Managing Drench Resistance for Sheep in Victorian Winter Rainfall Region

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 and test all drench groups.

A DrenchCheck­‐ Day 14 is used to check individual drenches at any time. Regularly do DrenchCheck-­‐Day 14s between the times that full resistance tests (DrenchTests) are performed.

Testing drench effectiveness with a DrenchTest

Checking for drench resistance with a DrenchCheck

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 and one group of sheep is left undrenched to act as a ‘control’ or comparison. Each of the groups is drenched (except the control group) 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 the fact sheet ‘Testing drench effectiveness with a DrenchTest’ on the WormBoss website.

The DrenchCheck­‐Day 14

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

The DrenchCheck‐Day 14 involves two WormTests: the first up to 10 days before drenching (usually at a routine WormTest time) and the second 14 days after the drench. Samples from individual dung piles (10–20) are used for this test, not a bulk collection.

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 advisor.

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 Victorian Winter rainfall regions.

• ‘Quarantine’ drench all sheep new to the property, including those returning from agistment.
  • Use a combination of no less than 4 unrelated drench groups with at least one of these being monepantel or derquantel. 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 or under veterinary advice, 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 would include most paddocks that have been grazed by home bred sheep for the last 3 months. 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 (especially if step 3 cannot be achieved). 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 a fully effective drench or combination of drenches for the strategic (summer) drenches: A fully effective drench is one that reduces the worm egg count in your sheep 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. Note: Drenches of less effectiveness (say 90–95%) may still be sufficient if sheep are treated in winter and returned to contaminated pastures (e.g. a pre-lambing drench, however, these drenches or combinations should not be used as a summer drench.)
2. Use a combination of two or more groups where possible; fewer worms are able to resist more than one group at a time, but these combinations must be tested in your flock.
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 later section, ‘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 (Smart-grazed) 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 by using WormTests to guide drench decisions, 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. Except for weaners (and when preparing low‐worm risk paddocks with smart grazing), don’t move newly-­drenched sheep into low worm‐risk paddocks. When the weaners finally leave their prepared paddock:
   • i) Give them an effective drench of a different group to that used when they entered the paddock.
   • ii) The next mob to use the paddock should not have been drenched recently, and their last drench should be a different drench group to what the weaners had when they entered the 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—about 3 months for long-acting and 1–4 weeks for mid-length treatments (depending on the particular product).

Persistent treatments 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 in the absence of susceptible ones. Secondly, persistent treatments have a longer time at the end of their protection period where the active concentration has dropped to a level where partially resistant worms may establish in the sheep, survive and start reproducing.

Persistent drenches appear to pose a particular risk for selection for resistant barber’s pole worm (Haemonchus). However, the situation is less clear for scour worms which reproduce more slowly and the interactions between using persistent products and selection for resistance are quite complex and incompletely understood.

Using primer and exit drenches with long­‐acting treatments

Primer drenches clear the sheep of any worms at the commencement of the long acting treatment 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. It is given at the same time that a long-acting product is given. A primer does not stop sheep accumulating resistant worms during the protection period of the long-acting treatment.

Exit drenches are used two weeks after the end of the actual protection period. 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.

The need for either treatment in this region will be dictated by the results of a drench resistance test and monitoring of worm egg counts during the period the long-acting treatment should be effective. WormTests at monthly intervals (30, 60 and 90 days) after a long-acting treatment are ideal. However, a primer drench should routinely be used with a benzimidazole (white drench) capsule.

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‐Day 14 (see above) after the next treatment. Also, conduct a similar test (collecting 20 individual samples rather than a bulk sample) at 60 days and 90 days after it is given to establish 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 60 days for a capsule (for moxidectin, where efficacy varies against worm species, WormTest at 35 days if scour worms are your prime consideration or 60 days in high-risk barber’s pole worm areas). For these tests, collect 20 individual samples rather than a bulk sample.

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.