Repellence of essential oil of nigella sativa l. seeds against anopheles Gambiae and identification of the active blend
Ndirangu, Githui Ephantus
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Anopheles gambiae mosquitoes are vectors of malaria because of their ability to transmit Plasmodium falciparum parasites. The major impact of malaria is in subSaharan Africa where at least 90% of the deaths from malaria occur. In Kenya, malaria accounts for 30% of all outpatients and 19% of all admissions to health facilities. Malaria can also affect the quality of labour negatively and also can lead to low productivity through absenteeism. Anopheles gambiae is the vector associated with stable malaria transmission in Africa because it is strongly anthropophilic, feeding exclusively on humans. One of the greatest challenges facing malaria control is the spread and intensification of parasite resistance to treatment. P. falciparum has become resistant to almost all malaria drugs including artemisinin and its derivatives. This means that there is need to come up with effective methods to control mosquito populations as well as diversifying methods of malaria treatment. There is no single mosquito control method which is effective in all situations. Today, the most effective insect repellent is DEET but it has been associated with medical complications when used for a long time. In many parts of the world plant-derived natural products have been used to repel mosquitoes and other insects. The objective of this study was to evaluate the repellence of the essential oil on Nigella sativa L. seeds using An. gambiae and identify the active constituents and blend. Nigella sativa L. seeds were ground and hydro-distilled. Then bioassays of essential oil was conducted on human subjects against newly emerged female An. gambiae using DEET as the positive control. It was noted that the repellence (98.81±1.19 and 100.00±0.00 at concentration of 0.01g/ml and 0.1g/ml respectively) of the essential oil against An. gambiae was comparable to that of DEET (100.00±0.00 and 100.00±0.00 at concentration of 0.01g/ml and 0.1g/ml respectively) at higher doses; however, it showed lower repellence (36.97±1.81 and 50.41±2.87 against 51.11±13.32 and 86.22±4.51 of DEET at concentration of 0.0001g/ml and 0.0001g/ml respectively) at lower doses. GC-MS and GC-EAD (Gas Chromatography-linked Electro Antennography) analyses of the essential oil led to the identification of eight bioactive constituents namely α-thujene (19), longifolene (38), 1, 2, 3, 4, 5-pentamethylcyclopentane (18), α-pinene (20), βpinene (22), tetradecane (24), p-cymene (11), and α-longipinine (37). Subtractive bioassays to characterize the constituents that contributed most to the repellence of the oil was then carried out. The most repellent blend was found to contain (+)-β-pinene (41), (-)-β-pinene (42), (+)-α-pinene (39), (-)-α-pinene (40), α-longipinene (37), tetradecane (24) and 1,2,3,4,5 pentamethylcyclopentane (18) (RD75 = 3.763), though less repellent than DEET (RD75=1.630). Bioassay of pure (+)-α-pinene (39) and (-)-αpinene (40) showed that (+)-α-pinene (39) was a better repellent than (-)-α-pinene (40). More studies need to be undertaken on the essential oil of N. sativa seeds to determine the optical stereo-chemistry of the α-pinene (20) and β-pinene (22) and also establish whether α-thujene (19) and longifolene (38) contribute to repellency or not against An. gambiae. These results form the basis of downstream development of the appropriate blends for personal protection against An. gambiae.