Sheep worms - testing for drench resistance and effectiveness

Introduction

Resistance now affects most drench groups including all the macrocyclic lactones (MLs; ivermectin, abamectin and moxidectin) and the drench combinations — in some cases including ‘triple combinations’; an ML plus a benzimidazole (white) and levamisole (clear drench).  The only groups to which resistance has not been detected in WA are monepantel and the new derqantel-abamectin combination.

Drench resistance

Drench resistance is a major limiting factor to profitable and sustainable sheep and wool production and it is important that we preserve the few remaining effective drenches. Resistant worms comprise only a tiny fraction of the total worm population when a new drench group is introduced. However, with repeated exposure in situations where resistant worms have a survival advantage over non-resistant worms, their proportion of the population increases. As resistance is genetically-based, this increase is permanent within a particular population, hence the resistance level does not decline to initial levels over time.

Resistance has remained common to the older drench types, even though their use has markedly decreased. Further, resistance to one drench type means resistance to others in its class, although there may be potency differences that allow some in the class to remain relatively effective even though others may be less effective.

It is important to understand the causal factors in each environment or region, so worm control programs can be modified to reduce the pressure for resistance development. Major factors include:

• Under-dosing: it is important that sheep weights are correctly estimated and the dose is set to treat the heaviest in the group.
• Frequency of use: the more that worms are exposed to drenches, the greater the chances that resistant worms will survive. Drench use should be restricted to treat worm disease outbreaks or in preventative programs.
• Long-acting drenches: drench products that remove ‘susceptible’ (non-resistant) worms living in the gut and also kill incoming susceptible larvae from the pasture bias the population in favour of resistant worms that can survive the treatment. The benefits of protection from the effects of worms must be balanced against the risks of resistance and long-acting products should be used only in a planned program to ensure some non-resistant worms survive.
• Use of less-effective drenches: the more worms that survive a drench, the larger the number left to produce eggs and increase the resistant population. This is especially important where there are few worm larvae from non-resistant worms to dilute resistant types (see below).
• Programs that allow only resistant worms to survive: drenching onto ‘worm safe pastures’ where there is no pick-up of larvae from drench-susceptible worms allows the resistant worms that survive treatment to become the major source of future populations. In WA, the classic situation is with ‘summer drenching’, where the downside of good worm control is an increased risk of resistance. Sheep worms – sustainable summer-autumn worm control outlines recommendations to manage drench resistance while achieving good worm control.
• Introduction from other properties: worms with a higher level or different pattern of resistance can be imported from other farms. Sheep should always be drenched with a combination of several drench groups whenever they are moved onto a new property.

Worm control programs should aim to minimise the development of drench resistance, through appropriate timing of drenches and the use of non-chemical approaches.

Drench resistance test

The drench resistance test — also known as the Worm Egg Count Reduction Test (WECRT) — is used to determine the effectiveness of the various drenches. This test can be used to check any drench group.

It is recommended that a drench resistance test be carried out about every two years to monitor drench performance. A veterinarian or animal health adviser should be contacted to plan the test and arrange the worm egg counting.

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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