Acaricides can be conveyed to ticks via the blood of their hosts. As fruit and kernel extracts from the Meliaceae family, and, in particular the tetranortriterpenoid azadirachtin (AZA) inhibits tick egg production and embryogenesis in the Ixodidae ticks, we investigated the effects of Neem Azal, an extract containing 43% AZA, given as a feed additive to lambs artificially infested with engorging adult Dermacentor vairiabilis ticks. After tick attachment, the lambs were allotted to three dietary treatments: AZA0 (control, n=10), AZA0.3 (n=5), and AZA0.6 (n=5), with feed containing 0%, 0.3%, and 0.6% AZA on DM basis, respectively. In half of the AZA0 lambs, ticks were sprayed on day 4 after attachment with an ethanol:water:soap emulsion containing 0.6% AZA (AZA0S). In spite of its very pungent odor, the neem extract was well accepted by all but one lamb. No differences were found between treatment groups in liver enzymes in blood, and there was no indication of toxicity. The plasma AZA concentrations after 7 and 14 days of feeding AZA were (4.81 and 4.35 microg/mL) for the AZA0.6 and (3.32 and 1.88 microg/mL) for the AZA0.3 treatments, respectively (P<0.0001). Treatments were not lethal to ticks, but tick weights at detachment were 0.64, 0.56, 0.48, and 0.37 g for ticks from the AZA0, AZA0.3, AZA0S, and AZA0.6 treatments (P<0.04), respectively, suggesting that blood AZA impaired blood-feeding. The highest mortality rate after detachment was for AZA0.6 (P<0.09). As AZA affects embryo development and ticks at the molting stages, we expect that following treatments of hosts for longer periods, one-host ticks will be more affected than the three-host tick D. variabilis.