Electrophysiological and behavioral characterization of bioactive compounds of the Thymus vulgaris, Cymbopogon winterianus, Cuminum cyminum and Cinnamomum zeylanicum essential oils against Anopheles gambiae and prospects for their use as bednet treatments

Abstract

Background: Laboratory and field studies showed that repellent, irritant and toxic actions of common public health insecticides reduce human-vector contact and thereby interrupt disease transmission. One of the more effective strategies to reduce disease risk involves the use of long-lasting treated bednets. However, development of insecticide resistance in mosquito populations makes it imperative to find alternatives to these insecticides. Our previous study identified four essential oils as alternatives to pyrethroids: Thymus vulgaris, Cymbopogon winterianus, Cuminum cyminum, Cinnamomum zeylanicum. The objectives of this study were to identify active compounds of these essential oils, to characterize their biological activity, and to examine their potential as a treatment for bednets.

Methods: We evaluated the electrophysiological, behavioural (repellency, irritancy) and toxic effects of the major compounds of these oils against Anopheles gambiae strain 'Kisumu'.

Results: Aldehydes elicited the strongest responses and monoterpenes the weakest responses in electroantennogram (EAG) trials. However, EAG responses did not correlate consistently with results of behavioral assays. In behavioral and toxicity studies, several of the single compounds did exhibit repellency, irritancy or toxicity in An. gambiae; however, the activity of essential oils did not always correlate with activity expected from the major components. On the contrary, the biological activity of essential oils appeared complex, suggesting interactions between individual compounds and the insect under study. Data also indicated that the three effects appeared independent, suggesting that repellency mechanism(s) may differ from mechanisms of irritancy and toxicity.

Conclusions: Based on the bioassays reported here, some of the compounds merit consideration as alternative bednet treatments.

Fig. 1

EAG response of Anopheles gambiae to 17 synthetic compounds of four essential oils. EAG amplitudes (mean ± SE) are control-adjusted and presented as relative response to the standard, 100 μM octanal. Each compound was tested on 28 female mosquitoes at 1 % (v/v) concentration in ethanol

Fig. 2

Repellent effect DEET, permethrin and four essential oils and their compounds on Anopheles gambiae. Response of 4–7-day-old, non-blood-fed, sugar-fed, Kisumu strain of female mosquitos at two different concentrations (C1 and C2 μl/cm² of product on chromatographic papers, refer to Table 1): a. corrected proportion escaping using Sun-Shepard’s formula (confidence interval calculated with the Wald method) by treatment concentration and b. dendrogram determined by hierarchical ascendant classification. 1) Pairwise comparison of proportion was done using Fisher’s test. Values in bold lettering were significantly different from the control with the Holm’s sequential Bonferroni correction method. *Pairwise comparison of proportion was done using Fisher’s test between one compound and the essential that it comes from. Values followed by a star were significantly different from the original essential oil with the Holm’s sequential Bonferroni correction method

Fig. 3

Irritant effect DEET, permethrin and four essential oils and their compounds on Anopheles gambiae. Response of 4–7-day-old, non-blood-fed, sugar-fed, Kisumu strain of female mosquitos at two different concentrations (C1 and C2 μl/cm² of product on chromatographic papers, refer to Table 1): a. corrected proportion escaping using Sun-Shepard’s formula (confidence interval calculated with the Wald method) by treatment concentration and b. dendrogram determined by hierarchical ascendant classification. 1) Pairwise comparison of proportion was done using Fisher’s test. Values in bold lettering were significantly different from the control with the Holm’s sequential Bonferroni correction method. *Pairwise comparison of proportion was done using Fisher’s test between one compound and the essential that it comes from. Values followed by a star were significantly different from the original essential oil with the Holm’s sequential Bonferroni correction method

Fig. 4

Toxic effect DEET, permethrin and four essential oils and their compounds on Anopheles gambiae. Response of 4–7-day-old, non-blood-fed, sugar-fed, Kisumu strain of female mosquitos at two different concentrations (C1 and C2 μl/cm² of product on chromatographic papers, refer to Table 1): a. corrected proportion escaping using Sun-Shepard’s formula (confidence interval calculated with the Wald method) by treatment concentration and b. dendrogram determined by hierarchical ascendant classification. 1) Pairwise comparison of proportion was done using Fisher’s test. Values in bold lettering were significantly different from the control with the Holm’s sequential Bonferroni correction method. *Pairwise comparison of proportion was done using Fisher’s test between one compound and the essential that it comes from. Values followed by a star were significantly different from the original essential oil with the Holm’s sequential Bonferroni correction method