Testing drench effectiveness with a DrenchTest


What is a DrenchTest?

A DrenchTest is used to assess the effectiveness of a number 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, especially in relation to combination/multi-active drenches you might test.

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-related services).

What equipment is required?

  • 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.
  • A small measuring container (for checking the drench dose).
  • Drench: about 350 ml of each drench (or combination) to be tested if using syringes; 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 some sheep.
  • Dung collection bags or bottles for individual sheep: 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 sheep for 2 weeks.
  • 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, assistant): see ‘Collecting, storing and transporting dung samples for worm tests’.

How is a DrenchTest conducted?

1. Decide when to conduct the DrenchTest

  • Choose a time of year when the sheep 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 be increasing 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

  • Sheep should be less than one year old and of an even line with respect to weight, age and sex. They should also have last been drenched on the same day with the same drench and have run in the same paddock together since the drench.

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 sheep for worms with a WormTest’).
  • A mob WormTest result of 500 epg is considered the minimum level required before conducting a DrenchTest.

4. Decide on drenches to test

  • Test the single drenches and combinations of drenches that you are considering using on the property in the coming few years. Do not include drenches that were ineffective at the last DrenchTest (unless in a new combination). You are advised to seek professional advice in selecting the drenches to test, especially since some drenches with lower effectiveness may be useful in a combination.

5. Select sheep for testing

  • 15 sheep are needed for each drench group plus 15 for an untreated control group. The total number of sheep equals (drench groups + 1) x 15. In the initial draft, include extra sheep in case some are deemed unsuitable during the allocation and drenching process.
  • Choose an even line of sheep from the mob, excluding heavy or light sheep and any sick, injured or unusual sheep.
  • 15 sheep 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 sheep at sampling time.

6. Conduct the test

  • Randomly allocate sheep to groups and find some of the heaviest:
  • 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 heaviest sheep in the mob you have kept for the test and weigh these when convenient during the group allocation process. Record the weights.
  • Run the whole group to be tested into the race progressively and mark or tag the first sheep so it is allocated to the first drench group, mark the second sheep to the second group and so on, so that sheep are allocated in turn to the different drench groups and the untreated control group (with any spare sheep left unmarked).
  • Draft the sheep into the separate treatment and control groups (set the spare sheep aside).

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

  • Using the weights of 5 of the heaviest-looking sheep in the test mob, calculate the dose of each drench to be tested (each group will be drenched to this ‘heaviest sheep 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 sheep) 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 some drenches can affect the seal in the syringe, making the drench leak or the plunger stick.

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 numbers, spray mark or other information to identify this treatment group along with the treatment and dose used.
  • Treat each of the 15 sheep very carefully, watching them for a few seconds to see if they spit the drench out. If any does, reject this sheep from the trial and substitute with one from the spares, treat and identify this new sheep as for the rest of the group. Once treated, ensure they are properly marked and release these sheep 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 sheep on this day.

9. Return the test sheep to the paddock

  • Any unused spares or rejected sheep should be returned to the original mob (drenching 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 sheep; however, they will need to be drafted from the other sheep in 10–14 days, and it is usually easier to keep them alone.

10. Collect dung samples from all sheep 10-14 days after drenching occured

  • Yard the test sheep and draft them into the separate treatment and control groups.
  • Assemble bags/bottles for samples from the number of sheep (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 sheep until the numbers of required samples is collected. Do not mix dung from different sheep (see WormBoss fact sheet ‘Collecting, storing and transporting dung samples for worm tests’). 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 sheep are spare in case dung can’t be found in an individual sheep; however, dung can usually be collected if a sheep 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 sheep (choose the drench that was used on the remainder of the mob that were not included in this DrenchTest).
  • Repeat for the next group of sheep and continue until all groups of sheep have been dung sampled and drenched, including the ‘undrenched control’ group.
  • Sheep 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, levamisole may reduce the barber’s pole worm infection by 98% but only reduce the black scour worm infection by 30%. The levamisole drench 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. Depending on the results (especially if many of the drenches have low effectiveness) more combinations may need to be tested to find one or more combinations that are 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’.


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