The first case in the UK of anthelmintic resistance to the group four sheep wormer Zolvix has been reported. Zolvix is the only group four wormer available to sheep farmers and is an orange coloured drench. It was launched eight years ago and veterinary advice is to only use the product as a quarantine drench for newly bought in stock and as a single-use wormer for lambs in mid-/late season.

It is understood that the farm where Zolvix resistance was reported had resistance to group one, two and three wormers and was relying almost exclusively on the group four product. The sheep were also put onto clean pasture with a low worm burden, so Zolvix-resistant worms were passed onto the pasture and multiplied.

Anthelmintic resistance issues with groups one (white), two (yellow) and three (clear) wormers are common. However, there are no known cases of resistance to the group five (purple) wormer Startect.

Advice published by the Sustainable Control of Parasites In Sheep this week urges sheep farmers to strategically use Zolvix and Startect to maintain populations of worms that are not resistant to wormers in groups one, two and three.

“The two newest wormer groups [four and five] should be incorporated into worm control programmes on all sheep farms, not left on the shelf until the others are no longer effective,” the advice reads.

It is also vital to ensure that the correct dose rate is used, by weighing animals and treating to the heaviest in a group. Calibrating your gun is also important to ensure they are getting the specified amount.