Reversing drench resistance

By Ben Allott

In Livestock9 Minutes
LIVESTOCK 􀀃Parasites Drenches used must be known to be effective against all worm species (at least

The steady progression of drench resistance in New Zealand sheep and cattle parasites has been highlighted in previous articles by myself and other Country-Wide contributors. In recent years farms with triple resistant parasites have been reported regularly across the country.

A common question I encounter from farmers is, “can I reverse drench resistance or am I stuck with it?” In the past my answer was the industry accepted view at the time, “Once present on a farm, resistance to anthelmintics can be considered permanent”.

More recently, published studies have given me much more confidence that if the right steps are taken, and the problem is identified early enough, reversing drench resistance is a real-world possibility and should be the stated goal of parasite management plans.

Let’s start with the good news and the motivation for change. In the article referenced (see Good oil), seven NZ farms were identified as having Teladorsagia (Ostertagia) worm populations resistant to more than one drench family. In year 1, faecal egg count reduction tests showed the average efficacy (across all seven farms) of the three major drench families against this worm to be: Albendazole 66%, Levamisole 66%, Ivermectin 55%.

After five years of being enrolled in a best practice parasite management programme the average efficacy across all seven farms had improved to: Albendazole 90%, Levamisole 85%, Ivermectin 86%.

These are truly game-changing improvements in efficacy over very short periods. How were these changes achieved?

First, the parasite management programmes implemented take a whole-of-farm approach. All aspects of livestock management were considered in a collaborative approach by the farmer, farm vet/parasitology adviser, farm consultant, livestock agent, and agronomist. Important factors considered and discussed include: stocking policies, grazing management, fertiliser use, and cropping/regrassing strategies.

Your animal health adviser should not be working in isolation. They should be aware and involved with discussing the wider production strategy of the farm so that their recommendations are effective, relevant to your system, and practical to implement.

Core principles

The parasite management plans differed between farms but were designed to align with the following core principles:

  • Always use effective anthelmintic products. Drenches used must be known to be effective against all worm species (at least 95%). This is far easier and cheaper to achieve when resistance is identified at an early stage when only single families are effective. It is much more challenging and expensive when combination drench resistance is found. Test your farm’s drench resistance status this season to identify resistance issues as early as possible.
  • Maximise the opportunities for the provision of refugia. If you do not really understand refugia you must get an understanding of it. The provision of adequate refugia while maintaining a high level of stock performance and welfare is the number one priority to achieving a reversal of drench resistance. In these seven farms the main way this was achieved was to minimise the treatment of adult sheep, ensuring treated lambs and untreated ewes grazed over the same pastures as much as possible.
  • Avoid over-use of anthelmintics. Every use of drench on the farm should be based on demonstrated need. Discuss with your adviser how you should be assessing if a stock class definitely needs a drench. Take the guesswork out of it. Ask your adviser to discuss options for: FEC-driven drenching, extending drench intervals in young stock while maintaining productivity, targeted selective treatment.

Adjust policies

  • Maximise the use of cross-grazing (cattle, deer, sheep) and the use of crops to minimise parasite challenge to young stock, then adjust drenching policies as a result of lower parasite challenge. When you implement stocking policy changes, grazing management changes, or integrate larger cropping areas to parasite contamination of feed, these are done with the key purpose of allowing you to reduce the amount of drench used in the system. If you implement these changes but do not reduce the amount of drench you use you will not see a reversal of resistant parasites.
  • Do not administer anthelmintic treatments at intervals shorter than 28 days to allow for some limited contamination of pastures with susceptible genotypes (refugia). This policy relates directly to lamb drenching.

Note: this principle does not state that 28d intervals is the optimum timing for every situation. In many cases, e.g. effective cross-grazing, lamb finishing on crop, or FEC-resistant sheep genetics, lamb drench intervals can be successfully extended past 28 days with minimal production loss. Discuss this with your adviser in detail.

Long-term impact

  • Reduce and avoid the use of anthelmintic products with persistent activity. The most common products that come to mind are long-acting injections, capsules, and oral products that have extended control claims. If you are in the habit of routinely using long-acting products across large numbers of stock for pre-lamb treatments in the spring, barbers-pole protection in the summer/autumn, or to reduce the number of times you get lambs in during the summer you need to consider the long-term impact on selecting for resistant parasites.
  • Use a knockout drench, containing a new anthelmintic class, to lambs in late summer to remove resistant genotype worms which have accumulated over previous treatments.
  • Ensure animals are not drenched on to new/clean pastures (low refugia areas) unless other strategies are in place to ensure adequate refugia is provided (e.g. treated lambs followed by untreated adult sheep).
  • Implement an effective quarantine process to prevent resistant parasites entering the farm with stock purchases.

The final comment I want to make is that while the above process may seem daunting it will be far easier to implement, with fewer drastic farm systems changes if it is tackled while drench efficacy is high.

Do not ignore drench resistance if your drench tests look good. It is far easier to maintain high efficacy than to reverse out of a problem once it is severe. Oh, and go book a drench efficacy test now.


For those interested in reading a more detailed article on the subject I recommend, “Evidence for reversion towards anthelmintic susceptibility in Teladorsagia circumcincta in response to resistance management programmes”. At the time of writing this article could be found in full, for no cost, at

  • Ben Allott is a North Canterbury veterinarian.