Testing drench effectiveness with a DrenchTest

A DrenchTest provides sheep and goat producers with the information about which drenches are no longer working effectively due to the worms on their farms having developed resistance. This information is used to protect the health of the sheep or goats by ensuring that worm burdens are treated with the correct product.

What is a DrenchTest?

DrenchTest is used to assess the effectiveness of drenches that you might use on your property in the next 2–3 years. The DrenchTest:

  • Examines each drench’s effectiveness for each worm type present.
  • Is the most accurate way to test for drench resistance.
  • Uses the procedure called a Worm Egg Count Reduction Test or WECRT
  • Should be conducted on each property every 2–3 years.

Contact a testing laboratory or an advisor for further advice before conducting a DrenchTest.

While producers can conduct their own DrenchTest, in most areas there will also be people who can do the on-farm actions of the test for you (such as testing laboratory staff, advisors, rural merchandise staff or contractors offering other sheep or goat related services).

What equipment is required?

  • An assistant is considered essential.
  • Drenching equipment: separate clean syringes are ideal for drench treatments, with small wide-mouthed containers to hold each drench. If a drench gun and backpack is used it is essential to thoroughly clean this between drench groups or use a separate gun and pack for each drench.

Note: Zolvix® and Startect® drenches should be placed into glass jars and have spare syringes for each as they are not compatible with certain plastics.

  • A small measuring container (for checking the drench dose).
  • Drench: enough for 20 animals of the maximum weight you will be drenching (this allows some spare doses). If using syringes, this is generally about 200–400 ml (depending on the dose rate of the particular product); double this if using a drench gun and pack (laboratories and veterinarians can often dispense the small amounts of extra drenches required).
  • Scales for weighing the animals.
  • Dung collection bags or bottles for individual animals: 10 or 15 for every group (ask your testing laboratory whether they do 10 or 15).
  • Identification equipment: coloured tags or spray mark suitable to distinguish each group of animals for 2 weeks. If animals have individual tag numbers these should be recorded, but colour coding makes the job easier.
  • Recording equipment: pen and paper—use the DrenchTest recording sheet provided at the end of this fact sheet and the recording sheet provided by your laboratory.
  • Other dung sampling equipment (lubricant, soapy water, gloves, table/trolley, esky): see ‘Collecting dung samples from individual sheep or goats’.

How is a DrenchTest conducted?

Any one mistake can ruin the results of a DrenchTest.

  • Set aside plenty of time—at least 3 hours for both the drenching and the collecting activities, as well as time to muster the animals.
  • Set out all the equipment close by, but protected from being knocked over by animals or you and your assistant.
  • Decide who will do what jobs, how and in what order and then stick to a routine—don’t do each other’s jobs, as this causes mistakes.
  • Go slowly, especially at the start, while you develop your routine.
  • Avoid having spectators and too much chatting, as these divert your attention.
  • Drafting and re-drafting will likely be required; ensure gates are secured between each group to prevent mix-ups and wrong treatments.

1. Decide when to conduct the DrenchTest

  • Choose a time of year when the animals are likely to be infected with all of the worm species of interest (e.g. barber’s pole worms and scour worms are not always abundant at the same time).
  • Avoid conditions (time of year and heavily contaminated paddocks) when worm burdens can increase rapidly between the start and finish of the test (as the undrenched group and groups that receive a less effective drench are more at risk).
  • Choose a day to start the test that results in the final collection day (10–14 days later) being when dung samples can reach the testing laboratory without weekend delays in the post.

2. Decide on the mob to be tested

  • Animals should be less than one year old and of an even line with respect to weight, age and sex. All animals should have received the same last drench, on the same day, and then been run together.
  • If there are not enough animals of the same age and type, a DrenchTest can still be done by using smaller numbers, but the procedure must be modified to include individual worm egg counts both before and after drenching to get the percentage reductions for each animal.
  • If there are not have enough animals to do all the drench groups, then separate DrenchTests in different seasons or years may need to be done. However a control group is needed in each case. Your professional adviser can help with the design.

3. Conduct a preliminary WormTest

  • At the time of year when the DrenchTest is planned, use a routine WormTest to decide whether the mob has enough worms for a DrenchTest. (See ‘Checking a mob of sheep and goats for worms with a WormTest’).
  • A mob WormTest result of 200 epg each for black scour worm and brown stomach worm and 500 epg for barber's pole worm is considered the minimum level required before conducting a DrenchTest.

4. Obtain drenches to test

  • It is useful to seek professional advice in selecting the drenches to test, especially since some drenches with lower effectiveness may be useful in a combination. For goats, combination drenches, sheep-only registered drenches and higher doses than on the labels, generally need a veterinary prescription.
  • For a comprehensive DrenchTest, WormBoss recommends testing these actives (groups):
    • levamisole (LV)
    • albendazole OR fenbendazole OR oxfendazole (one of the ‘white’ or BZ drenches)
    • abamectin (ML)
    • moxidectin (ML)
    • naphthalophos (OP)
    • closantel (SA)
    • monepantel (AD)
    • derquantel (SI) + abamectin (ML) (Derquantel alone, is not available for testing).

Note: praziquantel and triclabendazole are not included, as they do not target roundworms.

5. Select animals for testing

  • 10 to 15 animals are needed for each drench group plus 10–15 for an untreated control group. The total number of animals equals (drench groups + 1) x 10 or 15. In the initial draft, include extra animals in case some are deemed unsuitable during the allocation and drenching process.
  • Choose an even line of animals from the group, excluding heavy or light animals and any sick, injured or unusual animals.
  • Ideally, 15 animals are used initially for each group; however, a minimum of 10 will be sampled from each group in 10–14 days (ask your testing laboratory whether they want 10 or 15). If only 10 are needed, the extra 5 per group are ‘spare’ in case of deaths or escapes or inability to collect dung from animals at sampling time.

6. Conduct the test

  • Allocate enough time: at least 20–30 minutes per group is needed on the “drenching day” after the sheep or goats have been yarded.
  • Decide how each group will be identified from the other (such as coloured spray marks or tags) and record this next to the drench treatment group. Ensure that the identifying tag or mark will still be clearly visible in 2 weeks.
  • Look for 5 or more of the biggest animals in the mob kept for the test and weigh these. Record the weights. If numbers are restricted and hence the lines are uneven, then weigh every animal.
  • Run the whole group to be tested into the race progressively and mark or tag the first animal so it is allocated to the first drench group, mark the second animal to the second group and so on, so that animals are allocated in turn to the different drench groups and the untreated control group (with any spare animals left unmarked). Individual tag numbers of the animals can also be recorded.
  • Draft the animals into the separate treatment and control groups (set any of the spare animals aside). With more test groups, multiple drafting is likely to be required, i.e. draft off a couple of groups with the remaining ones still boxed together; when these initial groups are treated, draft off more groups until all groups have been drafted off and treated individually. Care is required to ensure all animals get their correct treatment.

7. Calculate doses of each drench being tested and check the drenching equipment

  • Using the weights of the heaviest animals in the test mob, calculate the dose of each drench to be tested (each group will be drenched to this ‘heaviest animal in the mob’ weight). Follow the manufacturer’s directions on the label of the drench container or as professionally advised. Record the dose rate (ml per animal) to be given alongside each drench treatment group on the recording sheet (different drenches may have different dose rates).
  • If using a drench gun and pack, ensure that they are thoroughly cleaned of previous drench and fill the pack with about double the volume of drench required for the first drench group. After setting the gun to the desired dose, measure a few doses into a measuring container and adjust the drench gun until it is correct and consistent.
  • If using syringes for each drench, these are already tested as accurate. Ensure recycled syringes are not cracked or leaking from the plunger. Have a container (with the drench in it) that can easily have the drench withdrawn with the syringe, rather than trying to get the syringe into the mouth of a large drench drum. A different syringe for each drench group is preferred. Note that Zolvix® and Startect® can affect the seal in the syringe, making the drench leak or the plunger stick, they also cause some plastic containers to crack or dissolve, so use glass jars for these.

8. Treat each of the drench treatment groups separately

  • Bring the first of the drench treatment groups into the race.
  • Check that the correct details have been recorded for the tag colour, individual tag number, spray mark or other information to identify this treatment group along with the treatment and dose used.
  • Treat each of the animals very carefully, watching them for a few seconds to see if they spit the drench out. If any does, reject this animal from the trial (remove the coloured tag or obscure the coloured mark if one had been applied, to ensure this animal is not sampled later) and substitute with one from the spares. Treat and identify this new animal as for the rest of the group. Once treated, ensure they are properly marked and release these animals from the race.
  • Thoroughly clean the drench gun and pack between treatment groups, or for syringes, use a new one for each group if possible or clean and check that it is not cracked from being chewed. Refill with the next treatment and set up the gun according to the dose calculated earlier.
  • Repeat the above steps for each drench treatment group.
  • The last group will be left undrenched. It is the control or comparison group.
  • Dung samples are not required to be collected from any animals on this day unless few animals are available and group sizes are small, in which case consult a professional regarding test design.

9. Return the test animals to the paddock

  • Any unused spares or rejected animals should be returned to the original mob (drench them if the remainder of the mob had been drenched at this time).
  • Mix all of the treatment and control groups together and graze them together for the next 10–14 days in a secure paddock convenient to the yards to avoid losses and so they are easily re-mustered.
  • The test mob can be boxed with other animals; however, they will need to be drafted from the other animals in 10–14 days, and it is usually easier to keep them alone.

10. Collect dung samples from test animals 10–14 days after drenching occurred

  • Allocate enough time: at least 20–30 minutes per group is needed on the “collecting day” after the sheep or goats have been yarded.
  • Yard the test animals and draft them into the separate treatment and control groups.
  • Assemble bags/bottles for samples from the number of animals (10–15) as directed by your testing laboratory.
  • Bring the first group into the race and take a dung sample from the rectum of each animal until the numbers of required samples are collected. Do not mix dung from different animals (see WormBoss fact sheet ‘Collecting dung samples from individual sheep or goats’). Place them in sample bags/bottles and then in another bag or box that clearly identifies the specific treatment (or control) group. If only 10 samples are required, the other animals are spare in case dung can’t be found in an individual animal; however, dung can usually be collected if an  animal is set aside for 15–30 minutes.
  • Keep the samples in a cool place or an esky during the collection process, with ice bricks in hot conditions.
  • Drench the control animals (use the drench that was used on the remainder of the herd that were not included in this DrenchTest).
  • Repeat for the next group of animals and continue until all groups of animals have been dung sampled and drenched.
  • Animals can be returned to the main mob (note that the mob has varied drenching times when considering when to drench next).

11. Complete the submission form

  • Ensure that the laboratory knows that these samples are part of a DrenchTest, that each treatment or control group of samples is identified as a group and that all groups are to have a larval culture done separately regardless of the worm egg count.

12. Package and send the samples

13. Receive the results

  • Final results will be available about 10 days after the samples were received by the laboratory (allowing time for the larval cultures and analysis to be completed).
  • The worm testing laboratory or veterinarian will analyse and interpret the test data.
  • Results will be provided for each drench and for each worm type present as a per cent reduction in the number of worm eggs.

14. Understand and use the results

  • A drench is classed as ‘effective’ only when it reduces the worm egg count by 98 per cent or more. At lower levels of effect, the worms on your property are drench-resistant.
  • Each worm type may be resistant or susceptible (not resistant) to different drenches and at different levels. For example, “drench x” may reduce the barber’s pole worm infection by 98% but only reduce the black scour worm infection by 30%. “Drench x” would therefore be an effective barber’s pole worm drench, but not an effective drench against black scour worm.
  • Drenches tested as less than 98% can be useful when used in combinations. Hence, why WormBoss recommends testing all single actives (except ivermectin) to allow you to calculate almost all combination products. If single-active drenches were tested, use the WormBoss Combination-Drench Efficacy Calculator to calculate the efficacy of combination drenches based on their single-active ingredients.
  • If only a limited number of drenches were tested and none or few were 98% effective, more will need to be tested to find those effective on your property.
  • Seek professional advice to interpret DrenchTest results and to decide suitable drenches for future use.

* Other WormBoss fact sheets are on the WormBoss web site under ‘Tests &Tools’.


DownloadDrench Resistance Test Recording sheet (93 KB)