When planning worm treatments it is critical to know the efficacy of the various drench options on your property. Knowing which drenches are effective on your property is essential if you are going to choose the correct drench to kill the worms present in your sheep.

The main way to assess drench efficacy is using a drench resistance test or faecal egg count reduction test (FECRT). You should contact your local veterinarian, sheep consultant or Department sheep officer when planning this test to ensure that the work involved yields the most useful results possible. They will give good advice on mob and drench selection for the test and other local requirements such as the number of animals and faecal sample collection techniques that are necessary.

A drench resistance test is best carried out on a group of wormy, young, undrenched sheep such as lambs approaching weaning age. If testing is only possible using sheep older than about six months of age then seek professional advice first to modify the methodology used.

An initial worm egg count of the selected sheep is important before the main test is started to check that sufficient worms of the necessary species are present to make the test worthwhile. A worm egg count of at least 300 eggs per gram (epg) is the general rule, but check this with your local veterinarian or sheep consultant. Once a suitable mob of test animals has been found they are randomly drafted in to groups of 15 animals.

One group is randomly allocated to each of the drench groups to be tested and an additional group is needed to provide untreated control animals. Sheep should be adequately identified to their groups and then receive an individual dose of their respective products. This is carefully calculated and checked on the heaviest individual within each group. Sheep in the control group remain untreated. After treatment all of the sheep can be run together or as part of any other mob of sheep until it is time for post-treatment sampling.

Between 10 and 14 days after treatment (this timing is critical for a good result) the sheep should be re-mustered and individual faecal samples collected from each group, including the controls. Individual worm egg counts for each sampled sheep and a bulk larval culture for each group should be carried out using the collected samples.

The results (drench efficacies) are obtained by comparing, for each worm species present in the test, the average number of eggs in the treated group sheep with the average number of eggs in the control group sheep.

The Drenchrite™ Drench Resistance Test is a product that allowed assessment of resistance to the BZ, Lev and BZ/Lev combination products by measuring the effects of these products on worm larvae.

This test is not used widely any more as it has difficulty in detecting ML resistance but ongoing development is continuing and it might become available again in the future.

Another test using worm larvae is available, through NSW Department of Primary Industries, to assess closantel efficacy.

It is also possible to get a 'quick and dirty' estimate of drench efficacy if a full drench resistance test is not possible at the time, by checking the worm egg count of a treated group of sheep before and after the drench.

Larval culture results before and after treatment can also add more value to the results obtained this way.

Also see:

• Professional service providers