Steve Love, State Coordinator – Internal Parasites, IandD NSW Primary Industries, Armidale

Recently I wrote about the first confirmed case of Zolvix (monepantel) resistance (Vet Talk, The Land, September 2013). This occurred on a goat farm in New Zealand and was published earlier this year, about 4 years after the world-launch of this new drench active in that country. There are reports of monepantel resistance on a further two goat farms in NZ.

Here I will discuss the matter a little further.

Goats generally metabolise (process and excrete) drenches (anthelmintics/wormers) faster than sheep, and this may be one reason why resistance often seems to develop faster on goat farms.

In the reported case, the efficacy of monepantel against small brown stomach worm and black scour worm in goats - and sheep – on the farm went to zero in less than two years. During this time the drench was used 17 times, but not on all animals on every occasion, perhaps a conscious effort on the producer’s part to maintain some worms in refugia i.e. worms not exposed to the drench and therefore not selected for resistance.

Has resistance to monepantel occurred earlier than it should have? I am not sure anyone yet knows what the mode of inheritance of monepantel resistance (e.g. dominant or recessive genes) is in worms – and it may be different in different worms - so that remains a moot point.

What has been the track record with other anthelmintics? According to parasitologist Professor Ray Kaplan, the time from initial drug release to the first reports in scientific journals of resistance to various drenches in sheep worms  were as follows: thiabendazole – 3 years; levamisole – 9 years, ivermectin – 7 years (4 years in Australia); moxidectin – 4 years (8-9 years in Australia).

However it is not so much about time as about how often a drench is used and the worms in refugia. In Western Australia for example, resistance of brown stomach worm to ivermectin was found after just 4 years, possibly due to few worms in refugia. Drenching occurred on  some farms only twice a year. Resistance of barber’s pole worm to ivermectin in northern NSW – where drenching is more frequent, but there are more worms in refugia? - happened just as quickly. However, the ‘ivermectin resistance’ gene in the case of barber’s pole worm is dominant, so one expects resistance to happen faster than if the gene was recessive.

Focussing now on management rather than the drench, I am wondering, for example, why the owner of the NZ goat farm did not know the drench was failing until it was completely dead. At this stage there is little publicly available detail on management aspects of this case, so I can only make educated guesses.

In particular, were drenching decisions based on results of regular worm egg count (WEC) monitoring, and was a DrenchCheck (WEC 10 days after drenching) done from time to time to check on drench efficacy?

Lest any are tempted to feel smug at this stage, a small minority of goat or sheep producers do these things, both of which are key elements of good worm control.

What about drench rotation, or better, using unrelated drenches concurrently i.e. in combination? This goat owner had exhausted all options, including ‘mectin’-based triple active drenches, but still had the new drenches Zolvix (monepantel) and Startect (derquantel + abamectin) up his (or her) sleeve(s).

Leaving aside the fact that neither are registered for use in goats, I am wondering why Zolvix was not used in rotation with Startect (not yet available in Australia, by the way) or why the two were not used concurrently each time the animals were drenched. Right now on this farm, Startect is the only drench left standing, and with the demise of ‘mectins’ on this property, the new active in Startect, derquantel, will get precious little support or protection from the abamectin component.

If used, how long will Startect last on this farm?   It took about a quarter of a century – after the release of the ‘mectins’ – to get the new drench families: the AADs (aminoacetonitrile derivatives, represented by monepantel) and the spiroindoles (represented by derquantel). Will they too in typically short time go the way of all drenches?

As to other elements of integrated worm control, I have no idea whether nutrition or grazing management or animal genetics were used to help with worm control.

Regarding biosecurity, were there good quarantine procedures to keep resistant worms out? Possibly these multi - / super – resistant worms were bred entirely on this farm but who knows?  And will these worms – by whatever means – end up on other farms?

The current advice in Australia is to treat sheep brought onto a farm with four unrelated drench actives, one of which is Zolvix. When and if we get Startect in Australia, I guess the recommendation will be Zolvix used concurrently with Startect along with at least one other unrelated broad-spectrum active.

Unfortunately this NZ story has a familiar ring to it.  The first published report of moxidectin- resistant sheep worms in Australia was in 2003 and related to barber’s pole worm on a property in northern NSW. Ivermectin was used once only and thereafter moxidectin was used exclusively up to 4-7 times per year. Several years later there were control failures (clinical haemonchosis), after which the owner sought advice.  The efficacy of moxidectin was 67 to 83% (tests on different occasions), abamectin 19% and ivermectin zero.

In case you think I am Kiwi-bashing here, I am sure we Australians – and others - are equally adept at breeding drug-resistant worms. At the very least, the dominance of the All Blacks rugby team saves me from over-weening national pride.

Hope springs eternal: perhaps we can do better.

WormBoss (wormboss.com.au) has excellent information (authoritative but user-friendly) on all the points discussed in this article (apart from how to play rugby).

Reference: Kaplan RM. Trends in Parasitology. 2004.