Livestock Solutions Logo

FarmPlanner LogoFind out more


Worm Control

Drench Resistance

Drench Resistance is caused by internal parasites developing inherited tolerance to commonly-used drenches. It results in animals with worms that don’t respond to drenching. Today, drench resistance is very common.

Theoretically, drench resistance occurs once a population of a species of worm can survive a dose of a drench that would have previously killed it. Initially resistant worms are rare in a population of worms (perhaps as few as 1 in a trillion).  When a sheep 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 sheep 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 to the treatment within the worm population increases.

Many diverse methods have been employed 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.


The majority of drench actives have been used extensively for decades and therefore have high levels of resistance

Effective Drench

Broad spectrum drenches from 1960 to 2014.11

The current drench resistance status in sheep


Drench Resistance - BZ


Drench Resistance - LEV


Drench Resistance - IVM


Drench Resistance - MOX


Drench Resistance - ABA


Drench Resistance - NAP

Figure 2: The current 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).

Prevalence and severity of anthelmintic resistance in ovine gastrointestinal nematodes in Australia (2009-2012)

Australian veterinary Journal 92,12:464-471. MC Playford, AN Smith, S Love, RB Besier, P Kluver and JN Bailey. Dec 2014

Introduction to Sheep Worm Monitoring

How do I avoid Drench Resistance? 

Practical tips for farmers keen to decrease the threat of Drench Resistance are to:

Develop a strategic plan for parasites

  • Only use drenches that are effective on your property
  • Use effective combination drenches 
  • Use short acting treatments and leave long acting treatments for a specific purpose or high risk periods
  • Avoid unnecessary drenching

Consult a veterinarian or advisor

'Quarantine' drench (need to contain 4 unrelated drench actives and ideally a new active) all sheep new to the property.

Current recommendations for quarantine drenches are to use:
    • Derquantel+Abamectin (Startect) plus a dual active combination (levamisole + benzimidazole)
    • Monepantel (Zolvix) plus a triple active drench (abamectin+levamisole + benzimidazole)
    • Leave sheep in yards or small paddock for 24-48 hours then move to a wormy paddock
Conduct worm egg counts to find out the actual worm burden before drenching

Conduct another worm egg count 10-14 after drenching to see if the drench worked

Use non-chemical methods to decrease the risk of parasitic disease

Refugia – What is it?

Refugia is one way we can slow down the onset and development of drench resistance. The aim of refugia is to avoid all of the worm population being exposed to a particular drench at the same time. Genes of susceptible worms are then allowed to persist into the next generation and the result is slower selection for the resistant genes.

So in practical terms, refugia occurs on your property when:

Drenching a wormy mob some animals are left undrenched.

Worms on the pasture cannot be drenched and therefore classed as in refugia

Worms in mobs not being drenched are in refugia.


When implementing refugia the goal is to avoid all excessive worm burdens and therefore it is critical to carry out regular worm egg counts and larval differentiations. Care must be taken when barbers pole worm is present to prevent a rapid excessive worm burden. Refugia is also best practiced on worm tolerant animals.

Dry pasture

Chart 1

Green pasture

Chart 2

Figure 1. Likely proportion of worms in the sheep versus on pasture in different climatic conditions.
Source: Lewis Kahn9

The term 'worms' is used to refer to all stages of the worm: eggs, larve and immature and adult worms. We have learnt about these stages early in the module and in 'talking parasites with sheep producers'.

When a drench is given to a mob, the main way that worms within the sheep will be in refugia is when some sheep are left undrenched. Worms on the pasture cannot be exposed to a drench, so are always in refugia.

Likewise, on a property basis, worms in mobs not being drenched are in refugia, compared with those in mobs receiving a drench9


Sheep Internal Parasites & Best Practice Worm Monitoring


Worms decrease appetite and prevent ewes gaining adequate nutrition. This is critical at joining when a rising plane of nutrition leads to higher ovulation rates and better conception.

The leading research has changed to provide the best  protocol for managing worm resistance. From, 3 principles have been recommended. Use all 3 principles for choosing drenches where possible.

They are equally important and greatly slow the development of drench resistance.

1 - Use drenches most effective on your property. Drenches that reduce worm egg count by at least 98% are preferred.

2 - Use an effective combination of two or more drench groups, either in a multi-active product or using more than one product concurrently (up the race with one and then the other) to combine different drench groups. 

3 - Use short-acting treatments and restrict the use of persistent products for specific purposes and high worm-risk times of year. 

4 - Rotating drench groups gives only a small benefit and only when stock are also rotationally grazed.

Rotating drench groups gives only a small benefit and only when stock are also rotationally grazed.


Successful worm control also involves:

Worm testing (FEC) at critical times
    • weaners, 4-6 weeks after the weaning drench 

    • during January-February for sheep showing signs of barbers pole (anaemia and lethargy)

    • all mobs in late January and early February (prior to second summer drench)

    • weaners, 4-6 weeks after Autumn break and for highly contaminated pastures/ high rainfall areas, test as soon as 2 weeks after the break

    • 4-6 weeks after short-acting drench

    • high risk mobs in July/August (i.e. youngest and oldest animals) then test the other mobs if high egg counts are found

    • ewes pre-lambing

    • any other non-routine times if parasitic challenge is suspected

Drenching at recommended times

    • November/ December for the first summer drench

    • lambs at weaning - pre-lambing

    • sheep going onto paddocks that are to be kept at low-risk for weaners

    • drench all introduced sheep with a quarantine drench
Managing Drench resistance
    • conduct drench tests each 2-3 years 

    • avoid unnecessary drenching 

    • use effective drenches i.e. drenches with greater than 98% efficacy

    • use multi-actives (drenches with more than one active ingredient) whenever possible

    • calibrate drench guns and dose to the heaviest sheep

Sheep Faecal Egg Count Sampling and Culturing

Further Reading

show more articles
A Better Way To Buy