You dosed them with Zolvix?
Mind if I ask why? I thought the general advice was only to use the new sorts of antihelmintics as a quarantine dose. Apologies if the sheep were new and you were quarantining them, but if you weren't, you do realise by bringing Zolvix into general rotation you are helping to select for the minutae of worms that are resistant to Zolvix, and it'll go the same way as the yellow, clear and white drenches, dont you?