on the back of their results, then they could achieve good worm control.
Actions include more attention to detail in flock management to reduce worm burden and also, should the stock need worming, in wormer choice.
“The results in 2020 have shown that there is an issue with wormer resistance in the industry and that we need to act now," he explained.
“Hopefully this project is helping farms to get a handle on their situation and is encouraging people to follow SCOPS principles."
He said he hoped it would be a prerequisite to farmers working more closely with their vets and animal health advisors in formulating a robust flock health plan.
Overuse of wormers in the past has not only led to resistance developing but had added to cost of production, Mr Hadwin said.
“Time and labour are among the highest costs on sheep farms. If you don’t need to be drenching lambs, why do it?’’
When time is limited, lambs are sometimes dosed simply because they had been gathered for another management task, he added.
“There is an element of ‘if the sheep are in for other things we might as well worm them’," he said.
“By planning ahead, and carrying out a FEC before handling, we know if the lambs actually needed worming."