Comparative Anthelmintic Efficacy of Thiabendazole and Mixtures of Phenothiazine and Phenzidole

Abstract

A RECENT report¹ claimed that a mixture of phenothiazine and phenzidole provided synergistic anthelmintic action on gastro-intestinal parasites in sheep. Previous claims of synergistic action for anthelmintics have been questioned by Gordon², who indicated that true synergism has not been demonstrated for anthelmintics. Work was conducted in our laboratories to evaluate the relative anthelmintic potencies of phenothiazine, phenzidole and thiabendazole and to determine whether synergistic action was demonstrable for combinations of phenothiazine and phenzidole. 130 penned sheep with natural infections of gastro-intestinal worms were assigned to control or medicated groups (5 sheep/replicate) on the basis of pre-treatment egg counts. The compounds were tested as drench suspensions in single doses at graded dose rates. Anthelmintic efficacy was assessed in critical tests by comparing the numbers of worms in treated and control sheep at slaughter one week after treatment.