Managing Drench Resistance in Sheep – Pastoral

Why manage drench resistance?

Drench resistance can occur in very dry areas and this is mainly due to:

• Importing sheep carrying drench-resistant worms from somewhere else.
• Drenching at a time when it is very dry and there are no worm larvae on the pasture to dilute the progeny of resistant worms surviving the drench.

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

Worms that survive treatment 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 were 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. In this region, a DrenchCheck-­Day 14 is the preferred method to check individual drenches at any time. DrenchCheck-­Day 14 should be considered when any drench is given and it is the most practical and cost-effective method of testing drenches in this region.

While a DrenchTest or Worm Egg Count Reduction Test (WECRT) is the most accurate test for drench resistance, this test is rarely feasible in this region as infections are often not high enough and when they are, they may be unexpectedly high and need swift treatment or are in lambs at weaning, which should not be put at risk in a DrenchTest.

The DrenchCheck­‐Day 14

This simple and inexpensive test gives an indication of drench effectiveness.

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.

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 – Pastoral regions.

• ‘Quarantine’ drench all sheep new to the property (particularly if sheep or rams are from a higher rainfall district where drench resistance in more common).
  • 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 3–4 months in cooler weather or 4–6 weeks when it is hot and dry (greater than 35°C during the day).
• After quarantine, release the sheep if possible 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.
• If possible, 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 drenches most effective on your property; ideally use those shown to reduce worm egg count by at least 98% as shown by a DrenchCheck-­Day 14. If drench effectiveness is unknown, conduct a DrenchCheck-­Day 14 after drenching. 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 drench groups where possible, as the chance of a worm being resistant to all group ingredients in the combination is much lower than for each individual group on its own.
3. Use short–­‐acting treatments where possible, and restrict the use of persistent products for specific purposes and high worm-risk times of year. In this region, long-acting products are rarely warranted, and then only in summer rainfall areas and when sheep are likely to be inaccessible for extended periods.
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 would ideally exclude any groups that were used the previous time. However, in practice, ensure the current drench 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, especially
   • Adults.
   • During droughts or prolonged dry 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 larger sheep are not under-dosed and smaller sheep are not excessively over-dosed.
4. Follow the label instructions to ensure correct dose and use of treatments (including complying with withholding periods).

Should long-acting treatments be used?

In this region, the only time a long-acting treatment should be considered is in the summer rainfall areas where there has been a history of barber’s pole worm outbreaks and extensive flooding threatens to isolate and crowd sheep for a number of weeks.

Fortunately, Pastoral producers often have some days’ notice of large floods, so in a situation where sheep are likely to be inaccessible for a month or more, the sheep can be treated with a long-acting product before being moved to a safer paddock. A fly preventative treatment is also warranted at the same time.

Do not use a long-acting drench more than once a year.