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Resistance of various parasites to different antiparasitic drugs by the parasites infecting sheep is not a contemporary problem but recorded more in recent times 1.In 1964, Drudge and co-workers first documented thiabendazole resistance 2.Thereafter, several authors reported benzimidazole resistance from various parts of the world.At the molecular level, resistance to benzimidazole is linked primarily to the mutation in three amino acids at position F167Y, E198A, and F200Y in the gene β-tubulin isotype 1 3,4,5.The mutations in the β-tubulin gene were reported in Teladorsagia circumcincta 5,6, Ascaris lumbricoides at position 167 7, and 200 8 from Brazil, Haemonchus contortus at position 198 and 200 from India 9, 10, 11 and at position 198 and 200 in China 12, Bunostomum trigonocephalum at position 200 13, Trichuris trichiura and Necator americanus from Southern Mozambique 14.Mutation at these positions alters the binding pocket formed by the β-tubulin for the benzimidazole drugs, thus reducing the binding affinity of the drug 15,16.Robinson et al. (2004) gave a possible explanation of the role of phenylalanine residue at position 200 interacting with albendazole based on the molecular docking study 17.The substitution of tyrosine for
K.A. et al. (Fri,) studied this question.