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First confirmed case of resistance to new drench in Australia - what does it mean?

by Stephen Love

Veterinarian / Parasitologist, NSW DPI Armidale

 A case of resistance to the sheep drench Zolvix® (monepantel (MPL), Novartis) was recently confirmed on a goat farm in NSW. This appears to be the first and only confirmed occurrence in any livestock in Australia. MPL is a brand new type of drench, released in this country in 2010. This case is a good reminder that producers need to focus on best practice worm management so as to keep drenches alive for as long as possible, as well as optimizing productivity and livestock health.

The world launch of MPL was in New Zealand in autumn 2009. Their first cases of resistance to the drench, also on goat farms, were reported in 2013. So far no cases have been reported in sheep.

It’s a long time between drinks when it comes to new drench actives coming onto the market. MPL, which is only registered for use in sheep, was the first entirely new type of drench to be released in Australia since the macrocyclic lactone (ML, mectin) drench family made its appearance in 1988.

In this Australian case of resistance, MPL was still highly effective against barber’s pole worm, but efficacy against black scour worm was declining and very low against brown stomach worm. It is unclear what this case means for the sheep industry, but we know that sheep, goats and alpacas share many of the same worms, so we have to assume these resistant goat worms can successfully transfer to other animals including sheep.

There are two ways of getting resistant worms:  importing someone else’s, or breeding your own.

So, how do you keep resistant worms out? A good quarantine procedure is needed. This includes using MPL as well as at least three other unrelated drench actives as a quarantine treatment. ‘Overkill’? No, because we know for example that there are now sheep worms able to resist even multi-combinations containing 3–4 actives.

On-farm you can slow the development of resistance to worms by using drenches only when required, at the right dose, and as part of a rotation of effective drenches or, even better, by using combinations of unrelated drench actives. To reduce reliance on drenches, also employ strategies to improve the ‘immunity’ of sheep to worms, notably through good nutrition and good genetics, the latter usually being achieved by using rams with favourable breeding values for worm egg count (ASBV WEC).

Reduce exposure to infective larvae on pasture using grazing management, i.e. getting time and nature on your side. Also, aim to keep some worms in refuge (‘in refugia’) by not exposing large proportions of your farm’s worm population to drenches at the same time. Drenching and immediately moving your sheep to cereal stubble, for example, is a ‘good’ way of having few worms in refugia; this gives very good worm control but at the expense of strong selection for drench resistance.

Follow the best program for your area: see ‘Your Program’ in 

Also, regularly monitor worm burdens using worm egg counts (WormTest) and keep tabs on drench efficacy by doing DrenchChecks from time to time: a WormTest 10 days after drenching, in the case of sheep.

For high quality, but easy to read information on all the strategies mentioned, go to It’s an indispensable resource for all who want to optimise sheep worm management.

It would be all too easy to blame the goat industry and this producer in particular for this first Australian case of MPL resistance.  But consider this: resistance was discovered on this farm because the owner does regular WormTesting to monitor worm burdens and to check drench efficacy. These are two of the hallmarks of best practice worm management, the sort of things only done by the top 5–10% of sheep and goat producers.

Others may have missed, ignored, or covered up the problem. This owner however immediately sought professional assistance, not only for his own sake, but with the welfare of the industry in mind as well.

Perhaps, then, the sheep and goat industries have an earlier warning than they otherwise might have had: take advantage of this by improving how we do worm control.