Benzimidazoles in a Wormy World

Abstract

Anthelmintics are the only weapons available to combat parasite menace in livestock. In fact, even if with the availability of any alternative control measures in future, viz., vaccines, biological control, resistant hosts etc., anthelmintics will definitely be playing its role in the so called “Sustainable Parasite Control Programme.” The black side of this story is the emergence of resistance to three major classes of anthelmintics, viz., levamisole, rafoxanide and benzimidazole. This is particularly important at present in small ruminants. Although faced with this obstacle it is important to remember that the majority of the anthelmintics are still efficacious in most livestock, particularly in large ruminants where only one report of anthelmintic resistance is reported so far from North India. One should not be complacent with this fact, but look at it from a different angle that the problem is emerging and looms large over large ruminants. Further, discovery of a new class of anthelmintics is very time and capital intensive and it is expected that a new drug with a different mode of action will only be available in the next decade. Therefore, the present-day chemicals will continue to be used for parasite control.
Benzimidazoles represent the only class of truly broad spectrum anthelmintics. With the discovery of thiabendazole in 1961 general pattern of benzimidazole as a class of low doses broad spectrum antheminthic with a high therapeutic index was established. The subsequent cascade of patents within the next 25 years led to the experimental or commercial development of further 15 benzimidazoles and benzimidazole prodrugs. The present presentation deals more closely with chemistry, mode of action, metabolism, veterinary uses and emergence of resistance in widely used benzimidazole anthelmintics. This is very much required because the present-day attitude of parasite control is to adopt technologies which maximise the drug efficacy and operations which compromise efficacy, be avoided. While repeated exposure of a parasite to anthelmintic must ultimately contribute to the development of resistance, it is essential to give the drug the best chance to work by taking actions which could prolong the effective life of these valuable resources.