If you’ve decided to test how efficacious wormer groups are within your flock, it is important to discuss the results with your vet and/or adviser and not to panic if the results come back showing multiple anthelmintic resistance (MAR) or even 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 for products within those groups.
Use the advice on this page and the SCOPS Multiple Anthelmintic Resistance Decision Support Tool.
It's understandable to assume tests showing MAR means products in groups one, two and three can no longer be used. But there are serious consequences of relying totally on groups 4-AD (orange) and 5-SI (purple). The reality in most cases is we can still use the older three groups on these farms, under a very 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 effectively, at which times of year against which worm species. And to also reduce the pressure on anthelmintics by reducing dependency on them.
What tests have been done? It is essential a test has been done rather than the assumption of MAR based 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? Was it done correctly and/or a reputable lab used?
What were the results? Interpreting the results can be tricky and SCOPS is constantly striving to improve the information available to farmers, vets and advisors in this area. 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 carried out to ensure efficacy 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 throughout the year, as the predominant worm species change due to 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 dominant worm species are different. The two most common worms on sheep farms – teladorsagia (the brown stomach worm) and trichostrongylus (the black scour small intestine worm) – often 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, further worm speciation testing in a specialised lab will be worthwhile. 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.
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.
For more detailed information, see SCOPS Technical Manual. Chapter 1.4.
