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Western Australia worms, flies and lice update - November 2019

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

Worms are on the slide across the state, and with harvest in full swing, many sheep will be moving onto crop stubbles. That will end the immediate worm-risk, but it’s important to make sure that drenches given in summer don’t increase the level of drench resistance.

Apologies to people who’ve heard it all before, but summer drenching is a two-edged sword. It can give great worm control because few worms survive on pasture to cause trouble next year. But as we’ve known for a long time, those few worms are resistant to drenches and become a major source of the farm’s worm population in the new year. The resistance level can rapidly increase after just a few years, so summer drenches—and other treatments—will no longer be as effective.

To expand on previous discussions, the main aims of summer drenches are:

  • Prevent immediate worm problems, and any reduction in growth rates of young sheep;
  • Ensure worm egg output onto pasture remains low into next autumn, to reduce winter worm populations;
  • Keep drench resistant worm numbers as low as possible.

Research over many years shows that we can achieve all these aims by concentrating worm control efforts on lambs and hogget-age sheep, with drench resistance management tactics reserved for the more worm-tolerant adult sheep.

Young sheep (orange and white ear tags):

Drench young sheep as they go onto crop stubbles, or a couple of weeks after the pasture dries off. In areas lucky enough to have perennial pastures, wait until the pasture gets as dry as it will in summer, but monitor worm egg counts to make sure worms don’t reach damaging levels before then. This is especially a risk with barber’s pole worm, which thrives on summer-green pastures.

The main exception to this recommendation is where lambs have been weaned onto low-worm paddocks (forage crops, cattle paddocks)—a worm egg count will quickly show whether a summer drench is necessary. Similarly, if you suspect hoggets have low worm burdens, check if the drench is needed.

Adult sheep

Worm burdens are usually low in these sheep, and even where they are routinely drenched into stubble paddocks, a worm egg count often shows it wasn’t warranted. The main exceptions are in late-lambing ewes that were wormy while lactating, and sometimes maiden ewes that haven’t developed their full worm immunity.

Strategies to manage drench resistance are based on ensuring the survival of some worms that have not been exposed to a drench. Their job is to dilute any resistant worms that survive on the farm (mostly after drenching the young sheep). There are 2 main options:

  • Drench adult sheep in autumn (early April at latest), not in summer. Although worm counts are generally low in early summer, they often rise as immature worms develop, to reach significant burdens by early autumn. These should be removed well before the season’s break kicks off the new worm-year.
  • Drench adult sheep as they go onto stubbles or dry pastures, but leave 10–20% of the mob undrenched. There should be plenty of ewes in good body condition (3.5 plus), and as these won’t be greatly affected by worms, not drenching them won’t reduce flock production. (Better still: do a worm egg count on a few mobs, and chances are that some will be so low that no drench at all is justified. In that case, check again in early autumn, to show whether counts have increased.)

Both strategies will reduce drench resistance development, so the choice can be based on the best fit with management programs. For early lambing ewes, an autumn drench could be given when they are scanned for pregnancy, or for later-lambing mobs when ewes are yarded to remove the rams.

The bottom line is, don’t drench all sheep on the farm in summer—the short term worm control benefit will be undone by the longer term increase in drench resistance.

What drench?

It’s absolutely critical that all drenches in summer and autumn are given with a product as close to 100% effective as possible. It’s simply a numbers game: the more resistant worms that survive, the more rapid the development of resistance.

If you don’t know how effective a summer drench has been—and resistance will develop one day to even the newest types—a worm egg count 14 days after drenching will tell the story. Either it’s a zero count, or very close, or you’ll need to re-drench (having learnt a valuable lesson regarding the effectiveness of the first drench).

All these recommendations and more are available on the WormBoss website, plus a complete run-down on the current drench options.

Esperance: Nicole Swan, Swan’s Veterinary Services (

Faecal worm egg counts (WormTest) in general have been low. A number of properties have been testing undrenched lambs to see if they have counts high enough to conduct a drench resistance test (DrenchTest). In all cases, counts have been too low.

A recent count of 465 eggs per gram (epg) was sent for larval culture. This mob had 24% barber’s pole. To date, we have not seen any clinical cases of barber’s pole this spring/summer, but with the recent rain and warmer weather, barber’s pole numbers could increase in the next few weeks.

As stubbles become available, farmers should monitor to see if a first summer drench is required. If so, this drench should be an effective product of at least 98% efficacy. If a drench resistance test has not been conducted recently, then it is recommended to use Startect© or Zolvix© combinations, as to date there has been no reported resistance to these drenches. As a general rule, lambs and hoggets should receive a first summer drench. If not drenched at this time, these classes of sheep usually have a significant burden at the end of summer.

If an adult mob requires drenching, it is important to leave a small (2–5%) number of sheep undrenched. This is especially important if most mobs on the farm are being drenched. Leaving a small number of sheep undrenched will ensure that if any worms survive in the drenched sheep, the eggs from these resistant worms will be diluted out on pasture by the mixed population of eggs from worms left in the undrenched sheep. This concept is called refugia. This ensures that we are not left with a population of only super resistant worms on the farm, against which no drench will be effective. In essence, leaving some sheep undrenched will slow down the progression of drench resistance.

If stubbles are not available, the same principles apply once pastures have dried off. If sheep are grazing perennial (green) pastures over the summer then regular monitoring especially 3–4 weeks following a rain event, is recommended.

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