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Monepantel-resistant goat and sheepworms, and drench-resistant cattle worms

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

July 2014

Sadly the first confirmed case of resistance to Zolvix (monepantel (MPL)) in the field has been reported. A case of MPL-resistant worms on a goat farm in New Zealand was published recently, less than four years after the world-launch of this new drench active in the same country.

Scientists Dr Ian Scott and others recently published the report in the journal, Veterinary Parasitology. Tests on the farm showed MPL had zero efficacy against small brown stomach worm and black scour worm in goats and sheep.

All the older broad-spectrum drench families had failed on this property, including triple combination products, so the farmer resorted to MPL, which is not registered for use in goats, using it and it alone 17 times in less than two years until it failed. Towards the end at least, MPL was used at a higher dose rate than that specified for sheep, which is normally required when using sheep drenches in goats.

The only drench that now appears to work on this farm is ‘Startect’, which contains derquantel, the newest drench active on the market. Startect is a combination of derquantel and abamectin, and was released in NZ a year or so after MPL. (Startect is not yet available in Australia). Given worms on this goat farm are resistant to virtually all drenches, including abamectin, it is unclear how long Startect will hold out. I don’t like the odds.

Lest there is a temptation to blame others, the sheep industry historically has been quite adept at breeding its own drug-resistant worms, with or without ‘assistance’ from goat producers. However, as with most drenches, MPL is ‘processed’ faster in goats than sheep, and using higher doses in goats may only partly off-set this. From this Dr Scott and colleagues argue that, although the property in question runs sheep and goats, it is likely that the goats contributed more in this case to the development of resistance to monepantel.

What can be done? Firstly, keep resistant worms out. Use an adequate quarantine procedure. No single drench is adequate. can help you with this.

Secondly, but just as important, try to slow down the emergence of resistance in your own ‘home-bred’ worms.  Consider the following.

Use the right drench at the right time. The right drench?   The one or ones that are shown by testing to be still highly effective on your farm. Use these in rotation, or better, in combination.

Timing? For the most part, this is based on regular worm egg count (WEC) monitoring (WormTesting). There may also be routine drenches, e.g. at weaning, depending on your region. (Check WormBoss).

Drenching unnecessarily results in stronger selection for drench-resistant worms, particularly when there are few worms ‘in refugium’ e.g. during droughts, or after moving sheep onto very clean paddocks, such as cereal stubbles.

Don’t rely on drenches alone. Employ ‘integrated parasite management’ (IPM). This includes improving flock immunity, by means of nutrition and genetics. Also use grazing management (worms just love set-stocking) to reduce worminess of pastures. Again, WormBoss has the good oil on this.

In short, use the three Ws: WEC before you consider drenching, WEC after you drench to see if the drench worked, and use WormBoss.

Are cattle producers sans souci? Drench-resistant cattle worms are an issue too. Surveys in Australia and elsewhere have shown this, and with each day there are more and more field reports of cattle drenches not working well. Begin by checking drench efficacy. Do a WEC on the day you drench or just before, and another 14 days later. As with sheep, consider combination rather than single-active broad-spectrum drenches, but this should be part of ‘IPM’ including, for example, grazing management.