Managing Drench Resistance in Sheep – NSW Non-seasonal Rainfall

Why manage drench resistance?

To stay profitable in the long-term, you will need to prolong the effective lives of old and new drench groups by using them well. (Drench groups are the ‘chemical families’ of drenches. Older groups can often be combined with newer groups to slow development of resistance).

Selection for drench resistance happens when worms in a sheep are exposed to a drench. Some worms can survive certain drench groups as they have genes for drench resistance. This may initially be just one worm in 100,000 or even 1,000,000 worms. Some worms present may be partly drench-resistant: they can survive lower (sub-lethal), but not full doses of the treatment.

Worms that survive treatment continue to produce eggs that give rise to infective larvae on a pasture. These are eaten by sheep and so the worm life cycle continues. In this way each treatment causes an increase in the proportion of the worm population that is either partly or fully drench-resistant.

If resistance to a drench group is already present, it will likely remain, even if the drench group is not used for years. Drench resistance probably cannot be prevented, but the rate at which it occurs can be greatly reduced.

The first step is to know what drenches are effective on your property.

How can the effectiveness of drenches be tested?

Each property has its own drench-resistance profile based on its own drenching history and that of properties from which sheep are sourced. The profile of neighbouring properties can be quite different.

The extent of resistance is only known by testing. Obvious worm control failures may only occur when resistance is quite advanced.

• A DrenchTest is needed to accurately test for drench resistance. Do these tests every 2–3 years. Test all single-actives that are likely to be used. Effectiveness of multi-actives or from giving single actives sequentially can be calculated from the efficacy of the single actives using the Combination Drench Efficacy Calculator.
• A DrenchCheck – Day 14 is used to check individual drenches at any time and where a DrenchTest is not practical because of small herd size. This is a guide only to drench efficacy and resistance and is best used to monitor drenches between the times that full resistance tests (DrenchTests) are performed.

The DrenchTest (WECRT)

DrenchTest is the common name for the Worm Egg Count Reduction Test (WECRT). This assesses the drench-resistance status of worms on a property.

WormBoss recommends testing actives from all drench groups; from these results, resistance to the multi-active products can be calculated.

Select a mob for the DrenchTest. From this mob, a group of sheep is used for each drench A sample is taken from sheep prior to the drench (Day 0) to act as a ‘control’ or comparison. Each of the groups is drenched and dung samples are collected from all of the sheep 14 days after the drench, for a WormTest.

The worm egg counts of each treatment group are compared with those of the undrenched control group. From this, the effectiveness of each drench against each worm type present is calculated.

Discuss the test with your adviser before setting up. For more details, including which drenches to test, see Testing drench effectiveness with a DrenchTest.

The DrenchCheck-Day 14

This simple and inexpensive test gives an indication of drench effectiveness and whether the drench should be properly investigated using a DrenchTest.

The DrenchCheck-Day 14 involves two WormTests: the first on the day of drenching (usually at a routine WormTest time) and the second 14 days after the drench.

The results from the two WormTests are compared to gauge the extent that worm egg counts have been reduced by the drench. Discuss the results with a worm control adviser.

For more detail see Checking for drench resistance with a DrenchCheck.

How can drench-resistant worms be kept out of your property?

Keeping drench-resistant worms out of your property is part of sustainable worm control.

Assume that purchased sheep are carrying worms with some degree of drench resistance to one or more drench groups (see Drench groups and actives at Appendices: Further information on sheep worm control for NSW non-seasonal rainfall region).

• ‘Quarantine’ drench all sheep new to the property.
  • Use a combination of no less than 4 unrelated drench groups with at least one of these being monepantel (e.g. Zolvix®) or derquantel (with abamectin—e.g. Startect®). This can be done using multi-active (combination) and/or single-active products concurrently—up the race with one product, then up the race again with the next.
  • Do not mix different drenches unless the label states you can, as different products may be incompatible.
• Quarantine the sheep after treatment.
  • Hold the sheep in quarantine in yards (small mobs) or a secure paddock (larger mobs) for at least 3 days to allow worm eggs present at the time of drenching to pass out of the gut.
  • Provide adequate feed and water.
  • Keep this paddock free of sheep, goats or alpacas for at least 3 months in summer or 6 months in cooler months.
• After quarantine, release the sheep onto a paddock that is likely to be contaminated with worm larvae due to grazing by other sheep. This will ‘dilute’ (lower the proportion of) resistant worms surviving treatment with worm larvae already on your property.
• WormTest the imported sheep 14 days after drenching for added confidence that treatment was successful.
• Get expert advice on up-to-date recommendations for quarantine treatments. These will evolve as the drench resistance picture changes.

How can the development of drench resistance be slowed?

Choosing drenches

Integrate all 4 principles where possible:

1. Use effective drenches: these are known to reduce the worm egg count on your property by at least 98% as shown by a DrenchTest. The more effective a drench is, the fewer drench-resistant worms will remain in the sheep after treatment.
2. Use a combination of two or more groups where possible, as fewer worms are able to resist more than one group at a time.
3. Use short-acting treatments where possible, and restrict the use of persistent products for specific purposes and high worm-risk times of year. See page 12, ‘How can persistent treatments be used effectively?’ There is little need to use mid-length or long-acting treatments if sheep are being moved to low worm-risk paddocks.
4. Rotate* among all effective drench groups each time a mob is drenched (and for each paddock where possible). An effective drench from a different group may kill worms that were resistant to the last treatment. These may be worms that survived treatment in the sheep or were picked up from the paddock.

*When rotating drenches, the current drench ideally would include no groups that were used the previous time. However, in practice, ensure it has at least one effective active from a drench group that was not used the previous time.

Using drenches

Follow all 5 principles where possible:

1. Avoid unnecessary drenching, especially
   • Adults.
   • During droughts or prolonged dry periods.
   • Immediately before or after moving sheep onto very clean, low worm-risk paddocks (such as ungrazed cereal stubbles or paddocks that have been sheep-free for extended periods). See points i) and ii) below for further discussion on this.
2. Calibrate drench guns to ensure the correct dose is delivered.
3. Calculate the dose based on the heaviest animals in the mob. Split mobs for drenching if there is a large weight range, so sheep are not under-dosed.
4. Follow the label instructions to ensure correct dose and use of treatments (including complying with withholding periods).
5. After sheep have been drenched, graze them initially on paddocks already contaminated with worms likely to be less resistant to drenches (except in the cases of lambing and weaner paddocks that specifically need to be low worm-risk). Eggs from surviving drench-resistant worms will be diluted by eggs and larvae already on the paddock and therefore not exposed to the drench (i.e. ‘in refugia’).

If sheep must be drenched onto low worm-risk paddocks do both of the following:

• i) When the sheep eventually leave these low worm-risk paddocks, treat them with an effective drench that is from a different group to the drench used when the sheep first went onto the paddock. The aim is to remove any drench-resistant worms surviving in the sheep after the first drench.
• ii) Ensure that the next time the paddock is grazed it is with a different mob of sheep. This second mob should have a moderate to high worm burden and their last treatment must be different from the treatment used on the first mob that grazed the low worm-risk paddock. This will dilute drench-resistant worms already on the paddock with more susceptible worms that the second mob is carrying. Note that grazing with cattle will not dilute the proportion of drench-resistant worms, but they will decrease the total number of worm larvae on this paddock.

How can persistent treatments be used effectively?

Effective persistent treatments kill immature and adult worms in the sheep at the time of treatment, as well as infective larvae eaten by sheep (with pasture) during the period of protection of the treatment—up to 3-4 months for long-acting and 1–4 weeks for mid-length treatments (depending on the particular product).

Both may increase selection for resistance to the actives in those treatments for two reasons. Firstly, worms are exposed to the active for longer. This favours surviving resistant worms, which then reproduce. Secondly, persistent treatments have a longer time at the end of their protection period where the active concentration has dropped to a level where partly resistant worms may survive and reproduce.

Use primer and exit drenches with long-acting treatments

Primer drenches clear the sheep of any worms that are resistant to the long-acting treatment. A primer drench is an effective short-acting drench (preferably a combination) that does not include the same group as the long-acting product. Give a primer at the same time that a long-acting product is given.

Exit drenches are used two weeks after the end of the actual protection period. This can be confirmed by doing worm egg counts at set times after the long-acting treatment is applied e.g. 30, 60 and 90 days, or at convenient management events such as prior to lamb marking and weaning. By this time, the persistent treatment has declined to very low levels in the sheep. The exit drench kills larvae that have survived the persistent treatment and developed into breeding adult worms. Another name for the exit drench is a ‘tail cutter’.

An exit drench (like the primer drench) is an effective short-acting treatment (preferably a combination) that is from a different group/s to the persistent product.

Mid-length treatments need exit drenches

Resistance can develop to mid-length treatments in the same way as to long-acting treatments. Primer and exit drenches are desirable with mid-length treatments, but they may not be cost-effective because of the relatively short protection period compared to long-acting products. However, the use of an exit drench is highly recommended two weeks after the end of the protection period stated on the label.

Check the persistence of a product

The effectiveness of the persistent product on your property will be shown by the length of the protection period actually achieved (rather than what is claimed on the product label). Persistent products that you plan to use should also be tested in a DrenchTest each 2–3 years. However, if you do not have current DrenchTest results and you plan to use a persistent product before your next scheduled DrenchTest, you should do a DrenchCheck- Day14 (see above) after the next treatment. Also conduct a WormTest at 60 days and 90 days after it is given to see how long it is effective. If it is shown to be ineffective at one of the earlier tests, then the later test/s will be of no value.

When you send the samples, request a larval culture if there is a positive worm egg count because

• resistance may only be present in one worm species
• if moxidectin was used, the protection period against different worm species differs
• if closantel is used, it is a narrow spectrum drench only for barber’s pole worm

If the treatment was fully effective, and you used a primer and exit drench, the product will probably have a similar length of effectiveness at the next use. However, it is best to check the effectiveness of long-acting products every year they are used by doing a WormTest at 30 and 60 days.

If a WormTest shows worm eggs are present before the end of the claimed protection period, drench resistance is likely. You should:

1. Immediately drench the sheep with an exit drench (as described earlier), keep them in their current paddock for a further 3–4 days (while most eggs pass in the dung), then move them to another paddock. This will stop more drench-resistant worm eggs from contaminating the pasture.
2. The next sheep to graze this paddock should have a moderate to high worm burden, with their last treatment not being from the same drench group as the long-acting product. This will help to dilute the resistant-worm eggs already on the pasture.
3. Seek professional advice on further use of products from this drench group and how they should be checked.

At any time that you are concerned that a mid-length or long-acting treatment is not providing protection, WormTest immediately and seek professional advice regarding drench resistance.