Never assume that a drench treatment will completely kill worms in your livestock. Drench resistance is a result of worms having genes that enable them to survive treatment. It is likely that these genes were present in some worms before a drench was ever used. Drench resistance is now very common and in many cases severe for some drench groups, making testing for drench effectiveness a vital component of a worm control program.
Drench groups are the 'chemical families' of drenches and some groups contain a number of drench actives. For example, the Benzimidazole group has the following actives: fenbendazole, oxfendazole and albendazole. When resistance is present for one of these actives, it is likely to be present for all other actives within the same group.
Selection for drench resistance happens when worms in the animal are exposed to a drench. Initially, there may be very few worms that survive the treatment (perhaps as few as 1 in a million), but these resistant worms lay eggs and their offspring constitute an increasing proportion of the worm population. In this way, each treatment causes an increase in drench resistance because only resistant worms survive to reproduce.
Resistance may develop faster with more drenching and use of persistent products. Drench resistance is unlikely to be reversible, so not using a drench for a while will not permanently result in the worm population becoming susceptible again. While ever drenches are being used, drench resistance cannot be prevented, but the rate at which it occurs can be greatly reduced.
The first step is to know what drenches are effective on your property.
Each property has its own drench-resistance profile based on its own drenching history and that of properties from which the animals are sourced. The profile of neighbouring properties can be quite different.
The extent of resistance is only known by testing. Obvious worm control failures may only occur when resistance is quite advanced.
DrenchTest is the common name for the Worm Egg Count Reduction Test (WECRT). This assesses the drench-resistance status of worms on a property.
You can test as many individual drench actives (you will need an off-label prescription from your veterinarian if testing actives not registered for use in goats) as you like in a DrenchTest, providing you have enough goats for the different groups.
Select a mob of sheep or goats for the DrenchTest. From this mob, a group is used for each drench and one group is left undrenched to act as a ‘control’ or comparison. Each of the groups is drenched (except the control group) and dung samples are collected from all animals 14 days later for a WormTest.
The worm egg counts and larval differentiations of each treatment group are compared with those of the undrenched control group. From this, the effectiveness of each drench against each worm type present is calculated.
Discuss the test with your adviser before setting up. For more detail see ‘Testing drench effectiveness with a DrenchTest’.
This simple and inexpensive test gives an indication of drench effectiveness and whether it should be properly investigated using a DrenchTest.
The DrenchCheck involves two WormTests with larval differentiation
The results from the two WormTests are compared to gauge the extent that worm egg counts (and in some cases, based on the larval differentiations) have been reduced by the drench. Discuss the results with a worm control advisor.
Keeping drench-resistant worms out of your property is part of sustainable worm control.
Assume that purchased sheep are carrying worms with some degree of drench resistance to one or more drench groups. See Drench groups and actives.
If sheep have come from high rainfall (>600 mm) or irrigation areas in eastern states, consider a liver fluke treatment using triclabendazole.
Use all 3 principles where possible.
They are equally important and greatly slow the development of drench resistance.
A small benefit can be gained by rotating drench groups providing you also rotationally graze stock across the property so that paddocks are exposed to sheep that have received different drenches. However, if you set-stock, drench rotation will not slow the development of drench resistance.
While not affecting resistance, it is essential to choose a drench with an appropriate withholding period (WHP) and export slaughter interval (ESI) according to the time left before the animals may go to slaughter, or their milk may be used for human consumption.
Search for drenches based on the worms or other parasites targeted, drench group or active and product name.
The drenching programs outlined above aim to provide good worm control with the minimum increase in drench resistance. This includes both keeping drench usage to the minimum necessary and ensuring that some non-resistant worms remain somewhere in the worm population on the property, so they can dilute resistant worms that survive a drench. (This is known as the ‘refugia’ principle, that is, ensuring some non-resistant worms are in refuge from drenches.)
In Western Australia, the long, hot, dry summer causes worm eggs on the pasture to die, with only the worms in the sheep surviving to the next year. While young sheep should receive a summer drench, the resistant worms inside the sheep at the drench time will be the only survivors over summer. From autumn, successful development of their eggs to larvae will resume, increasing the number of resistant worms on the farm.
To counteract this, it is recommended that adult sheep do not receive a summer drench; instead they are drenched in autumn. In this way, they carry the less resistant population of worms inside them over summer. From early March onwards, before they receive their autumn drench, some of the eggs from these worms can develop to the larval stage, providing a population with more susceptible worms compared to those carried by sheep that received a summer drench.
Move the summer-drenched sheep onto the pastures contaminated by the adult sheep so that the resistant worm eggs the summer-drenched sheep leave are diluted among the less-resistant worm eggs left by the adult sheep. This will reduce the overall proportion of resistant worms on the farm.
Ironically, drenching sheep onto ’worm-safe’ pastures at any time of year can have the similar effect of promoting drench resistance—while recommended for highly worm-susceptible sheep (especially weaners), some caution is needed for use of ‘worm-safe’ pastures as a routine.
Follow all 4 principles where possible:
Persistent or long-acting treatments are rarely required in this region. Only use them if professionally advised to do so.
Effective persistent treatments kill immature and adult worms at the time of treatment, as well as infective larvae eaten by animals (with pasture) during the period of protection of the treatment—for sheep, this is about 3 months for long-acting and 1–6 weeks for mid-length treatments (depending on the particular product).
Both may increase selection for resistance to the actives in those treatments for two reasons. Firstly, worms are exposed to the active for longer. This favours surviving resistant worms, which then reproduce. Secondly, persistent treatments have a longer time at the end of their protection period where the active concentration drops to a level where partly resistant worms may survive and reproduce.
The most commonly used persistent drenches contain the actives moxidectin or closantel. Some moxidectin and closantel products have a “Do Not Use” statement preventing use in animals that may be used to produce milk for human consumption.
Primer drenches clear the animal of any worms that are resistant to the long-acting treatment. A primer drench is an effective short-acting drench (preferably a combination) that does not include the same group as the long-acting product. Give a primer at the same time that a long-acting product is given.
Exit drenches are used two weeks after the end of the actual protection period. By this time the persistent treatment has declined to very low levels. The exit drench kills larvae that have survived the persistent treatment and developed into breeding adult worms. Another name for the exit drench is a ‘tail cutter’.
An exit drench (like the primer drench) is an effective short-acting treatment (preferably a combination) that is from a different group/s to the persistent product.
Resistance can develop to mid-length treatments in the same way as to long-acting treatments. While primer and exit drenches are desirable with mid-length treatments, they are rarely cost-effective because of the relatively short protection period compared to long-acting products.
The effectiveness of the persistent product on your property will be shown by the length of the protection period actually achieved (rather than what is claimed on the product label). Where the persistent product contains an active/s available in other products as a short-acting formulation (e.g. albendazole and abamectin) or with mid-length activity (e.g. moxidectin) then a DrenchTest can simply include these drenches rather than the persistent products.
The schedule to test the length of protection provided by persistent products on your property depends on if you know the efficacy of the drench active.
When you send the samples, request a larval culture if there is a positive worm egg count because:
If the treatment was fully effective, and you used a primer and exit drench, the product will probably have a similar length of effectiveness at the next use. However, it is best to check the effectiveness of long-acting products every year they are used by doing a WormTest at 30 and 60 days.
If a WormTest shows worm eggs are present before the end of the claimed protection period, drench resistance is likely. You should:
At any time that you are concerned that a mid-length or long-acting treatment is not providing protection, WormTest immediately and seek professional advice regarding drench resistance.