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Emily King
Manager, Woolgrower Education & Capacity Building
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Worm management and drench resistance

Posted by Bestprac on Jul 03 2012


  • Green pastures mean that worms will be active, but early diagnosis using worm egg counts and effective drenches (passing resistance tests) should reduce impacts on growth rates, wool cuts and staple strength.
  • Whilst resistance is the inevitable outcome of using worm drenches, strategies can slow its development.
  • Tools such as the Making More From Sheep manual and WormBoss can help woolgrowers manage worms and other sheep parasites for maximum profitability.

With lambs or weaners on the ground, and many regions having had a wet summer, woolgrowers should look at their worm control and drench resistance plans to reassess the risk of heavy worm burdens in late autumn and winter.

The Healthy and Contented Sheep module from the Making More From Sheep manual includes procedures and tools to guide woolgrowers on managing the health and wellbeing of their flocks.

For worm control, the important actions to consider include:

  • use of worm egg counts (WEC) as the basis for when to drench
  • testing the drench resistance status of your property so only effective drenches are used
  • selecting sheep for increased resistance to worms (low WEC) and lower dag, or purchase rams from studs that have made progress in this trait without compromising wool (and meat) quality
  • drenching timed strategically so the total number of drenches is less
  • management systems to minimise disease in the highest-risk mobs:
    • Grazing to provide weaners with low-worm- risk paddocks
    • Use ‘smart grazing’ – a strategy for the control of worms in weaners during their first winter – see:
    • Rotate sheep with cattle
    • Use intensive grazing management with highly effective drenches to control barber's pole worm
    • Ensure weaners and lambing ewes have adequate nutrition so they can handle worm burdens.

Integrating a number of these management tools is recommended by WormBoss regional control programs, rather than waiting for failing drenches.

Worm control programs are part of farm management programs on most sheep properties. Each state has detailed programs designed to minimise production losses and delay drench resistance.

Two broad programs include the winter rainfall areas where Teladorsagia (brown stomach worm) and Trichostrongylus spp (black scour worm) are the important worms; and the summer rainfall areas where Haemonchus contortus (barber's pole) is the most important worm.

The Healthy and Contented Sheep module provides guidelines for worm control in each of the winter rainfall regions of southern Australia and the summer rainfall regions.

In all areas, local climate and management practices make a significant difference to worm control. Consult WormBoss (see below) and seek advice from a veterinarian, sheep health consultant or State Department of Agriculture/Primary Industries adviser to design a program specific for your property. Local knowledge from other sheep producers can also help. A good program is flexible enough to accommodate climate and seasonal fluctuations.

Detection and management of drench resistance

Drench resistance is widespread in all sheep growing regions of Australia except some consistently dry pastoral areas.

Sheep producers should test the drench resistance status of the worms in their sheep every 2-3 years, depending on how many drench groups are effective. Drench resistance occurs if drenches are less than 98 per cent effective. But, in reality, resistance starts when efficacy falls below 100 per cent.

Initially, a simple worm test 10-14 days post drenching will indicate if a drench works (and if any sheep were misdrenched) as long as the sheep producer knows the worm count at drenching. To properly assess drench resistance using a worm egg count reduction trial (WECRT), the following guidelines are recommended: 

  • Select undrenched lambs or weaners (young sheep give the best correlation between worm burden and WECs) with WEC greater than 250–300 epg (scour worms) or 800 epg (in barber’s pole regions).
  • The number of groups of sheep needed depends on how many drenches and combinations are being tested. An additional group that remains undrenched is used as a control group. Sheep are randomly allocated to groups and each group clearly identified with coloured ear tags. About 15 sheep should be allocated to each group.

Weigh the lambs and calibrate the dose of drench based on the heaviest in each group. Treat each sheep, except those in the control group, then run the sheep as one mob.

Collect dung in the sample bottles provided 10-14 days after drenching from at least 10 lambs per group, making sure to keep samples separate. Collect dung directly out of the rectum, not off the ground. Collect as much dung as possible, as it will be used for growing worm larvae (larval cultures) to determine what worms are present. Mark the bottles clearly and send them to the lab immediately.

Drench efficacy is calculated by comparing the average WEC of each group with the control WEC. Efficacy for each type of worm can be compared if larval cultures are done on each group.

Discuss results with your veterinarian or livestock adviser:

Important strategies to minimise drench resistance

  • Always drench sheep introduced to the property with monepantel and a combination of three unrelated drenches (“mectin”, “white”, “clear”) – check WEC 10 days after arrival drench to ensure 100 per cent worm kill.
  • Test drench efficacy and use effective (proven by resistance test) drenches at critical times, such as summer drenching.
  • Rotate drench groups, including combinations, to avoid reliance on one group.
  • Minimise drench frequency by only drenching after a WEC and by using rams selected for low WEC.
  • In most southern regions (other than Mediterranean climates), preferably use one summer drench, but monitor using WECs.
  • Reduce exposure of young sheep to heavily contaminated pasture by:
    • 6-monthly swapping sheep and cattle or smart grazing or grazing young sheep on new pastures
    • Rotational grazing (>50 days rest) in combination with a highly effective drench with a short grazing time (less than 21 days) – this appears to be very effective at controlling barber's pole in weaners over the November to March period in summer rainfall regions, but is less effective for other worms.
  • Ensure sheep maintain body condition targets (BCS 3) and consider protein supplements for lambing/lactating ewes in summer-rainfall regions. Good nutrition appears to improve sheep resilience and immunity to worms.
  • In southern regions with low worm survival over summer, research suggests leaving a small proportion (five per cent) of adult identified sheep undrenched. This strategy is not recommended for high-rainfall regions where worm larval survival in summer is higher. Advanced management techniques like these are best used with advice from a professional veterinary consultant.

Wormboss logo


For the most comprehensive and effective management of worms, consult WormBoss which represents AWI’s collective knowledge of the management of worms and the reduction of their resistance to drenches.

WormBoss is an online tool, available at, which has been developed by AWI and the Sheep CRC to help producers optimise the timing and effectiveness of drenching and other worm management practices.

The aim of WormBoss is to reduce drenching and minimise the onset of drench resistance while increasing animal productivity and profitability.

WormBoss was created by leading Australian parasitologists, researchers, extension officers, consultants, drench manufacturers and drench resellers, and it is continually being updated.

WormBoss is currently composed of two main sections:

Ask the Boss

Ask the Boss is designed to help producers consider all the key issues when making a decision to drench or not drench a mob of sheep. In the Ask the Boss section, users enter key information such as the their regional location, the class of sheep in question, any worm egg counts, the existence of drench resistance data, climatic issues, and the availability of low worm paddocks. WormBoss uses this information to generate a concise report that highlights key issues and options that woolgrowers can use to make decisions and take action against worms.

A reference section

The reference section contains the world’s most comprehensive guide to Australian worms and worm-related issues with information on:

  • worm management
  • available drenches and their properties
  • common sheep worms, their life cycles, effects, diagnoses and treatments
  • sheep breeds and their relationships to sheep worms.

Free monthly email alerts about worms in your region

One of the most useful features of WormBoss is the free monthly WormBoss email newsletter that provides the latest sheep worm control information and news, as it happens and when it is important for you.

This free monthly email newsletter includes a summary of regional sheep worm issues and news from around Australia. Content is coordinated by a technical committee of sheep parasitologists from WA, Queensland, NSW, SA, Victoria and Tasmania, and features input from private veterinarians and consultants and government veterinarians from around Australia.

To subscribe to the monthly WormBoss email newsletter, please contact and quote “WormBoss” in the subject line.

More information: See the Healthy and Contented Sheep module of the Making More From Sheep Manual at for further procedures and tools.
Visit WormBoss at


Last changed: Jul 05 2012



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