Managing Drench Resistance for Goats in Rangelands Region

Why manage drench resistance?

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

• Importing goats (or 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. 

Never assume that a drench treatment will completely kill worms in your goats. 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, albendazole. When resistance is present for one of these actives, it is likely present for all other actives within the same group.

Selection for drench resistance happens when worms in the goat are exposed to a drench. Initially, there may be very few worms that survive the treatment (perhaps as few as 1 in 100,000) 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 goats 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 is the preferred method to check individual drenches at any time. DrenchChecks 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.

Testing drench effectiveness with a DrenchTest

Checking for drench resistance with a DrenchCheck

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 (sometimes based on the larval differentiations) have been reduced by the drench. Discuss the results with a worm control advisor.

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 goats (and sheep) are carrying worms with some degree of drench resistance to one or more drench groups.

1. ‘Quarantine’ drench all goats (including bucks) new to the property.
   • Discuss with your veterinarian which drench groups and how many can be used, their dose rates and withholding periods, including those drench groups not registered for use in goats, but which can be used with an off-label veterinarian’s prescription. The quarantine treatment should ideally consist of:
     • Meat and fibre goats: four drench groups are recommended, preferably including one from the most recently available products.
     • Dairy goats whose milk will be for human consumption: the number of registered drench actives is limited to two (fenbendazole and abamectin).
   • Do not mix different drenches unless the label states you can or under veterinary advice, as different products may be incompatible. Otherwise, use drench products concurrently—up the race with one product, then up the race again with the next.
2. Quarantine the goats after treatment.
   • Hold the goats 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 goats onto a paddock that is likely to be contaminated with worm larvae due to grazing by other goats or sheep. This would include most paddocks that have been grazed by home bred goats or 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 goats 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.

ParaBoss Product Search Tool will search 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.

Should persistent treatments be used?

In this region the only time a persistent (also called long-acting treatment; an unregistered drench for goats) 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 goats for a number of weeks.

Fortunately, Rangeland producers often have some days’ notice of large floods, so in a situation where goats are likely to be inaccessible for a month or more, the goats can be treated with a long-acting product before being moved to a safer paddock.

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