The strongly seasonal nature of Mediterranean environments lends itself to a routine drenching program, as dry summer and autumn pastures are unfavourable for worms, and sheep drenched then do not pick up new worm burdens. This is the basis of the highly efficient ‘summer drenching’ program. However, summer drenching has been confirmed as the main factor causing the high levels of drench resistance in WA, as any worms surviving these drenches (i.e. resistant worms) are the source of future worm populations.
To reduce the risk of drench-resistance without affecting the effectiveness of worm control, ‘summer drenching’ should be replaced by ‘Summer-autumn drenching’. (See DAFWA FarmNote No. 348: ‘Sheep worms — a change to summer autumn worm control’ by clicking here, then Animal Health, then Parasites.)
Under this program, drenches for adult sheep are delayed until autumn, and only weaner and hogget-age sheep are drenched in summer. Most adult sheep have low worm egg counts in early summer and a drench at that time is not warranted; delaying a drench until late March or April allows some less-resistant worms to survive. Provided that the worm population on the property includes sufficient less-resistant worms to dilute resistant worms, the overall resistance level on the property will be reduced.
Why check worm burdens in sheep?
Checking worm burdens with a worm egg count (WormTest) indicates whether worm control is on track (no significant effect on sheep) or whether treatment is needed. Worm egg counts are important in several situations:
How are worm burdens tested?
Checking worm egg count using WormTests is a critical part of the WormBoss worm control program.
WormTests are mostly done by private veterinarians or DAFWA laboratories. In addition, many farmers have done DAFWA worm egg count courses and carry out tests on their own flocks. Some local worm egg counting service providers are also operating—it is important to be sure that these have received appropriate training and are part of an accreditation program, and to seek professional advice where worm egg count results are not simple to interpret.
Which mobs and how many should have a WormTest?
It is best to test each mob individually as it is difficult to extrapolate worm egg count results from one mob to another, even of the same age or class. There are usually differences between paddocks in the favourability for worm survival, the number of worm eggs being deposited by different mobs, and the time since a drench was given. In some cases, a drench may be given to some mobs but others WormTested.
When should WormTests and drenches be routinely done?
In WA, the routine ‘Summer-autumn drench’ program is recommended for the main sheep areas (the South-West Medium to High Rainfall Zone). In the Low Rainfall Cereal Zone, routine treatment is needed only for younger age classes. It is critical that all drenches given in summer and autumn are given with a fully-effective drench, as otherwise drench resistance is likely to increase more rapidly.
Routine WormTesting is recommended mostly in the months when sheep are grazing green pasture, as worm eggs and larvae require moisture to develop. For opportunities to fine-tune programs with WormTests, see the section below: ‘When are other WormTests done and drenches given’.
When are other WormTests done and drenches given?
The timing of WormTests and drenches will vary between farms and seasons. Use the Drench Decision Guide (see below) to weigh up important factors when deciding when to drench or WormTest on your property. These factors include signs of worms, time since last drench, WormTest results, time of the year, and condition of sheep and pastures.
Worm control can be made more efficient using WormTests over a couple of years to determine whether a routine drench or a WormTest will usually be appropriate at a particular time of year. Examples are:
If drenching is done for other reasons (such as an early drench before holidays or harvesting), use the Drench Decision Guide to decide when to drench or WormTest again.
What samples should be collected for WormTests?
Sheep do not need to be yarded for a WormTest. Collect warm, fresh dung from the paddock (but make sure that ewe and lamb samples are not mixed).
To conduct a WormTest obtain sandwich bags (not ziplock) for individual animal samples and then follow the instructions provided by your testing laboratory or advisor. As a guide, collect 20 individual samples from mobs. The laboratory will then ‘bulk’ these samples using an identical amount of dung from each sample.
Avoid delays in transit (when worm eggs can hatch) by collecting and posting early in the week. Also ensure samples are kept cool (refrigerate but do not freeze) before sending, include an ice brick in transit in very hot weather and exclude as much air from the sample bags as possible.
The WormBoss Drench Decision Guide
The Drench Decision Guide helps to simplify decisions on whether and when to drench. There is a version of the Drench Decision Guide for each WormBoss region.
The Drench Decision Guide will recommend:
You can use the Drench Decision Guide at any time, whether you are contemplating drenching a mob now or in coming weeks. Not all situations require a WormTest: the Drench Decision Guide will recommend when these should be done.
See the Drench Decision Guide.