by vet Chris Bailey
Worm control in sheep has changed significantly over recent decades. For many years, the routine approach was to dose lambs every few weeks through the summer and assume the problem was controlled. That system is no longer sustainable. Wormer resistance is now widespread across the UK, and many farms already have reduced efficacy to one or more wormer groups. The aim today is not simply to kill every worm, but to control disease and production loss while protecting the wormers we still have.
The scale of resistance should make all sheep farmers stop and think. UK survey work has found resistance to white wormers on the majority of farms tested, with resistance also common to yellow and clear drenches. English and Welsh studies have reported resistance to white wormers on approximately 76–96% of farms, yellow wormers on 55–60%, and clear wormers on 57–67%. This does not mean every product has failed on every farm, but it does mean no farm should assume a wormer is working unless this has been checked. SCOPS recommends post-treatment faecal egg count testing to monitor efficacy, usually around seven days after a yellow drench and around 14 days after white, clear, orange or purple drenches.
The SCOPS principles give a practical framework for summer parasite control. The first step is avoiding unnecessary treatments. Not every lamb needs worming just because the calendar says so. Faecal egg counts are a useful way of checking whether worm burdens are building and whether treatment is needed. They are not perfect and should be interpreted alongside growth rates, pasture history and clinical signs. Weighing lambs, or at least handling and assessing batches regularly, is also important. A group of lambs growing well may not need treatment, even after several weeks at grass.
When treatment is needed, product choice matters. Alternating wormer groups sensibly through the season can help reduce selection pressure, but this does not mean changing product randomly every time. A white wormer may still have a place early in the season where Nematodirus is the main concern and where it is known to be working. Later in the summer, yellow or clear drenches may be appropriate depending on previous egg count results, farm history and resistance testing. The key point is to know what works on your own farm.
Preserving “refugia” is one of the most important SCOPS ideas. Refugia means leaving some worms unexposed to treatment, either in untreated sheep or on pasture. This sounds counterintuitive, but it dilutes resistant worms that survive treatment. If every lamb is dosed and moved to clean pasture, the only worms left are those that survived the drench.
Targeted selective treatment can be very useful. Rather than dosing every lamb, treat those that need it: poorer growers, dirty lambs, lambs in poorer condition, or a representative group where egg counts and growth data suggest a worm problem. On many farms, treating the worst affected 20–30% at certain points can maintain performance while reducing wormer pressure. If all lambs in a group are treated, they should not be moved straight onto clean pasture. In most cases, it is better to return them to previously grazed fields for a few days, helping dilute resistant worms that survive treatment.
Zolvix, the orange group 4-AD wormer, also has a role when used correctly. SCOPS recommends newer wormer groups, including orange and purple drenches, as a one-off mid- to late-season break drench in lambs. The aim is to remove worms that have survived exposure to older white, yellow and clear wormer groups earlier in the season. Used in this way, Zolvix can help lamb performance while slowing resistance to older products. It should not become another routine drench.
Sustainable worm control means using the right wormer, at the right time, in the right sheep, and checking