Control of anthelmintic resistant endoparasites in a commercial sheep flock through parasite community replacement

An effort was undertaken to replace a community of sheep endoparasites that had been classified as resistant to levamisole and albendazole with a comm unity of more susceptible parasites using a dilution approach that could be integrated into the management of a commercial hock. For this study, pastu res on this sheep farm were divided into two areas: north and south. Strate gically timed anthelmintic treatments combined with pasture management redu ced to nondetectable levels the endemic community of anthelmintic resistant parasites in this flock and on these pastures by early summer. A group of 102 ewes, lambs, and rams were experimentally infected with third stage lar vae from the more susceptible community of parasites. These sheep then seed ed the south pastures with the new parasite community, while sheep on the n orth pastures maintained the endemic resistant community. Despite its insen sitivity as a technique for detecting anthelmintic resistance, fecal egg co unt reduction tests at the end of the grazing season indicated that the mor e susceptible parasites were present on the south pastures while resistant parasites were present on the north. The following grazing season, similar protocols were used to introduce the more susceptible parasites onto the north pastures. At the end of the grazi ng season, fecal egg count reduction tests indicated that the new community of parasites had become established on both groups of pastures of the farm . (C) 2001 Elsevier Science B.V. All rights reserved.