Managing drench resistance

WormBoss worm control program for goats

Australian smallholders

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.

Using drenches

Follow all 5 principles where possible:

1. Avoid unnecessary drenching, especially
   • during droughts or prolonged dry periods
   • immediately before or after moving goats 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.
   • adult dry goats with low worm egg counts (refer to the Drench Decision Guide) or if WormTests are not practical then adult dry goats showing no clinical signs of worms based on eye mucous membrane colour (FAMACHA^©) and adequate Body Condition Scores. 
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 kidding, weaning or winter weaner paddocks, do both of the following:

i. When the goats 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 goats 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 goats. 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.

Using Barbervax^® vaccine for barber’s pole worm

The use of Barbervax should slow the rate of development of drench resistance because fewer drenches will be used. It is unlikely that barber’s pole worm will develop resistance to this vaccination.

Note: This vaccine is only recommended in districts where barber's pole worm is a high risk for some months of the year. For more details see the Barbervax vaccination program.

How can persistent treatments be used effectively?

Note: There is no research to confirm the length of protection for goats provided by persistent products.  It is well reported that, in relation to sheep, goats are able to more rapidly metabolise drenches and are less fat.  Both of these features will reduce the exposure of worms to drench actives and likely shorten the length of protection provided by the persistent product.

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, 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 ingredient for longer. This favours surviving resistant worms, which then reproduce. Secondly, some persistent treatments have a period 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.

Cattle pour-on or injectable products, or horse paste tubes should not be used on goats.

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 10, 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 was used, it is a narrow  spectrum drench, only for barber’s pole worm.

Note: Neither moxidectin nor closantel are registered for use in goats (see above for veterinary prescription).

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 animals 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.

At any time that you are concerned that a mid-length or long-acting treatment is not providing protection, WormTest immediately and seek veterinary advice regarding drench resistance.