Sensory and behavioural responses of the malaria mosquito Anopheles gambiae to human odours
Malaria is one of the most serious human diseases, affecting between 300 and 600 million people per year and killing, on average, two children per minute. In tropicalAfricathe mosquito Anopheles gambiae Giles sensu stricto is responsible for much of the transmission of malaria parasites between humans. This mosquito species preferably feeds on human blood, rests inside human houses and breeds close to human dwellings, making it an effective malaria vector. The major cues guiding Anopheles gambiae females to their human hosts are volatiles emanating from the human body. The main aim of the present thesis was to investigate the chemical components in human emanations that play a role in the host-seeking behaviour of this mosquito species and how these human odours are perceived by the olfactory system of the mosquito. The knowledge obtained can be applied in developing odour-baited traps that can be used to protect humans from being bitten by mosquitoes or to decrease the chance of being bitten by mosquitoes and to provide an alternative for the traditional but questionable "human landing" method in the investigation of mosquito population size.Glass beads to which skin emanations from human hands had been transferred elicited a level of attraction similar to a human hand ( Chapter 2 ). The attractiveness of these handled glass beads faded away four hours after transfer onto the beads. The headspace of handled glass beads elicited a dose-dependent EAG response. Glass beads provided a suitable neutral substrate for the transfer of human odour to enable the investigation of behavioural and electrophysiological activities of An. gambiae exposed to these odours and to allow chemical analysis of the human skin emanations by gas-chromatography-mass spectrometry performed in a twin-project.To study the chemical basis for the inter-individual differences in human attractiveness to mosquitoes , emanations from 27 human individuals, collected on glass beads, were tested against ammonia in a dual-choice olfactometer to establish the degree of attractiveness to An. gambiae ( Chapter 3 ). There were clear differences in the trap entry response as well as in the attractiveness relative to that of ammonia between the skin emanations of different volunteers. Consistency of the differences was observed when emanations of the three most and the three least attractive volunteers were tested pair-wise. Emanations from males elicited a higher trap entry response than those from females. Odours of younger volunteers significantly raised the trap entry response and were preferred over odours from older volunteers. Electroantennogram responses to skin emanations from volunteers with different behavioural attractiveness were not in all cases positively related to the behavioural response level, suggesting the involvement of repellent components.Stockings worn by humans were previously shown to be highly attractive to females of An. gambiae . The headspace of nylon stockings was collected and analysed with gas chromatography coupled on-line to electroantennography (EAG). EAG responses were detected consistently at 23 retention times, and 14 compounds that elicited such EAG responses were tentatively identified. These compounds, however, were not of typically human origin ( Chapter 4 ).Ammonia, L-lactic acid and a mixture of carboxylic acids were previously found attractive to An. gambiae. These compounds are all present in human skin odours, therefore a mixture of these components was studied in a dual-choice olfactometer ( Chapter 5 ). Ammonia was an attractant on itself, whereas lactic acid alone was not attractive. Carboxylic acids, offered as a mixture of 12 compounds, were repellent at the concentration tested. The addition of ammonia to the carboxylic acid mixture overruled the repellent effect of the latter. Combining ammonia with either lactic acid or the carboxylic acid mixture did not enhance the attractiveness of ammonia alone. However, a synergistic effect was found when ammonia, lactic acid and the carboxylic acids were applied as a blend.Human odour compounds that elicited electrophysiological or behavioural responses were tested in combination with ammonia + L-lactic acid against ammonia alone ( Chapter 6 ). The results showed that C3-C8 and C14 carboxylic acids augmented the attractiveness of ammonia + lactic acid at certain concentrations, whereas alcohols，ketones，4-ethylphenol and indole only reduced the attractiveness at the concentrations tested. For some compounds, no effect was found at any of the concentrations tested.Based on the behavioural and electrophysiological findings, a field study in The Gambia (West Africa) was carried out to investigate the efficiency of mosquito traps baited with synthetic odour blends or human odour ( Chapter 7 ). This study showed that odours released from counterflow geometry (CFG) traps baited with up to nine compounds that were mixed during release were in many cases more attractive than odours from a tent occupied by a human. Carbon dioxide substantially increased the catch of the CFG traps for all mosquito species. CFG traps baited with the mixture of ammonia + lactic acid + 3-methyl butanoic acid + CO 2 resulted in the highest catches for most mosquito groups; the mixture is considered to be a promising candidate odour blend in the control of nuisance mosquitoes. Experiments with traps indoors showed that one odour mixture, consisting of ammonia + lactic acid + CO 2 + geranyl acetone + indole + 4-ethyl phenol was more attractive for An. gambiae than the control odour; this mixture holds promise for further experiments under conditions of higher An. gambiae abundance and for implementation in vector control programs.Using a single sensillum recording method, an electrophysiological study on the olfactory neuron responses of female An. gambiae mosquitoes was undertaken ( Chapter 8 ). Six functional types of sensilla trichodea and five functional types of sensilla basiconica (grooved peg sensilla) were identified. "Generalist" ORNs that are tuned to a broad range of odours were found in sensilla trichodea subtype E, whereas "moderate specialist" ORNs that are tuned to a narrow range of odours were found in subtype C and grooved peg sensilla, with two "extreme specialist" ORNs tuned to only one odour. There was overlap in response spectra between sensilla trichodea E and C or grooved peg sensilla, but no overlap was found between sensilla trichodea C and grooved peg sensilla except that both responded to ammonia. Neurons associated with the same sensillum tended to respond to similar odour stimuli but with different sensitivities. Neurons in grooved peg sensilla were tuned to more polar compounds including the important behavioural attractant ammonia and its synergist lactic acid, responses to which were only found in grooved peg sensilla. Phenols were among the most effective stimulants for several neuron types belonging to different functional classes. Across-fibre patterning is the most plausible coding principle operating in the olfactory system of this mosquito species.After a blood meal, female mosquitoes minimise host seeking activity and rest during egg maturation. To investigate whether the sensitivity of olfactory neurons changed after a blood meal and whether these changes correlate with the observed behavioural change, we compared the responses of ORNs in sensilla trichodea and grooved peg sensilla 2 - 24 h post blood meal with that of mosquitoes that had not fed on blood ( Chapter 9 ). Three instead of two functional types of sensilla trichodea E were found following a blood meal. A functional type that had not been detected in mosquitoes deprived of blood was found repeatedly. The most responsive neuron of the "new" functional type of sensillum showed a high sensitivity to indole. This neuron was also highly responsive to C6-9 carboxylic acids and moderately responsive to the human-specific odour compounds 7-octenoic acid and 3-methyl-2-hexenoic acid. These results indicate that changes in sensitivity and response profile of ORNs as a result of a blood meal are involved in modulating behaviour of An. gambiae females.The main conclusions from this thesis can be summarised as follows. This thesis provides additional evidence that chemical cues play a substantial role in the host attraction of An. gambiae (Chapter 2) and that skin emanations alone contribute significantly to inter-individual differences in attractiveness of humans to mosquitoes (Chapter3). The GC-EAG method can be used in the detection of kairomones used by An. gambiae , but a suitable substrate for collecting odours is essential (Chapter 4). Synergism was demonstrated to operate between ammonia, lactic acid and a mixture of carboxylic acids in attracting females of An. gambiae (Chapter 5) and olfactometric studies demonstrated the dose-dependent effects of human odour components to An. gambiae in addition to ammonia and lactic acid (Chapter 6). The results of our field study provided evidence that mosquito traps baited with synthetic mixtures were superior to those baited with a human being, suggesting great potential of these traps in future malaria control programs (Chapter 7). Based on the response to several compounds, olfactory receptor neurones were classified into functional groups, providing fundamental information for future studies of these neurons (Chapter 8). Qualitative and quantitative changes were found in olfactory neuron responsiveness before and after a blood meal, suggesting the involvement of the peripheral nervous system in the modulation of mosquito behaviour observed in different physiological stages (Chapter 9).
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| Format: | Doctoral thesis biblioteca |
| Language: | English |
| Subjects: | animal behaviour, anopheles gambiae, attractants, culicidae, host-seeking behaviour, malaria, man, odours, smell, diergedrag, gedrag bij zoeken van een gastheer, geurstoffen, lokstoffen, mens, reuk, |
| Online Access: | https://research.wur.nl/en/publications/sensory-and-behavioural-responses-of-the-malaria-mosquito-anophel |
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