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Worm testing

Aside from drenches at one or two strategic times the mob’s average worm egg count should be the basis for other drenching decisions.

Structured reading

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Checking a mob of sheep for worms with a WormTest
How-to guide to collect and submit samples for a mob worm test to a laboratory.

Worm egg counting
How worm egg counting is carried out.

Assessing worm burdens without a WormTest
Other ways to assess whether sheep have worms and what level of worms exist.

Collecting dung samples from individual sheep (optional)
How-to guide on collecting dung samples from individual sheep (for drench resistance tests or genetic assessment of worm resistance).

Worm testing for ram breeders (optional)
How-to guide for ram breeders who want to gain worm egg count or dag breeding values for individual sheep.

Question and answer

For those who prefer a problem based approach to learning, answer the following questions.
Each of the questions below links further down the page to the answers.


  1. How many individuals should be sampled for a WormTest?
  2. Name a standard time when you should WormTest.
  3. What 2 things does a larval culture tell you and how these help you make a drenching decision?
  4. Name a situation when you would drench without a WormTest?
  5. You can use the WormTest from one mob to make drenching decisions about similar mobs. For every mob you test how many other mobs could this represent?
  6. If only very few sheep (less than 2%) in a mob appear to be badly affected by worms: ? What could be a cause of this?  What action would you take? 


You can also click on each question below to go to WormBoss pages with related information.

1. How many individuals should be sampled for a WormTest?

Collect the number of samples per mob as recommended by your laboratory (ideally this would be from at least 20 sheep—if you are doing your own worm egg count on farm, try the following ‘bulk collection’ method.

Bulk collection method

  • When conducting your own worm egg counts on farm:
  • Collect three pellets per adult pile (or the equivalent amount if soft or runny) or five for weaners.
  • Collect from at least 20 dung piles.
  • Where Haemonchus (barber’s pole worms) are an issue, and if the mob has over 200 sheep, collect from each of 40 dung piles.
  • Collect all dung into one container
  • The dung then needs to be mixed extremely thoroughly.
  • Conduct your worm egg count using a sub-sample from the bulk mixture.

2. Name a standard time when you should WormTest.

WormTests can be done at any time, however there are certain routine times to WormTest:
Note: a larval culture (larval differentiation) is particularly useful in areas or seasons in which summer rainfall occurs and when re-infection with scour worms or barber’s pole worm might occur.

  • WormTest  weaners no more than 5–6 weeks after their first summer drench, especially in wet summers. This is usually the weaning drench for spring-drop lambs; for lambs born in autumn (often early May)WormTest 4–5 weeks after their weaning drench.
  • During January–February for sheep showing signs of barber’s pole worm (anaemia and lethargy)—aside from known barber’s pole worm areas this can also occur in wet summers or irrigation areas.
  • All mobs in late January/early February, just prior to a possible second summer drench. This will usually be 6–8 weeks after the first summer drench.
  • Weaners, 4–6 weeks after the autumn break and thereafter through winter. However under high risk conditions (pastures highly contaminated with worms/higher rainfall areas/wetter than normal) test as soon as 2 weeks after the break.
  • 4–6 weeks after any short-acting drench.
  • Higher risk mobs in July/August (usually youngest and oldest). Test other mobs if high worm egg counts are found. These results will give a check on peak winter egg counts.
  • Ewes pre-lambing (provided it is at least 8 weeks post-autumn break for adults and 6 weeks for maidens). This is especially important for ewe mobs that are struggling with low condition score (less than 2.5) and/or grazing pastures of less than 1200kg DM/ha (3–4 cm pasture height).
  • As suggested by the Drench Decision Guide.

3. What 2 things does a larval culture tell you and how these help you make a drenching decision?

A WormTest refers to a ‘Worm Egg Count Test’ or ‘WEC test’; it will identify the number of worm eggs in faeces, which is a good indication of the worm burden of the sheep. Some laboratories can also perform a ‘Larval Culture’ (also called a ‘Larval Differentiation’) to identify the types of worms present and their proportion (the importance of this varies according to your location).

4. Name a situation when you would drench without a WormTest?

A) When giving a quarantine drench.

B) When giving a strategic drench. The timing of strategic drenches depends on the region and the class of sheep, as their use is closely associated with times when sheep are most susceptible to worms or  when development of eggs to infective larvae on pasture is likely to be extremely low (to reduce pasture contamination) or high (to pre-empt likely immediate problems). Strategic drenches are given regardless of the average worm egg count of the mob.

There are six common strategic drenches; not all are used in every region. The WormBoss programs outline which strategic drenches to use in each region.

  • Pre-lambing ewes
  • Lambs being weaned
  • Winter weaners going into low worm-risk paddocks
  • Smart grazing
  • Summer drenches—temperate winter rainfall regions 
  • Summer-autumn drenches—Mediterranean climatic regions

Details of when/how to use strategic drenches are in Your Program.

5. You can use the WormTest from one mob to make drenching decisions about similar mobs. For every mob you test how many other mobs could this represent?

Ideally each mob should be tested individually, as 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. However, if there are a number of mobs that have the same drenching history, same class of sheep and very similar paddock type (including recent level of contamination from worms) then one mob can represent two others (i.e. test one in every three similar mobs).

Testing representative mobs saves the cost of testing all mobs, but carries the risk that the result may not be representative.  If in doubt, test additional mobs. Testing individual mobs is suggested for Western Australia, South Australia, Victoria and Tasmania.   

6. If only very few sheep (less than 2%) in a mob appear to be badly affected by worms: ? What could be a cause of this? What action would you take?

If less than 2% of a mob are showing signs that include pale inside eyelids and gums, bottle jaw, lagging or collapse, you should treat these affected sheep immediately with a drench suitable for barber's pole worm, but also sample and test the remainder of the mob now. If these signs are not visible, but a few sheep are scouring badly, test the mob now. In both cases, use the results with your Drench Decision Guide to decide whether the whole mob should be treated.

When less than 2% of a mob show signs of worms these are the possible causes:

  • These sheep were not drenched when the rest of the mob were (e.g. didn't get mustered, were missed or spat out the drench in the race, strayed in from another mob). Their worm egg count may be a lot higher than the rest of the mob.
  • These sheep are suffering from some other illness or injury that has reduced their immunity and they have acquired a larger worm burden than the rest of the mob. Signs of another problem may be evident. 
  • These sheep are the most susceptible in the mob because of lower worm-resistance and the mob has been tested just when these first sheep are starting to show signs.
  • These sheep are not actually affected by worms at all. Instead, anaemia could be the result of liver fluke and scouring could be from larval hypersensitivity, coccidiosis or excess lush feed.

In all of the above situations, a WormTest on the remainder of the mob (don't include these badly affected sheep) to determine the worm egg count will inform your decision of whether to treat the rest of the mob.


Links to the learning topics for Victorian Winter Rainfall

  1. Introduction
  2. Grazing management
  3. Breeding for worm resistance
  4. Worm testing (you are currently on this page)
  5. Drenching
  6. Drench resistance management
  7. Sheep worms