Managing drench resistance


  1. Give all introduced sheep (including rams) a quarantine drench.
  2. Avoid unnecessarily drenching sheep when conditions are very dry or in droughts.
  3. When a drench is used, follow the guidelines on choosing and using drenches.
  4. DrenchTests will rarely be feasible in this region. If a drench is warranted consider a DrenchCheck-Day10 to check the effectiveness of that drench.

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 active because they have genes conferring resistance to that active. 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 active 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-Day10 is the preferred method to check individual drenches at any time. DrenchCheck-Day10s 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-Day10

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

The DrenchCheck-Day10 involves two WormTests: the first up to 10 days before drenching (usually at a routine WormTest time) and the second between 10 and 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 the fact sheet ‘Checking for drench resistance with a DrenchCheck-Day10’.

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.

  1. ‘Quarantine’ drench all sheep new to the property.
    • Use a combination of no less than 4 unrelated drench actives with at least one of these being the newest drench actives: monepantel (Zolvix®) or derquantel (with abamectin—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 or under veterinary advice, as different products may be incompatible.
    • If sheep have come from high rainfall (>600 mm) or irrigation areas in eastern states, consider a liver fluke treatment using triclabendazole.

  2. Quarantine the sheep after treatment.
    • Hold the sheep in quarantine in yards (small mobs) or a secure paddock (larger mobs) for 1–3 days (1 day if feed is green high quality, 3 if it is dry low quality) 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.
  3. 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.
  4. WormTest the imported sheep 14 days after drenching for added confidence that treatment was successful.
  5. 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

Use all 3 principles where possible.
They are equally important and greatly slow the development of drench resistance.

  1. Use drenches most effective on your property. Drenches that reduce worm egg count by at least 98% are preferred. The more effective a drench is the fewer drench-resistant worms will remain in the animals after treatment. If drench effectiveness is unknown, conduct a DrenchCheck after drenching.
  1. Use an effective combination of two or more drench groups, either in a multi-active product or using more than one product concurrently (up the race with one and then the other) to combine different drench groups. The higher the efficacy of each drench group and the more drench groups included in the combination, the greater the benefit for slowing drench resistance. The chance of a worm being resistant to all active ingredients in a combination is much lower than for each individual active on its own. For goats, be aware of what drench groups are registered or permissible with a veterinarian’s prescription.
  1. Use short-acting treatments and restrict the use of persistent products for specific purposes and high worm-risk times of year. Persistent products provide a long time during which ingested resistant larvae can survive and reproduce. There is little need to use mid-length or long-acting treatments if animals are being moved to low worm-risk paddocks.

A small benefit can be gained by rotating drench groups providing you also rotationally graze stock across the property so that paddocks are exposed to sheep that have received different drenches. However, if you set-stock, drench rotation will not slow the development of drench resistance.

While not affecting resistance, it is essential to choose a drench with an appropriate withholding period (WHP) and export slaughter interval (ESI) according to the time left before the animals may go to slaughter, or their milk may be used for human consumption.

Search for drenches based on the worms or other parasites targeted, drench group or active and product name.

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 animals onto very clean, low worm-risk paddocks (such as  ungrazed cereal stubbles or paddocks that have been free from sheep or goats 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 that heavy animals are not underdosed, and light animals are not overdosed.

4. Follow the label instructions to ensure correct dose and use of treatments.

5. After animals have been drenched, graze them initially on paddocks already contaminated with worms, not on paddocks that are being specifically prepared as low worm-risk. Eggs deposited on pasture from surviving drench-resistant worms in the animals will be diluted by eggs and larvae already on the paddock (these should be susceptible, or at least, less drench resistant). 

If animals must be drenched onto low worm-risk paddocks, such as lambing, weaning or winter weaner 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.

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, 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.