Abstract:
Malaria, a tropical disease caused by Plasmodium parasites and spread by the bites of
infected female Anopheles mosquitoes, remains a major public health issue. Mosquito
repellents are used as a control measure.Existing synthetic repellents present challengesof
high insecticide resistance, allergic reactions, and non-biodegradable residues. This has
necessitated the search for alternative repellent compounds from natural sources.This
study was designed to evaluate the repellent activity of selected ethnomedicinal plants
against wild adult female Anopheles gambiae (s.l.), and characterise isolated bioactive
compounds.
Dry powdered plant parts of Citrus limon Linn. (seeds), Citrus paradisi Macf. (seeds),
Citrus sinensisL.(seeds), Jatropha curcas L.(seeds), Dennettia tripetala G. (fruits), and
Afromomum melegueta K. (fruits) were successively extracted with n-hexane (Hex), ethyl
acetate (EA) and methanol by cold maceration. Phytochemical screening of the extracts
was done using standard methods. The extracts were screened against A. gambiae for their
repellent activity using human bait method, whileN,N-diethyl-meta-toluamide(DEET) and
acetone were used as positive and negative controls, respectively. Extracts with the
highest percentage repellency, calculated using standard formula, were subjected to
bioassay-guided fractionation and separation using chromatographic techniques. Isolated
compounds were evaluated for cytotoxicity using brine shrimp lethality assay (safety cutoff, LC50> 100µg/mL), and characterised by spectroscopic (IR, 1D and 2D NMR) and
mass spectrometric analyses. Repellent activity data were analysed using one-way
ANOVA at α0.05.
Extraction of the plant materials yielded eighteen extracts. Phytochemical screening of the
extracts revealed the presence of alkaloids, flavonoids, tannins, glycoside, saponins, and
anthraquinones. The screened extracts, tested at 1.5, 2.5, and 5mg/mL, gave percentage
repellency of ≤ 68.1, ≤ 74.9, and ≤ 97.5%, respectively (DEET, 100%). At 5 mg/mL, the
percentage repellency of the three most active extracts (C. limon (Hex, 97.5%), D.
tripetala (EA, 87.7%), and C. limon (methanol, 86.8%) were not significantly different
(α˂0.05). Citrus limon (Hex) fractionation yielded 10 fractions with percentage repellency
of ≤ 89.5 %, whileD. tripetala (EA) yielded eleven fractions which were pooled into four
(DTH1, 55.7%; DTH2, 60.6%; DTEA, 77.2%, and DTM, 71.1%). n-Hexane-100% and
Hex/EA (9:1) fractions of C. limon, DTEA and DTM fractions of D. tripetala showed the
most repellency effects of 89.5%, 71.1%, 77.2% and 71.1%, respectively.
Chromatographic purification of n-hexane-100%, Hex/EA (9:1) and DTEA gave seven
compounds. They were identified as palmitic acid (a), 14-oxotricosanoic acid (b),n-octyl
stearate(c), 15-(heptanoyloxy) pentadec-9-enoic acid (d), 6,8-dimethoxy-3-undecyl-1H-
[2]benzopyran-1-one (e), 1,2,3-propanetriyl tris(5-eicosenoate) (f), and α-linoleic acid (g)
with percentage repellency of 65.9, 63.6, 51.7, 77.9, 75.2, 37.0, and 57.3%, respectively.
Cytotoxicity evaluation of compounds a-e gaveLC50ranging from 63.2 to 171.1 µg/mL,
with compounds c (106.1 µg/mL) and e (171.1 µg/mL) being adjudged safe. Compoundd
showed the highest repellent activity against A. gambiae mosquito.
Extracts of Citrus limon and Dennettia tripetalacontain potentially useful mosquito
repellent compounds. n-Octyl stearate and 6,8-dimethoxy-3-undecyl-1H-[2]benzopyran-1-
one (both from C. limon) may serve as leads for the development of novel bio-degradable
and safe mosquito repellent compounds.
