The first case in the UK of anthelmintic resistance to the group four sheep wormer Zolvix has been reported. Zolvix, an orange-coloured drench, is the only group four wormer available to sheep farmers.

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 the problem with Zolvix was reported had resistance to group one, two and three wormers and was relying almost exclusively on the group four product.

Clean pasture

The sheep were also put on to clean pasture with a low worm burden, so Zolvix resistant worms were passed on to 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 only group five (purple) wormer Startect.

Advice published by the Sustainable Control of Parasites In Sheep (SCOPS) 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.

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