The current study was aimed at exploring the activity of combined crude aqueous methanol extracts (CAMEs) of Nicotiana (N.) tabacum leaves and Azadirachta (A.) indica seed kernels to control levamisole (LEV) and ivermectin (IVM)-resistant Haemonchus (H.) contortus in sheep. The in vivo fecal egg count reduction test (FECRT) demonstrated rampancy of resistance among H. contortus populations against LEV and IVM. There was a significant difference (P<0.05) between groups treated with the combined plant extracts compared to other groups treated with solitary N. tabacum and A. indica. The recorded FECR% at low (1+1g kg^-1 BW) and high (2+2g kg^-1 BW) doses was 52.71 and 94.59, respectively for the combined extracts, whilst for N. tabacum at low (2g kg^-1  BW) and high (4g kg^-1 BW) doses, the FECR% was 87.5 and 88.6, respectively. Moreover, the FECR% for A. indica was 45.62 and 85.14 at low (2g kg^-1 BW) and high (4g kg^-1 BW) doses, respectively. There was a dose and time dependent ovicidal and adulticidal activity of CAMEs with estimated LC₅₀ values of 0.523, 0.566 and 1.169 μg ml^-1 for the combined extracts, solitary N. tabacum and A. indica, respectively in egg hatch assay (in vitro test). The estimated LC₅₀ values in adult motility test (in vitro test) 10 hours post-exposure were 0.17, 0.20 and 0.80 mg ml^-1 for the combined extracts, sole N. tabacum and A. indica, respectively. Accordingly, the aforementioned combined extracts at high dose were more efficacious than solitary N. tabacum and A. indica and, thus, can be used to restrict rampancy of LEV and IVM-resistant H. contortus populations parasitized in sheep.

Key words: Antinematicidal resistance, Combined plant extracts, Haemonchus contortus, Ivermectin, Levamisole, Sheep