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Summer drenching in Mediterranean climate regions: two paths to sustainability

By Brown Besier, Veterinary Parasitologist

December 2017

The “summer drenching” program has long been a mainstay of sheep worm control in winter rainfall regions. However, in areas where summers are very hot and dry, it is also a major causal factor for drench resistance, with a consequent reduction in the long-term effectiveness of worm control.

Many graziers in Western Australia now drench their adult ewes in early autumn, rather than in early summer, to reduce the resistance risk. Aside from this change, research has also confirmed that if summer drenching is continued, then it is more sustainable if some sheep are left undrenched. Provided these two strategies are used only for adult sheep, both will still provide effective worm control.

Summer drenches, good and bad

Drenching sheep (and goats) in summer is the basis of effective worm control in all winter rainfall regions in Australia, as fewer worm eggs are passed onto the pasture in autumn, which reduces winter worm problems.

However, it has long been known that summer drenching can speed the development of drench resistance. Where there is close to a total die-off of worm larvae on the pasture over summer, such as in Mediterranean climatic zones or on cereal crop stubbles after harvest, any worms surviving drenches are drench-resistant, and are the main source of the worm population in the following months.

This explains the historic development of widespread resistance to most drench groups in these environments, even where relatively few drenches are given each year.  Unless there is a change in drenching policy, the level of resistance increases rapidly as larger numbers of resistant worms survive successive treatments.


“Refugia” to the rescue?

The key to managing drench resistance is to keep resistant worms to a low proportion of the total population. This involves diluting them among a worm population with fewer resistant worms that have been kept “in refugia” from drenches, whenever treatments are given.

“In refugia” (in refuge from) means they have not been exposed to a recent drench—worms are either in the egg or larval phase on the pasture or are in sheep that have not been drenched since those worms were ingested as larvae.

This occurs naturally for much of the year because worm larvae (which sheep pick up as they graze) survive on the pasture for many weeks (aummer) or months during cooler seasons, and can re-infect sheep soon after drenching. In southern parts of the winter rainfall regions—typically where green feed persists—some larvae may survive on pastures year-round, including through summer. The development of drench resistance potentially occurs more slowly in these zones, provided that the number of treatments is kept to the minimum.

In regions with harsher summer periods, worm larvae die on the pasture over summer, and active planning is needed to maintain sufficient worms in refugia to reduce the selection pressure for worm resistance from summer treatments without jeopardising worm control.

Sustainable programs—two options

At the outset, please note that these strategies are designed specifically for adult sheep, usually ewes. Summer drenching is still recommended for the two youngest year-groups on the property (lambs and hoggets), as they are more susceptible to worms and need to grow without any potential impediment. The strategies are also specifically for regions where barber’s pole worm is not a significant problem.

Recent research has led to two main strategies for sustainable worm control. These are aimed at adult sheep mobs that were last drenched back in winter or spring—a time when there are higher numbers of infective larvae on pasture—so any worms they carry will be less drench-resistant than worm populations remaining after a summer treatment.

  • Drench in autumn, not summer: Shifting the annual treatment to early autumn allows worms to remain in the adult sheep (with little ill-effect) over summer. While few, if any, eggs develop to larvae over the hot summer, in early March seasonal conditions will generally become favourable for a small percentage of worm eggs to develop to the larval stage. This reinfects the pastures with a population of worms that contains many more drench-susceptible worms than if the sheep had been drenched in summer. As the aim is to keep worm burdens to a minimum before the season’s break, a drench should be given strategically between mid-March and mid-April unless a WormTest shows the sheep have very low counts.  Many sheep farmers in WA have used this strategy for some years, with no reported problems.
  • Leave a percentage of sheep untreated when a summer drench is given.  Sheep in good body condition (3.5 +) are rarely affected by the small burdens of worms typically seen in summer, and can be safely left undrenched. Drenching the rest of the mob reduces the overall worm egg output, and makes sure that any poorer-condition sheep don’t suffer from worms. (This strategy is known as “targeted treatment”, and similar strategies have been developed in several countries, including New Zealand, the UK and parts of Europe.)

The choice of strategy depends on the convenience for sheep management, such as when sheep will be in the yards for other reasons, or when the time and labour for drenching is available. In both cases, mobs that are completely or partly undrenched will carry far fewer resistant worms than mobs or individuals that received a summer drench.

These undrenched or partly drenched mobs should be moved around the property over autumn and winter. The less-resistant worm population that develops from these sheep will dilute the high proportion of resistant worms—albeit a smaller number—that develop from the drenched mobs or individuals.

In a nutshell, when, where and who?

Some basic rules for implementing autumn drenching or targeted treatment will greatly slow the development of drench resistance and ensure sustainability without reducing worm control effectiveness:

Where: Winter rainfall regions with hot dry summers, especially Western Australia, South Australia and other areas where worm larvae don’t survive over summer. Most will be major cereal crop areas where sheep are often drenched onto the stubbles in early summer. These strategies are not recommended where barber’s pole worm is a significant threat.

Sheep class: Adults (older than 2 years/4-tooths +); not lambs or hoggets.

Sheep body condition (sheep left undrenched in summer): Body condition score 3.5 or more. They are easily identified when moving along a race, and the required number (see below) is left undrenched.

When to “Autumn drench”: Either drench all adult sheep in the mob from mid-March to mid-April, or refine this by checking worm egg counts and then only drench mobs with an average of more than 100 eggs per gram (but this is not usual except in very dry areas).

When to use “Targeted treatment”: Drench adult mobs in summer, but leave between 10% and 50% undrenched:

  • When a WormTest has not been carried out:
    • If the average body condition score is about 2.75, leave 10% undrenched.
    • If the average body condition score is about 3.0 or more, leave 20% undrenched.
  • When a WormTest has been carried out:
    • If the mob average WEC is less than 50 epg AND the average body condition score is 3.0 or more, then leave 50% undrenched.
    • If the mob average WEC is greater than 50 epg AND the average body condition score is about 2.75, then leave 10% undrenched.
    • If the mob average WEC is greater than 50 epg AND the average body condition score is about 3.0 or more, then leave 20% undrenched.