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Tackle the rise of resistant worms in cattle

Drench-resistant worms. If it’s something you suspect your cattle may have, you’re not alone! The landscape of drench resistance in Australia is changing and with a predicted monster of a worm season ahead, it’s something you need to act on now.

The thing about worms is that resistance can be passed on to the next generation of worms, eliciting a whole new vicious cycle of resistance. Anthelmintic (drench) resistance occurs when a genetic change or mutation happens in individual worms. For example, for some worms where the genetic changes have occurred, they can increase the rate of excretion of drench from their cells. Others carry genes that enable the worms to detoxify the drench, making it ineffective, or that alter or by-pass the biochemical pathway targeted by the drench.

So, where do you start?

Figure 1. Graph of an effective drench (orange) that kills on average over 95% of worms in a mob compared to a low efficacy drench (blue) that kills fewer than 95% of worms. Source
Figure 1. Graph of an effective drench (orange) that kills on average over 95% of worms in a mob compared to a low efficacy drench (blue) that kills fewer than 95% of worms. Source

Step 1. Confirm you have drench resistance

The benchmark for efficacy of cattle drenches is a 95% reduction in worm egg counts for a particular species (although this may differ from the actual reduction in number of worms in the animal).

Complete a farm-based test. These involve faecal worm eggs counts (WECs) and include the below options.

Click the links to see what you will need and easy to follow steps on how to conduct each test.

Step 2. Identify what is causing the resistance

Many factors can influence the development of resistance, including the worms themselves and their environment, however, the way that cattle are managed also plays an important role.

Which cause is likely related to your situation?

  • Frequent use of the same chemical. This includes the frequent use of chemicals from the same chemical group. Find the chemical groups and actives here.
  • Under-dosing with drench. This allows the more tolerant pests in a population to survive. Common causes of under-dosing include under-estimating the weight of animals being treated, poor application technique, and mis-calibrated application equipment. Untreated cattle licking animals recently treated with a pour-on product can also result in under-dosing.
  • Unintentionally exposing non-target parasites to chemicals (e.g. pour-on products to treat worms can also affect tick and lice populations).
  • Treatment of cattle when the number of drench susceptible worm eggs and larvae on the pasture is low. Otherwise known as refugia, it is important to know what stage the worm is at both within the cattle and on the pasture to know when the best and most effective time to drench is. Find the life cycle of worms here.


Step 3. Put the right management strategies into place

There are several strategies to manage resistance, but by far the most effective measure is to minimise the number of times that drenches are applied.

Strategies for delaying the emergence of drench resistance include:


Reduce the need for treatments by using non-chemical worm control methods to reduce larval contamination on pastures.

Use the appropriate drench for the species of worm present. Different worm species have developed resistance to different chemical classes.


Time treatments for lower risk classes or management types so that you drench only when necessary, such as when faecal worm egg counts are high, poor weight performance is observed or worms are found post mortem.


Maintain refugia (a population of worms that have not been exposed to drenching) by ensuring populations of worms, not recently exposed to drenches, are present when other mobs are drenched.


Consider a combination drench to maximise efficacy, and reduce the build-up of resistant worms within the population. 

Consider other parasites when treating, to avoid unintentional exposure of non-target parasites to chemicals. It is best to use different actives for internal and external parasites.


Follow product label instructions to avoid under-dosing.

For more information on worm management on cattle visit