Why manage drench resistance?

Never assume that a drench treatment will completely kill worms in your livestock. Drench resistance is a result of worms having genes that enable them to survive treatment. It is likely that these genes were present in some worms before a drench was ever used. Drench resistance is now very common and in many cases severe for some drench groups, making testing for drench effectiveness a vital component of a worm control program.

Drench groups are the 'chemical families' of drenches and some groups contain a number of drench actives. For example, the Benzimidazole group has the following actives: fenbendazole, oxfendazole and albendazole. When resistance is present for one of these actives, it is likely to be present for all other actives within the same group.

Selection for drench resistance happens when worms in the animal are exposed to a drench. Initially, there may be very few worms that survive the treatment (perhaps as few as 1 in a million), but these resistant worms lay eggs and their offspring constitute an increasing proportion of the worm population. In this way, each treatment causes an increase in drench resistance because only resistant worms survive to reproduce. 

Resistance may develop faster with more drenching and use of persistent products.  Drench resistance is unlikely to be reversible, so not using a drench for a while will not permanently result in the worm population becoming susceptible again. While ever drenches are being used, drench resistance 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 the animals 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 (for goats, some of these will require an off-label prescription from your veterinarian). Effectiveness of multi-actives or from giving single actives sequentially (i.e. up the race with one and up the race again with the other active) can be calculated from the efficacy of the single actives using the Combination Drench Efficacy Calculator.
• A DrenchCheck 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.

You can test as many individual drench actives (you will need an off-label prescription from your veterinarian if testing actives not registered for use in goats) as you like in a DrenchTest, providing you have enough goats for the different groups.

Select a mob of sheep or goats for the DrenchTest. From this mob, a group is used for each drench and one group 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 animals 14 days later for a WormTest.

The worm egg counts and larval differentiations 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 detail see ‘Testing drench effectiveness with a DrenchTest’.

The DrenchCheck

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

The DrenchCheck involves two WormTests with larval differentiation

• The first up to 10 days before drenching (usually at a routine WormTest time)
• The second at 14 days after the drench. The second WormTest should be based on individual samples and not the Bulk Collection Method.

The results from the two WormTests are compared to gauge the extent that worm egg counts (and in some cases, based on the larval differentiations) have been reduced by the drench. Discuss the results with a worm control advisor.

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.

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.

2. 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.

3. 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.

How can persistent products be used effectively?

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

Effective persistent treatments kill immature and adult worms at the time of treatment, as well as infective larvae eaten by animals (with pasture) during the period of protection of the treatment—for sheep, this is about 3 months for long-acting and 1–6 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 drops to a level where partly resistant worms may survive and reproduce.

The most commonly used persistent drenches contain the actives moxidectin or closantel. Some moxidectin and closantel products have a “Do Not Use” statement preventing use in animals that may be used to produce milk for human consumption. 

Use primer and exit drenches with long-acting treatments

Primer drenches clear the animal 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. By this time the persistent treatment has declined to very low levels. 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. While primer and exit drenches are desirable with mid-length treatments, they are rarely 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). Where the persistent product contains an active/s available in other products as a short-acting formulation (e.g. albendazole and abamectin) or with mid-length activity (e.g. moxidectin) then a DrenchTest can simply include these drenches rather than the persistent products.

The schedule to test the length of protection provided by persistent products on your property depends on if you know the efficacy of the drench active.

• Where the DrenchTest results indicate that the active/s are effective on your property (i.e. reduced worm egg count by at least 98%) then conduct a WormTest at 30, 60 and 90 days after treatment. If it is shown to be ineffective at the earlier test, then the later tests will be of no value.
• If you do not have current DrenchTest results you should do a WormTest at 14, 30, 60 and 90 days after treatment. 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. Spell the pasture for at least 2 months to allow many of the drench-resistant larvae to die. The next animals to graze this paddock should have a moderate 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 veterinary advice on further use of this product.

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.