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Home News Feature articles Diagnosing and Dealing With Drench Resistance

Diagnosing and Dealing With Drench Resistance

24 Apr 2023

What is Drench Resistance?

Drench resistance, also known as Anthelmintic Resistance (AR), is caused by internal parasites developing inherited tolerance to drenches. It results in animals with worms that don’t respond to treatment and is recognised as an important problem in the livestock industries around the world.  New anthelmintic (drench) molecules are released on the market very rarely, so producers have been forced to use various management procedures to cope with the fact that their drenches no longer work. 

Worms killed by a drench are said to be ‘susceptible’ to treatment. Drench resistance occurs once a population of a species of worm can survive a dose of a drench that would have previously killed it. 

Many diverse methods can be used to slow the onset of parasite resistance to drenches, including pasture management, animal management, quarantine treatment and providing ‘refugia’ for worms by Targeted Selective Treatment.


Figure 1: Creating ‘low-risk’ pastures will decrease the need to drench cattle and slow the development of anthelmintic resistance

Practical definition of drench resistance

The industry definition of drench resistance, as measured in a Faecal Egg Count Reduction test (FECRT), is a reduction in worm egg count of less than 95 per cent. This definition is important to understand because, at this level of efficacy, drench resistance would often not be causing clinical worm problems, such as scouring, anaemia or obvious weight loss. However, it could be associated with loss of production (milk, wool, body weight).

By the time obvious drench failure occurs, (usually at about 70% efficacy) then resistance is very well established within the worm population. This is one of the reasons why the reversal of drench resistance is very unlikely, even if the drench group is not used for a long period of time. Preventing or reducing the onset of drench resistance is the best option.

Drench resistance is ‘genetic’ (controlled by genes). Resistance to each group of drenches is controlled by different genes, meaning that resistance develops to each drench group in each species of worm separately.

Being a genetic trait, drench resistance is also ‘heritable’ (can be passed on from one generation or worms to the next).  Initially, resistant worms are rare in a population.  When an animal is treated, the resistant worms survive and, if they find a mate, can reproduce.  The resultant offspring are resistant and if they survive as larvae on the pasture and infect another animal they will make up a greater proportion of the worm population than their parents did.  Over time, and with continued treatment, the overall resistance level within the worm population increases.

The rate of development of drench resistance can be influenced by many factors such as:

  1. The chemical group and ‘persistency’ of the product involved
  2. The frequency of treatments
  3. The worm species involved
  4. Environmental factors such as climate and ‘refugia’

Australian sheep drench resistance status

Figure 2. National drench resistance status in sheep. Range of efficacy (%) and mean efficacy (%) for abamectin (ABA), benzimidazole (BZ), ivermectin (IVM), moxidectin (Mox), levamisole (Lev) and naphthalophos (Nap). (Source: Playford MC, 2014, Australian Veterinary Journal)

Other publicly-available sources of information on sheep drench resistance can be seen at:

  1. SheepTRAX (provided by Zoetis Australia)
  2. Tridectin efficacy portal (provided by Virbac Australia)
  3. Dynamic Ag study in Western Victoria (2012-2018)
  4. Australian Wool Innovation-sponsored study in Southern Tablelands of NSW (2020)


Since goats share the same worms as sheep, resistance patterns are similar to the above. However because goats have a different metabolism to sheep, on-farm resistance is usually much greater.


There are currently no public databases showing drench resistance status in Australian beef cattle. However, data collated from regional studies and tests conducted at the Dawbuts laboratory show important trends that producers can use for selecting products.

Cooperia (small intestinal worms) and Haemonchus (barber’s pole worms) are often resistant to mectins, but susceptible to levamisole.

Ostertagia (brown stomach worms) have shown some resistance to levamisole, mectins and white drenches.

  1. Cotter and Besier (2011) Drench resistance in beef cattle in Western Australia
  2. Bullen et al. (2016) Australian Veterinary Journal- resistance in dairy cattle in Victoria

Choosing a drench

When choosing a drench you need to know:

  • The specific properties and potential uses of all of the different drench groups
  • The current drench resistance status against the worm species on your property

Drenches are often classified on the basis of their range of activity and/or the class or type of active ingredient(s) that they contain. Broad-spectrum drenches provide activity against most of the important worms of sheep, provided they are susceptible to the drench.

Narrow-spectrum drenches generally just have activity against one or two species of sheep worms. It is preferable to use these against specific worms (e.g. an infestation of barbers pole worm) rather than a broad-spectrum drench. Long acting treatments, such as oral and injectable moxidectin products, closantel products and controlled-release capsules, have persistent efficacy against susceptible target worms.

The majority of products are administered as oral drenches but injectable products and controlled-release capsules (where available) are also used. The withholding period and possibly export slaughter interval of the different products should be considered before using any particular drench.

Introducing new stock

To prevent the introduction of resistant worms, take all new stock directly to yards, and administer a quarantine treatment. For sheep and goats, use at least 4 effective drench families including a ‘new’ active (see WormBoss Product search tool). An example would be Macrocyclic Lactone, Benzimidazole, Levamisole (+ Monepantel or Derquantel).

Cattle should be treated with a combination drench containing levamisole at the time of introduction, as well as other treatments for external parasites as determined by your biosecurity plan, e.g. treat cattle coming from Theileria-endemic areas with a product registered for bush ticks prior to or on arrival.

Keep new stock in yards or small paddocks for 24-48 hours with feed and water, then place in a “wormy” paddock, to dilute any surviving resistant worms. Alternatively, stock can be put into a paddock that is going to be ploughed and cropped, as this will also disrupt the life cycle of any surviving resistant worms.

Non-chemical options for cattle

Current advice from parasitologists is to reduce the number of drenches used per year, to decrease the likelihood of inducing resistance to anthelmintics. Also, after treating cattle, take samples for Drench Checks to ensure the treatment has worked. This will mean you can avoid dud products in future.

The NSW Dept. of Primary Industries state in their PrimeFact on cattle worm control that adult cattle are drenched too frequently, especially in western areas. However, younger cattle, particularly in higher rainfall zones, may not be drenched enough for optimal production and welfare. NSW DPI propose the following as “Integrated Pest Management” measures-

  • pasture management to provide safe pasture for susceptible stock
  • rotation of stock- using older cattle or sheep to decrease pasture larval burdens
  • improving immunity of stock to worms by good nutrition & supplements
  • decreasing dung pats on pasture- this can be done by harrowing, cropping or dung beetles

‘Safe’ pastures (low-risk pastures)

Young stock, especially weaned young stock and yearlings are particularly susceptible to roundworm infection. These stock should have priority access to “safe pasture”- that is pasture that has a low larval worm burden. The best ways of preparing low worm pastures are by cropping, making hay or spelling the pasture over the summer. Other measures that can be used are alternate grazing with other stock (e.g. sheep/cattle) for several months, or with older dry stock which due to their acquired resistance to worms do not produce many worm eggs.

High risk pastures are those that have been recently grazed by young stock (under two years of age). These pastures are likely to have high larval worm burdens and so stock placed on it will pick up lots of worms. It is best to think of preparing a pasture for young stock (especially weaners) several months ahead of the weaning date.

Drench Tests or Worm Egg Count Reduction Test (WECRT)

The most practical way to assess drench efficacy is using a worm/faecal egg count reduction test (WECRT or FECRT).

You should contact your local veterinarian, advisor or State Department vet or livestock officer when planning this test, to ensure that the work involved yields the most useful results possible. They will give good advice on mob and drench selection for the test and other local requirements such as the number of animals and faecal sample collection techniques that are necessary.

Principles of this test are the same for sheep, goats and cattle.

Figure 3: Marking sheep using coloured zip ties to show different treatment groups;

A drench resistance test is best carried out on a group of wormy, young, un-drenched stock such as lambs kids, or calves approaching weaning age. If testing is only possible using older stock, then seek professional advice first.

Pre-test An initial worm egg count of the selected mob is important before the main test is started, to check that sufficient worms of the necessary species are present to make the test worthwhile.

A worm egg count of at least 300 eggs per gram (epg) is the general rule, but check if your lab is able to offer sensitive tests that allow for lower starting worm egg counts. For cattle, 150 epg is adequate.

Once a suitable mob of test animals has been found they are randomly drafted into groups of 15 head.

Note that current guidelines do not require an additional group of untreated control animals.

Day 0- Animals should be adequately identified to their groups. Dung samples are then taken and all stock receive a dose of their respective products. The dose is calculated based on individual weights, or the heaviest individual within each group. After treatment, all of the stock can be run together or as part of any other mob until it is time for post-treatment sampling.

Day 14- At 14 days after treatment (this timing is critical for a good result) the mob is re-mustered and individual faecal samples collected from each animal. Worm egg counts for each sampled animal and a bulk larval culture for each group should be carried out.

Figure 4: Taking dung samples from rectum of sheep 14 days after treatment.

Report– after worm egg counts and larval culture and differentiation are completed your lab will calculate the efficacy of each drench against each species of worms and send you a report summarising these results.

You will then know which drenches work and don’t work on your property at different times of the year when the various species of worms appear.

Figure 5: Collecting samples into sample collection trays, ready to ship to the laboratory.

Use the WormBoss combination drench calculator to estimate the efficacy of various combination products.

Practical tips for livestock producers to decrease the threat of Anthelmintic Resistance

  1. Develop a strategic plan for parasites
  2. Consult an advisor
  3. Conduct Faecal Egg Counts to find out the actual worm burden before drenching
  4. Check that drenches work (using FECRT or Drench Checks)
  5. Use non-chemical methods (flock and paddock management) to decrease the risk of parasitic disease

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