Method for Worm Egg Count Reduction Test (WECRT)
Select appropriate sheep
The best sheep for the test are undrenched lambs, at between three and five months of age, ideally around weaning. If only older sheep are available (and have adequate worm egg counts) the test may need to be modified, it is best to discuss this with your vet or adviser. Sheep drenched within a two month period should not be used for a test.
Do a preliminary worm egg count
Before starting the drench resistance test, collect faecal samples from 10–20 sheep in the flock for a worm egg count, using the paddock method (see Sheep worms – faecal worm egg counts). This will indicate whether the worm burden is high enough for a test (ideally a minimum 300 eggs per gram), a worm larval culture and differentiation will be conducted to determine if enough worms of the right species are present in the flock to continue with the test.
Very little resistance has been found in barber’s pole worm in WA and if present (as indicated by the worm larval culture), drenching with closantel to remove this worm will prevent interference with the test for scour worm resistance.
Decide which drenches to test
Deciding which drenches to test will depend on results from previous tests and the drench usage pattern on your property. Again, discuss this with your local vet or adviser prior to doing the test. The following can be used as a guide:
- BZ/LEV (white/clear) combination
- Abamectin
- Moxidectin
- ‘triple combination drenches’ with abamectin (for example Hat-Trick, Q drench, Pyrimide, Triguard, Trifecta)
- OP combinations with BZ and LEV (for example Rametin Combination, Polevault)
- Derqantel-abamectin combination (Startect)
- no drench – this is a ‘control’ group and must be included in each WECRT.
(There is no point in testing benzimidazoles, levamisole or ivermectin by themselves as resistance is present to these on almost all WA sheep properties. Startect is included as although no resistance has yet been reported in WA, it may be less effective where there is severe resistance to abamectin.)
Set up groups for drench resistance testing
Firstly, select sheep by drafting off a group of similar size (that is, discarding especially heavy or light sheep). Secondly, draft them into separate groups of 12–15, with each group representing a drench to be tested and one undrenched (control) group. Only 10 sheep will be sampled later in the test but more should be treated initially to allow a few spares in case faecal samples cannot be obtained from some individuals.
Identify the sheep in each group so that they can be sampled again in 10 to 14 days using a different coloured spray mark or numbered ear tags.
Drench each group
Weigh some of the largest sheep and calculate the dose of drench for each group using the weight of the heaviest sheep. Drench each group with the appropriate drench. Do not drench the sheep in the control group. (Note: if a closantel drench is required to remove barber’s pole worm, then give this to all animals including the control group.)
It is important to ensure that all of the sheep tested receive the correct dose. Check dose calculations and calibrate drench guns (or use syringes) to ensure the right dose is given. Drench the sheep carefully to make sure all animals receive the full amount of drench.
Return sheep to the paddock
Following treatment, the sheep can be run together or as part of any other flock of sheep until it is time for post-treatment sampling.
Collect faecal samples for worm egg counting
Between 10 to 14 days after treatment (the correct timing of this post-treatment sampling is important to get a useful result), re-muster the sheep and collect an individual faecal sample from each animal. Place this into its own container making sure that the group is clearly identified on the container. About 10 grams (approx. two heaped teaspoons) of faeces is needed. Fifteen sheep were initially treated in each group to allow some spares in case a post treatment faecal sample could not be obtained from some individuals. A minimum of 10 sheep in each treatment group (including 10 for the untreated control group) must be collected for the test. However, if possible 12 samples per group should be submitted as some samples may be insufficient in weight to test.
Submit the faecal samples for worm egg counts and larval culture and differentiations (the latter are done at specialist laboratories).
Important: Give a clean-out drench with a fully effective drench to all sheep in the treatment groups not likely to have been highly effective, including the control group.
Interpreting results
For each drench group, the average number of worm eggs of each of the main worm species is compared to the average number of eggs for each worm species in the faeces from the control group sheep. This indicates the effectiveness of the various products that have been tested. A veterinarian or adviser can do this calculation and help to interpret the results.
A fully effective drench is one that shows at least 95% reduction in the number of worm eggs for a particular worm species, compared to the undrenched control group.
Testing drench effectiveness
A general indication of the effectiveness of a particular drench can be achieved by taking worm egg counts before and after a drench is given, where it is not possible to do a full drench resistance test at an appropriate time. The first worm egg count should be carried out about a week before the drench is planned to be given to allow time for the results to be completed, as they may indicate that a drench is not needed. This is done by using the ‘paddock method’ to collect 10–20 individual fresh faecal samples from the flock.
Treat the flock if the worm egg count result indicates a need for drenching. Then, 10 to 14 days after drenching, collect another 10–20 fresh faecal samples from the same flock for a second worm egg count.
Comparing the two worm egg counts will indicate the percentage reduction due to this drench against the particular worm population present. However, it does not strictly indicate the level of drench resistance, as this is specific to a worm species. Ideally, larval cultures are performed on both the pre- and post-treatment faecal collections so that particular worm species can be identified (for example to differentiate between the scour worms and barber’s pole worm).
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