Repellency of Skin-Surface Lipids of Humans to Mosquitoes

Evaluation of the toxicity and repellence of an organic fatty acids mixture (C8910) against insecticide susceptible and resistant strains of the major malaria vector Anopheles funestus Giles (Diptera: Culicidae)

Background

Malaria vector control relies principally on the use of insecticides, especially pyrethroids. Because of the increasing occurrence of insecticide resistance in target vector populations, the development of new insecticides, particularly those with novel modes of action, is particularly important, especially in terms of managing insecticide resistance. The C8910 formulation is a patented mixture of compounds comprising straight-chain octanoic, nonanoic and decanoic saturated fatty acids. This compound has demonstrated toxic and repellent effects against several arthropod species. The aims of this study were to measure the insecticidal effects of C8910 against an insecticide susceptible (FANG) and a pyrethroid resistant (FUMOZ-R) laboratory strain of An. funestus as well as against wild-caught An. funestus material from Zambia (ZamF), and to investigate the repellent effects of two formulations of C8910 against these strains.

Methods

Toxicity against adult females was assessed using a range of concentrations based on the CDC bottle bioassay method and repellence of three different C8910 formulations was assessed using standard choice-chamber bioassays.

Results

C8910 proved equally toxic to adult females of the FUMOZ-R and FANG laboratory strains, as well as to adult females of the wild-caught (ZamF) sample. None of the C8910 formulations tested gave any conclusive indication of repellence against any of the strains.

Conclusion

C8910 is equally effective as an adulticide against pyrethroid resistant and insecticide susceptible An. funestus. However, the formulations tested did not show any consistent repellence against laboratory reared and wild-caught female samples of this species. Nevertheless, C8910 shows potential as an adulticide that can be used for malaria vector control, particularly in those instances where insecticide resistance management is required.

Laboratory Studies of Aedes aegypti Attraction to Ketones, Sulfides, and Primary Chloroalkanes Tested Alone and in Combination with L-Lactic Acid

The attraction of female Aedes aegypti to single compounds and binary compositions containing L-lactic acid and an additional saturated compound from a set of ketones, sulfides, and chloroalkanes was studied using a triple-cage dual-port olfactometer. These chemical classes were studied because of their structural relation to acetone, dimethyl disulfide, and dichloromethane, which have all been reported to synergize attraction to L-lactic acid. Human odors, carbon dioxide, and the binary mixture of L-lactic acid and CO₂served as controls for comparison of attraction responses produced by the binary mixtures. All tested mixtures that contained chloroalkanes attracted mosquitoes at synergistic levels, as did L-lactic acid and CO₂. Synergism was less frequent in mixtures of L-lactic acid with sulfides and ketones; in the case of ketones, synergistic attraction was observed only for L-lactic acid combined with acetone or butanone. Suppression or inhibition of attraction response was observed for combinations that contained ketones of C7-C12 molecular chain length (optimum in the C8-C10 range). This inhibition effect is similar to that observed previously for specific ranges of carboxylic acids, aldehydes, and alcohols.

Behavioural and electrophysiological responses of the malaria mosquito Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) to human skin emanations

Behavioural and electrophysiological responses of Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) to human skin emanations collected on glass beads were studied using a dual-port olfactometer and electroantannography. Glass beads to which skin emanations from human hands had been transferred elicited a level of attraction similar to a human hand. The attractiveness of these handled glass beads faded away 4 h after transfer onto the beads. Storage at -20 degrees C for up to 8 weeks showed a decreased but still attractive effect of the beads. In a choice test between one individual and four others, the emanations from the reference individual were significantly more attractive in three out of four cases. The headspace of handled glass beads elicited a dose-dependent EAG response. The substances causing EAG activity could be removed partially by dichloromethane, ethanol and pentane-ether. Glass beads provide a suitable neutral substrate for the transfer of human odour to enable chemical analysis of the human skin emanations for identification of kairomones of anthropophilic mosquitoes.

Contribution of fatty acids to olfactory host finding of female Aedes aegypti

Single carbon to 18 carbon n-aliphatic carboxylic acids were tested for their attractive effects on female Aedes aegypti in a Y-tube olfactometer. Each acid was tested over a wide range of concentrations together with L-(+)-lactic acid, the indispensable synergist for other attractive components emitted from human hosts. The attractiveness of lactic acid was significantly augmented when combined with fatty acids of chain length C(1)-C(3), C(5)-C(8) and C(13)-C(18), respectively. The addition of the C(9) and C(11) acids reduced the attractive effect of lactic acid. According to experiments showing a further increase of attractiveness by adding a second fatty acid, we suggest two groups of attractive carboxylic acids: C(1)-C(3) and C(5)-C(8). The addition of a fatty acid from one group to a mixture of lactic acid and an acid from the other group augmented the attraction to the mixture. Together with ammonia, a previously demonstrated attractant for Aedes aegypti, lactic acid plus two fatty acids from the different groups formed the hitherto most attractive, artificially composed blend. Two of the carboxylic acids which were found to be attractive together with lactic acid were also tested alone and in combination with CO(2), the major attractant in human breath. In both cases no attractive effect of the carboxylic acids could be observed.

Influence of human skin surface lipids on protection time of topical mosquito repellent

Skin surface lipids were extracted from volunteers who had been ranked according to the duration of protection from mosquitoes by diethyltoluamide. These lipids were analyzed by GLC-mass spectrometry for their fatty acid contents. Correlations were found between total skin lipid content and protection time of diethyltoluamide and between certain fatty acid concentrations in the skin lipids and the protection time of diethyltoluamide.

Correlation between mosquito repellent protection time and insensible water loss from the skin

Insensible water loss and mosquito repellent protection time have been found to be relatively characteristic of any given individual. In a population having a biologic distribution of repellent protection period against mosquitoes, an inverse linear correlation was observed between repellent duration and insensible water loss. Doubling of insensible water loss was associated with a 20% decrease in repellent duration. Repellent penetration or evaporation but not attractiveness to mosquitoes are explanations for the observed relationship.