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16 March 2021

Using a faecal egg count reduction test (FECRT) to test for wormer resistance on farm is an important step, say the Sustainable Control of Parasites in Sheep (SCOPS) group – but it is important not to panic if the results come back showing triple resistance.

A triple resistance result, indicating inefficacy of products in group 1-BZ (white), group 2-LV (yellow) and 3-ML (clear), can be incredibly daunting, but does not mean the end of the road.

Lesley Stubbings of SCOPS explains:

“The first knee jerk reaction to tests showing triple resistance is to assume products in groups one, two and three can no longer be used. However, we have already seen the consequences of relying totally on groups 4-AD (orange) and 5-SI (purple), with reports of resistance to group four on farms where this has happened.

“The reality in most cases is we can still use the older three groups on these farms, albeit within a much more detailed and carefully considered plan. It is vital for the farmer, vet and/or advisor to work together, digging deeper to understand more about which groups work and when. Then work hard to reduce the pressure on anthelmintics by implementing options such as targeted treatments, quarantine and grazing management. What we mustn’t do is panic.”

The approach on each farm will be completely different, but similar questions should be asked in all scenarios.

What tests have been done? It is essential a test has been done rather than the assumption of triple resistance be based just on poor lamb performance following treatment. Jumping to such a conclusion is particularly dangerous. If tests have been done, was it a full FECRT or a more simple drench test, and how reliable was it? Investment in reliable tests is crucial. More on the SCOPS website here.

triple worm resistance

What were the results? Interpreting results can be tricky and SCOPS believes more work is needed in this area to help farmers, vets and advisors. A test only means resistance has been detected at 90-95% efficacy. This means 5-10% of worms were not killed by the treatment. It does not mean this active cannot be used at all. What it does mean is that selection pressure must be reduced (for example, by paying more attention to levels of refugia) and routine drench testing is essential to ensure it is not deteriorating.

When was the testing done? A test showing triple resistance at one point of time during a year is very different to testing at different times during the year. The resistance profile on a farm changes during the year, as the predominant worm species change in different seasons. For example, a 1-BZ (white) wormer can be used on most farms in the spring for nematodirus but is often not effective at other times of year when the worm species are different. We also often find the two most common worms on sheep farms – Teladorsagia (the brown stomach worm) and Trichostrongylus (the black scour small intestine worm) – have very different resistance profiles.

Were any samples checked for the worm species involved? This is not easy to do, nor is it cheap, but before jumping to the conclusion that all three groups are totally ineffective, it is essential to do some speciation by sending faecal samples to a specialised lab. This will highlight the variation in resistance profile between worm species, so a plan can be created to use different wormer groups at different times.

Ms Stubbings concludes:

“The bottom line is that with all this information in place, it is normally still possible to utilise all the anthelmintic groups, together with management strategies to maintain worm control for most flocks. Inevitably this means the product mix on a farm becomes more complex, but that is where a well thought out plan, with regular testing and good advice, comes into its own.”

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