Production on sheep farms in New Zealand is reliant on the effective management of roundworms, with effective drenches, or anthelmintics, playing a key role in controlling these parasites.

Resistance by roundworms to drenches poses a significant threat to the ongoing sustainability of sheep farming operations and welfare of livestock. Industry figures suggest that drench resistant parasites may be found on more than 60 percent of New Zealand sheep farms¹, and a veterinary study indicates that drench-resistant parasites are found on an increasing number of New Zealand sheep farms, including triple drench (11 percent) and moxidectin (30 percent) resistant roundworms.² A 2017 to 2019 survey by Taihape Veterinary Services found 55 percent of 18 farms tested had triple drench resistant parasites while moxidectin resistance was found on 62 percent of 16 farms.³

Triple drenches and moxidectin based products are cornerstones of many sheep roundworm parasite control programmes. Strategies that help extend the useful life of these older anthelmintics, while they are still effective, should be adopted as soon as possible. This involves the use of a novel anthelmintics such as Zolvix™ Plus from Elanco.

Dr Dave Leathwick of AgResearch, one of the world’s leading parasitologists, studied the use of a fully effective new anthelmintic in a five-drench lamb drenching programme4 via computer modelling. This model simulated roundworm parasite dynamics to predict the development of drench resistance (defined as less than 95 percent efficacy). It tested variations on a traditional five drench preventative programme for lambs in which either none or the first, third or fifth treatment was substituted with a different anthelmintic class.

The results indicated that replacing one of the five treatments with a fully effective different anthelmintic slowed the development of resistance to the older anthelmintic class. Replacing one of the late summer/autumn routine drenches provided the most benefit in terms of slowing resistance to the other products used in the programme (including combinations).

The study concluded that strategic use of a novel drench, such as Zolvix Plus, as a knockout drench has the potential to greatly extend the effective life of existing anthelmintics.

Always refer to the registered label directions before use. Good agricultural practice is essential for optimal worm control.

To learn more about using Zolvix™ Plus as a knockout drench, contact your local PGG Wrightson Technical Field Representative.

¹Anon. (2017). Wormwise Handbook.
²McKenna, P.B. (2018). Update anthelmintic resistance. Vetscript 31(May):44–45.
³Grazing Gazette New Zealand. Official Newsletter of the Sheep and Beef Cattle Veterinarians Branch of the New Zealand Veterinary Association. Issue number 56. December 2019; pp 29-31.
Leathwick, D.M. et al. (2009). Managing anthelmintic resistance: Modelling strategic use of a new anthelmintic class to slow the development of resistance to existing classes. New Zealand Veterinary Journal 57(4):181–192.
Zolvix Plus Broad Spectrum Oral Anthelmintic for sheep and cattle contains 25 g/L monepantel and 2 g/L abamectin.