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Drench Resistance Tests - as little as 9 cents a sheep

by Deb Maxwell, Operations Manager, ParaBoss, May 2015


While a Drench Resistance Test should be done each 2–3 years on properties in high to moderate rainfall regions, they are not used by many sheep producers on the basis of cost or difficulty. When viewed in perspective across a flock they are excellent value for money, and by following the WormBoss instruction sheet, they are not difficult to do, especially if you and a neighbour both do a test and you help each other out.

If a property has 3,000 sheep and does a Drench Resistance Test each 3 years with 8 drench groups tested, the cost is about 9 c per sheep per annum (based on a total cost of $800). In a flock of 1,000 sheep, this cost is higher, at about 26 c/sheep/year. With both flock sizes, this cost is generally less than the cost of one drench; and of course if you have more than 3,000 sheep the cost/sheep is even less.

The Drench Resistance Test results provide essential information for your drench decisions that will ultimately save you more than you have spent, through more effective drench use.

A Drench Resistance Test, also called a DrenchTest, is the gold standard for assessing the effectiveness of a number of drenches against the worms on your property.

It is more accurate than a DrenchCheck-Day10 (or DrenchCheck), which is a simple, but useful, way to follow up on how your last drench performed. DrenchCheck-Day10s are generally a better option on properties in lower rainfall areas where drenching occurs less frequently, but where drench resistance occurs nevertheless, or on properties with fewer sheep.

A Drench Resistance Test

  • 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

What needs to be considered before the test?

Decide which drenches will be tested. WormBoss recommends testing an individual drench active from each drench group. This allows the likely effectiveness of various combination drenches to be calculated and avoids the cost and effort of testing all the possible product combinations you might use in the next few years.

The ideal choices are one active (or two for MLs) from each of the following groups:

  • either albendazole, fenbendazole or oxfendazole (BZ)
  • levamisole (LV)
  • naphthalophos (OP)
  • abamectin (ML)
  • moxidectin (ML)
  • closantel (SA)
  • monepantel (AD).

Ideally, derquantel (SI) should also be included, but can only be tested in combination with abamectin, as this is the only commercially available form.

Decide what time of year to test. The ideal time is when all of the worm species in your area are active; this ensures that the drench resistance of each of the worm types can be tested at no extra cost.

There are two periods you should avoid; one is when worm risk is very high because the sheep need to be allowed to gain at least a moderate level of infection before drenching, and then if a particular drench active being tested has low efficacy, the test group (and the undrenched control group) are put at risk.

The second time to avoid is when one worm species is far more active than the other; typically, in the summer rainfall areas, barber’s pole worm egg counts are far in excess of the scour worms during summer and autumn, and vice versa in winter in the more uniform and winter-rainfall areas of Australia.

The ideal times can be late autumn or even winter/spring in the summer rainfall areas and late autumn/early winter for the winter rainfall areas.

Decide which sheep to test. Sheep under one year old are ideal, but not too soon after weaning or during southern winters when young sheep can be very susceptible to worms. They should also be the same sex and of even weight and age. Their recent drench and paddock history should be the same, that is, they should have last been drenched on the same day with the same drench and have run together since that drench. Also, consider the paddock history of where they have grazed since the last drench to make sure the test mob’s worm infection represents the whole farm.

You will need 15 sheep for each drench active you are testing, plus another 15 for the control group that won’t be drenched. Try to start with at least double that amount so you can select a fairly even line to use in the test.

Once your mob is chosen, there are two important jobs

  • Obtain the necessary drenches you wish to test.
  • Monitor the average worm egg count of the chosen mob to see when the test can be started.

Obtaining your drenches can be the hardest part of this test. You are unlikely to have all these single actives on hand, and you only require a small amount of each, so buying a whole drum is not feasible.

One option is to contact your testing laboratory; they may already have kits available for purchase that have the small amount of each drench needed. The second option is to obtain small amounts of other drenches from your neighbours—but check that their product has not expired, which could confuse the results. Also ensure it is poured into clean, labelled containers, not contaminated by other product, and that you have more than enough to treat the 15 sheep.

Decide when to start the test. Monitor the mob using a standard worm test procedure, with samples collected in the paddock. Once the average mob egg count is above 300 eggs per gram for each of the worm species of interest then the test can be conducted. Plan to do the initial drenching and follow up sample collection when you will have enough time and samples can be promptly received and tested by the laboratory. Consult your laboratory to ensure they will be able to test the large number of samples just after your planned collection date.

How is the test carried out?

The first stage of the test involves drafting off the required number of sheep, plus a few spares, dividing these into the groups of 15 and group-identifying them with a different coloured tag for each group or a record of individual ear tag numbers.

One group acts as a comparison or ‘control’ group and should be left undrenched and the other groups should receive their specific drench. Allow at least an hour or two, so that the job can be done properly and mistakes avoided.

These sheep are now returned to a paddock and must be kept together for the next 10–14 days. Ideally, they are kept separate from the remainder of their mob, so they can easily be gathered for the second stage of the process.

It is critical that the second stage of the test is done no sooner than the 10th day after the drench, but no later than the 14th day; plan for the 10th day so that if delays occur you have a few days up your sleeve.

During the second stage, collect individual dung samples from the tested sheep. Collect from a minimum of 10 sheep (number depending on your laboratory recommendations) from each drench and control group.

The faecal sample from each sheep should be put into an individual container marked with the drench group the sheep was allocated to (or the undrenched group). Keep each group of samples separate.

Send all of the samples to the laboratory where worm egg counts will be done on each sample and a larval culture done for each group. Your laboratory will calculate the drench efficacy results for you and can also be asked to use these results to calculate the likely efficacy of various combination drenches.

A detailed procedure for carrying out a Drench Resistance Test can be found in the WormBoss Tests and Tools section.