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Queensland worms, flies and lice update - December 2020

QLD WormBoss Worm Control Programs

QLD WormBoss Drench Decision Guides





Brisbane: Maxine Murphy, Veterinary Parasitologist (

There is an adage that says, to control barber’s pole, drench three weeks after rain.

Given the weather forecasts are for storms over the Darling Downs and Maranoa in the next few days, let’s look at this advice.

The three-week time interval is the time taken for ingested larvae to mature into egg-producing adults. So three weeks after rain is good, but would you drench ‘to be sure’ and hope it will solve the issue, or do you have time to do a WormTest, as the worm burden could be still low? This is where your knowledge of rainfall, evaporation rates, and temperatures on your farm becomes important for you to determine the risk.

It is very difficult on-farm to determine the size of the building worm burden in the live animal, so counting worm eggs passed out with the dung is the easiest way, albeit an indirect way. So should you ‘set and forget’ after just one WormTest, particularly if the result is low?

A WormTest will tell you about the larval intake three weeks ago, plus the size of the already existing adult burden from any previous infestations, but it does not give you any information about the numbers of larvae picked up in the last 20 days. During warm, wet conditions, this could be significant.

The size of the infecting dose of larvae, or the reinfection rate, will determine if the size of the future worm burden will rise, fall or be stable. Overlaying the dose rate is the host’s response to the establishment of larvae in its gut. Some sheep are genetically more resistant to worms than others. Typically, 20% of the mob will carry about 80% of the worm burden, so if you are doing a test to determine if the mob needs to be drenched, sampling from the tail group is essential, as this group will cause the whole mob to be drenched. However, in peak worm season, all members of a mob will probably be infected to some extent and need drenching.

A moderate to high egg count taken during wet, warm conditions will usually necessitate a multi-active short-acting drench, or a long-acting drench if the situation suggests that sheep could become confined on higher ground in a ‘quarantine hotel’ type situation. A short-acting drench will remove most of the current adult population — assuming for the time being that the worms aren’t resistant to the drench — and maybe some of the immature population. So the mob, after a drench, could still have some adult worms and a population of larvae that will mature into adults in the next week or so. A long-acting drench will provide extended protection until sheep can be removed to fresh grazing. Relying on a long-acting product during peak worm infestation without knowing its resistance status on your property is really, very risky.

For producers with small numbers of sheep or goats that are individually managed, the more susceptible ones in the group can be removed to a less contaminated environment and checked regularly with FAMACHA.

So, while one WormTest is important, the follow-up increases the value of the first especially during warm, wet weather. Also, a test taken within 10–14 days of the first will give information about how well the drench killed (DrenchCheck).

For December 2020 state outlooks, please follow the links below:
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