A survey on resistance to anthelmintics in sheep stud farms of southern Brazil.

A survey to investigate the status of anthelmintic resistance in 29 sheep studs in southern Brazil was conducted from March 1992 to December 1993. Compounds from three drug families (macrocyclic lactone, levamisole and benzimidazole) were evaluated concurrently on 22 of the 29 studs. On seven of these properties, resistance to all three families was declared or suspected; at 15 of the 22 studs, ivermectin was the only compound found to be effective in reducing faecal egg counts. Resistance to levamisole was detected on 22 of the 23 studs where it was evaluated and was suspected in the remaining one. The position of benzimidazoles was similar, resistance being declared or suspected on all 28 studs where they were tested. Results of larval cultures indicated that Trichostrongylus, Ostertagia and Haemonchus were the most prevalent nematode genera in the survey, with Trichostrongylus and Haemonchus being the genera associated with anthelmintic resistance.