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Use refugia to prolong drench life

By Deborah Maxwell, Brown Besier and Lewis Kahn

What is refugia?

Refugia is simply the Latin term for ‘in refuge’. When managing drench (anthelmintic) resistance, the aim is to avoid all of the population of worms being exposed to a drench and subject to drench resistance development at the same time. Ideally, part of the population remains unexposed to the drench and is said to be in refugia; it is available to dilute resistant worms so these never become a significant proportion of the farm worm population.

The population of worms on a property is composed of stages within the sheep and stages on the pasture. When pastures are green and temperatures are mild, worms within the sheep potentially represent only a minor proportion of the total worm population, but when pastures are dry and temperatures too hot (e.g. summer in Western Australia) or too cold (e.g. winter in the Northern Tablelands of New South Wales) to allow development of worm eggs, then worms within the sheep will represent a very large proportion of the worm population.


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

The term ‘worms’ is used here to refer to all stages of the worm: eggs, larvae, and immature and adult worms. 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. In contrast, 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 drench.

Why is refugia important?

Prolonging drench life on a particular property is about slowing the development of drench resistance. The important aspect of drench resistance is the proportion of the worm population (not the number of worms) with drench resistance. Having enough worms in refugia is a powerful way to slow development of drench resistance. Worms in refugia ingested by sheep after a drench will dilute the remaining resistant worms, so these do not become a significant proportion of the total worm population. However, there is a conflict between managing drench resistance and gaining good worm control. Maintaining too many worms in refugia to dilute drench-resistant worms also means more worms to infect sheep after drenching. Nevertheless, with some understanding of when and how to use refugia, you can achieve both good worm control and slower development of drench resistance.

Drench resistance is also specific to a drench group (or active family, and to a particular worm species. Benefits from using worms in refugia to dilute resistant worms in drenched sheep rely on them being the same species and susceptible to the same drench that was just used.

What is important when using refugia?

  1. Understand the seasonal environmental pattern that influences worm larval development and survival so as to know the regions and times of year when refugia management is most important.
    • In regions where there is regular rainfall, where periods of hot and dry or cold are short, or where there are perennial pastures persisting through hot summer months, there is almost year-round development and survival of larvae on pasture. In these regions, there is less need for deliberate management to maintain worms in refugia. This is because worms on pasture remain a reasonable proportion of the total worm population year round.
    • In regions where there are regular and prolonged periods of dry, or very hot or very cold weather, no larvae will develop or survive during these times. Under these conditions, the proportion of worms on pasture in refugia falls to very low levels and strategies to maintain worms in refugia are especially important.
    • The key factors in development of eggs to larvae are temperature and rainfall at the time as well as after eggs are deposited on the pasture.
  2. Distinguish between the worm species.
    • Refugia strategies are most applicable for scour worms. They need to be applied with more caution where and when barber’s pole worm is a major risk. 
  3. Apply refugia strategies in sheep best able to tolerate worms.
    • Adult sheep in good body condition are the best class when you are going to leave a proportion undrenched. 
    • Do not apply refugia strategies with lambs, weaners and hoggets.
  4. Monitor worm egg counts
    • Use a WormTest in flocks when you are using refugia strategies to check that excessive worm burdens don’t develop.

What are the practical strategies for using refugia?

Translating these principles into action is easy with some forward planning for the following situations where using refugia is beneficial for slowing the development of drench resistance. 

  1. In Mediterranean and hot, dry summer-dry climates
    1. Use an ‘autumn drench’ program for adult sheep:
      • Do not give a summer drench (on dry pastures or crop stubbles) to adult sheep unless you have a WormTest indicating that worm egg count exceeds 200 epg.
      • Give an autumn drench for adult sheep in early April. This allows minor, but sufficient, pasture contamination in March from worms that have not recently been exposed to a drench.
      • Ensure that sheep that did receive a summer drench (weaners and hoggets) are rotated through pastures occupied by adult sheep that did not receive a first summer drench.
      • Worm egg counts can be taken to check the need for an autumn drench. Although most flocks will warrant treatment to prevent excessive pasture contamination, a drench may be unnecessary in some flocks.   
  2. In regions where scour worms are dominant (including Mediterranean climates)
    1. When drenching and planning to move adult sheep onto a low worm-risk paddock
      • Move these sheep onto the paddock one week before their drench is given. (This allows some pasture contamination by less-resistant worms, to provide a small population in refugia, to offset the selection pressure of the intended drench.)
    2. In mobs given summer drenches or mobs that have been drenched and run on low worm-risk pastures since then
      • Move these mobs to paddocks that have been occupied by wormy sheep (undrenched or drenched with a different drench type). (This allows pick-up of less-resistant worms to dilute resistant worms surviving the previous drenches.)
    3. Prepared ‘low worm-risk’ paddocks, once their benefit has been obtained or pastures that carried young sheep that had been summer drenched
      • Move wormy mobs (that were undrenched or were drenched with a different active to the sheep that used the paddock) onto these pastures once their worm-free benefit is no longer required. (This provides some pasture contamination with less-resistant worm, creating a worm population in refugia.)
    4. Consider a targeted treatment* strategy
      • Deliberately leave at least 10% (and ideally 15%–20% or more*) undrenched when a treatment is given to the mob so they continue to deposit eggs from worms not exposed to the drench. This dilutes eggs from resistant worms as the mob grazes; so planned paddock moves to ensure dilution is not necessary.
      • Use this in adult sheep only, not in lambs or weaners.
      • Sheep left undrenched should be those in good body condition score (Score 3 or more).
      • Do not use this strategy where there is a concurrent Barber’s Pole worm risk.

*Targeted treatment (or ‘targeted selective treatment’) research has been completed in Western Australia and western Victoria, and has been shown to be effective and safe. It has been used successfully by some producers for many years and reportedly has led to a significant reduction in the development of drench resistance. Research has not long been completed and the details of finding the best proportion of the mob to leave in various situations, as well as rules indicating which sheep can miss the drench will be released through WormBoss in early 2016.


Each of the WormBoss programs includes recommendations on how to use refugia to assist in the management of drench resistance.

Drench resistance can also be managed in a number of other ways. Click here for more information.