Synergistic larvicidal and repellent effects of essential oils of three Origanum species on Rhipicephalus annulatus tick

Ticks are of great economic importance worldwide, both because they represent major obstacles to livestock productivity and because of their ability to transmit diseases to humans and animals. Although synthetic acaricides are the most common method for tick control, their overuse has led to the development of resistance as well as unacceptable residual levels in animal products and in the environment in general. There is therefore an urgent need to identify alternative treatments. Among such alternative approaches for tick control is plant essential oil (EO) therapy. In the present study, we investigated the synergistic effect of EOs of three oregano species—Origanum onites, O. majorana and O. minutiflorum—against Rhipicephalus annulatus larvae. Gas chromatography–mass spectrometry profiles of the three EOs revealed that carvacrol was their major component, with a concentration of 86.2% in O. majorana, 79.1% in O. minutiflorum and 77.4% in O. onites. The results of larvicidal assays revealed that the doses that lead to the death of 50% of the ticks (LC₅₀) were 22.99, 25.08 and 27.06 µL/mL for O. majorana, O. minutiflorum and O. onites EOs, respectively, whereas the doses that lead to the death of 99% (LC₉₉) were 41.26, 43.62 and 48.96 µL/mL. In addition, the LC₅₀ and LC₉₉ of the three oils combined was lower (viz., 4.01 and 6.97 µL/mL) than that of each oil alone. The tested EOs were also able to repel larvae of R. annulatus to varying degrees, with O. onites oil exhibiting the greatest repellent effect, as shown by the lowest RC₅₀ dose, followed by O. minutiflorum and O. majorana. Interestingly, this means that the oil that was least effective in killing the larvae was the most effective in repelling them. The calculated synergistic factor of any combination was higher than 1 which means that combinations have a synergistic effect. In conclusion, the combination of all three oils showed higher toxic and repellent activities than either oil separately or combinations of any two oils, suggesting synergistic effects with low doses. Further studies including field trials and the establishment of the mode of action and side effects are urgently needed to expand on these findings, and other tick stages such as adults should also be tested.

     

D-limonene nanoemulsion: lousicidal activity,stability, and effect on the cuticle of Columbicola columbae

The current study was conducted to investigate the efficacy and stability of D-limonene (DL) and its nanoemulsion (DLN) against pigeon feather lice (Columbicola columbae) and their mode of action. DL pure form and DLN were prepared and characterized freshly and after storage for 50 days. In vitro bioassay on live lice was conducted with different concentrations of DL, DLN, and deltamethrin (DM). The results revealed significant mortality rates in the DL-, DLN-, DM-treated groups when compared with the control (p < 0.05). The scanning electron micrographs of lice treated with DL and DLN revealed collapsed bodies with destruction in the cuticle of the mouthparts and damaged antennae. The 50 days stored DLN showed stability in their effectiveness when compared with the freshly prepared formulation. DL and DLN caused significant inhibition (p ≤ 0.05) in acetylcholinesterase activity (AchE). Malondialdehyde level (MDA) was significantly increased while glutathione was significantly decreased in DL- and DLN-treated lice. In conclusion, DL and DLN have significant lousicidal activities. DLN showed better stability than DL after storage for 50 days. In addition, the mode of action of DL may associate with its effect on the cuticle of the lice body, inhibition of AchE, and increasing oxidative stress in the treated lice.