This study was carried out to assess the efficacy of Nicotiana (N.) tabacum leaves to control benzimidazole-resistant Haemonchus (H.) contortus in sheep. Fecal egg count reduction test (FECRT) and egg hatch assay (EHA) revealed catastrophic levels of resistance in H. contortus of sheep against oxfendazole, a member of benzimidazole group of antinematicidals. Antinematicidal efficacy of N. tabacum leaves was evaluated employing EHA, adult motility test (AMT) and FECRT. There was a dose and time dependent antinematicidal activity of crude aqueous methanol extract (CAME) of N. tabacum leaves with estimated LC₅₀ values of 0.566 and 1.91 mg ml^-1 in EHA and AMT, respectively. There was, however, no significant-difference (P>0.05) in fecal egg count reduction (87.5 vs 88.6%) in sheep at low (2g kg^-1 BW) and high (4g kg^-1 BW) doses of CAME of N. tabacum leaves.  Administration of N. tabacum leaves at low dose (2 g kg^-1 BW) did not exhibit untoward effects in animals; therefore, may be used in sheep harboring antinematicidal- resistant populations of H. contortus.

Key words: Antinematicidal resistance, Benzimidazole, Haemonchus contortus, Plant extract, Sheep, Tobacco