When to Drench and When to WormTest for Sheep – Western Australia Winter Rainfall

Annual drenching programs (‘Summer-autumn drenching’)

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’ at www.agric.wa.gov, 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:

• To confirm whether signs of ill-health are likely to be due to worms. Many signs are not specific to worms, e.g. weight loss and poor growth rates, a tail in the mob, scouring and possibly deaths.
• To check whether moderate worm burdens are causing production loss, even though signs of worms are not present. Reduced weight gains and wool growth occur well before signs of ill-health are seen.
• To show whether the number of worm eggs being passed onto pasture is too high for a particular time of year. The level of pasture contamination with worm larvae in autumn largely determines the extent of worm problems in winter and spring.

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 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?

Routine drench times

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.

The South-­West Medium to High Rainfall Zone

• Lambs at weaning
• Weaners in early summer, after the pasture has dried off (this can often be done as the lambs go into a crop stubble paddock)

Note: If weaning occurs when the lambs are on dry pasture or are to be placed onto a crop stubble, the weaning and summer drench coincide and only a single drench is needed.

• Hoggets (last year’s lambs, including maiden ewes) in early summer, as for the weaners.
• Adult sheep in autumn, between the end of March and the end of April. Worm burdens in sheep of this age rarely justify a summer drench, but this can be easily checked with a WormTest (especially maiden ewes) if you are concerned.
• Late-lambing ewes (lambing later than mid-June) pre-lambing if a WormTest is not conducted (see Routine WormTest times, below). In early-lambing ewes, the March-April drench removes the need for a specific pre-lamb treatment.
• In barber’s pole worm areas, a long-acting drench should be considered for ewes lambing in May or June.

Low Rainfall Cereal Zone

• Lambs at weaning or as lambs are moved onto a crop stubble if this is within a few weeks of weaning.
• Hoggets (last year’s lambs) in early summer, unless WormTests over some years indicates there is rarely a need.

Routine WormTest times

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’.

The South-­West Medium to High Rainfall Zone

• Weaners, 6 weeks after their weaning drench, unless a summer drench is to be given at about that time
• Hoggets (last year’s lambs)
  • 6 weeks after the season’s break, or by the end of June at latest then 6 weekly to the end of spring
  • 6 weekly after any drench is given in winter or spring (unless a drench is to be given at about that time)
• Late-lambing ewes (lambing later than mid-June) WormTest 3 weeks before lambing is due to start.

Low Rainfall Cereal Zone

• Adult sheep in late March to April
• Check whether a drench is necessary (testing for a couple of years will indicate the need for a routine drench).

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:

• Ewes in early summer and mid-autumn in the Medium-High Rainfall Zone: where a summer drench is not justified, but an autumn drench is usually needed. There may be variations to this pattern.
• Pre-lambing drenches for ewes: often given routinely, but in fact are not always necessary.
• Drenches to hoggets in the Low Rainfall Zone: in drier areas, these may not be warranted.

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.

Barber’s pole worm in this region is usually sporadic and short-lived. If summer and/or autumn are unusually wet, check worm egg counts each 4–6 weeks through to early winter to identify unusual increases in barber’s pole worms before they cause production loss and deaths. If worm egg counts exceed 1000 epg (or a little lower if sheep are in poor condition), drench with a short-acting drench effective against barber’s pole worm or closantel (generally effective in this region). Test again in 4–5 weeks.

If your property faces a significant barber’s pole worm risk for several months each year, seek professional advice regarding an effective program, which may include the Barbervax® vaccine.

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 a kit from your laboratory provider or 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 following fact sheet is on the WormBoss website: ‘Checking a mob of sheep for worms with a WormTest’.

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. It considers

• whether signs of worms are present
• the class of sheep
• the WormTest results
• the condition of the sheep
• the condition of the pasture
• the likely worm contamination of the paddock

The Drench Decision Guide will recommend

• whether to drench now
• whether to use a persistent drench
• when to WormTest again

How to use the Drench Decision Guide

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.

Each Drench Decision Guide is available as a separate 2-page printable version, or can be used directly online.

Using the print version:

1. Start on the page that shows the ‘Drench Decision Guide Questions’.
2. Read Question 1.
3. Follow the ‘go to’ information on the right for the answer that applies to your sheep.
4. Only go to the question or recommendation to which you are directed by your answer.
5. When you are directed to a letter, this is the final recommendation, and is shown on the next ‘Recommendations’ page.
6. Also, read the important information in the green boxes.

Using the web version:

1. Firstly, refer to the Drench Decision Guide, which is provided separately.
2. Start on the page that shows the ‘Drench Decision Guide Questions’.
3. Read Question 1.
4. Follow the ‘go to’ information on the right for the answer that applies to your mob.
5. Only go to the question or recommendation to which you are directed by your answer.
6. When you are directed to a letter, this is the final recommendation, and is shown on the next ‘Recommendations’ page.
7. Also read the important information in the green boxes.

The region’s Drench Decision Guide is available by clicking the link below: