Investigations on the efficacies of anthelmintics are few and anthelmintics have been used without efficacy assessments in most farms in Nigeria. There is a recent interest in this area worldwide as reports of anthelmintic resistance (AR) abound in the literature even to the newest broad-spectrum invermectins. Anthelmintic resistance can be described as a heritable change in the ability of individual parasite to survive the recommended therapeutic dose of an anthelmintic. Anthelmintic resistance is thought to exist when there is a greater frequency of individual within a population able to tolerate a given dose of the anthlmintic than those in the same population that respond to the same treatment. In the present study, the efficacies of three commonly used anthelmintics against gastrointestinal nematodes were assessed in naturally infected sheep in Ijebu-lgbo, Ogun-state, Nigeria.
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