Managing Drench Resistance for Sheep in Western Australia 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. Some groups can be combined with others 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‐Day14s between the times that full resistance tests (DrenchTests) are performed.

Testing drench effectiveness with a DrenchTest

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’ in Appendices: Further information on sheep worm control for Western Australia winter rainfall regions – WormBoss

• ‘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 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 a day to allow worm eggs present at the time of drenching to pass out of the gut.
  • Provide adequate feed and water.
  • If feasible, 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 10–14 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?

Timing of drenches

The drenching programs outlined above aim to provide good worm control with the minimum increase in drench resistance. This includes both keeping drench usage to the minimum necessary and ensuring that some non-resistant worms remain somewhere in the worm population on the property, so they can dilute resistant worms that survive a drench. (This is known as the ‘refugia’ principle, that is, ensuring some non-resistant worms are in refuge from drenches.)

In Western Australia, the long, hot, dry summer causes worm eggs on the pasture to die, with only the worms in the sheep surviving to the next year. While young sheep should receive a summer drench, the resistant worms inside the sheep at the drench time will be the only survivors over summer. From autumn, successful development of their eggs to larvae will resume, increasing the number of resistant worms on the farm.

To counteract this, it is recommended that adult sheep do not receive a summer drench; instead they are drenched in autumn. In this way, they carry the less resistant population of worms inside them over summer. From early March onwards, before they receive their autumn drench, some of the eggs from these worms can develop to the larval stage, providing a population with more susceptible worms compared to those carried by sheep that received a summer drench.

Move the summer-drenched sheep onto the pastures contaminated by the adult sheep so that the resistant worm eggs the summer-drenched sheep leave are diluted among the less-resistant worm eggs left by the adult sheep. This will reduce the overall proportion of resistant worms on the farm.

Ironically, drenching sheep onto ’worm-safe’ pastures at any time of year can have the similar effect of promoting drench resistance—while recommended for highly worm-susceptible sheep (especially weaners), some caution is needed for use of ‘worm-safe’ pastures as a routine.

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 drench 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 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, try to ensure it has at least one effective active from a drench group that was not used the previous time.

Using drenches

Follow all 4 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).
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 excessively over-dosed.
4. Follow the label instructions to ensure correct dose and use of treatments (including complying with withholding periods).

How can persistent treatments be used effectively?

Persistent or long-acting treatments are rarely required in Western Australia. Only use them if professionally advised to do so.

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; also, larvae from resistant worms will become adult worms when non-resistant ones are killed. Secondly, some 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 products may be recommended where there is a severe worm risk that cannot be managed otherwise. However, worm control of a level similar to persistent products can usually be obtained by grazing management (pasture planning), provided this recognises the time required to allow worm larvae to die off before sheep are re-introduced. The cost-benefit should also be considered, in comparison to grazing management.

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 recommended. However, a primer drench should routinely be used with all slow release capsules.

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.

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.