Human skin emanations in the host-seeking behaviour of the malaria mosquito Anopheles gambiae

Malaria is an infectious disease caused by a parasite ( Plasmodium spp.) that is transmitted between human individuals by mosquitoes, belonging to the order of insects, Diptera, family of Culicidae (mosquitoes) and genus of Anopheles (malaria mosquitoes). Mosquitoes feed on humans (and other animals) because they need blood for their reproduction. Like most other haematophagous insects, only the female mosquitoes bite and use the protein-rich blood meal for egg development. Whilst feeding on a person infected with malaria, the mosquito can be pick up the parasites from the blood stream. After a developmental period in the mosquito, the parasites can be transmitted to another person when the mosquito takes a next blood meal. Thus, malaria transmission depends largely on the characteristics of the mosquito population. Knowledge about the ecology, behaviour, infection level and size of the mosquito population is essential for the development, implementation and evaluation of control programs. Development of an adequate trapping device for monitoring the mosquito population is of high priority for ecological and epidemiological studies.Malaria is one of the most important human parasitic infectious diseases and one third of the world population is under threat of the disease. Most victims are found in the sub-Saharan countries of Africa. The Afrotropical malaria mosquito, Anopheles gambiae sensu stricto, is the most important vector since it strongly prefers to feed on humans. Like most anopheline species, An. gambiae s.s. is nocturnal and its host-seeking behaviour is mainly odour-mediated. Consequently, odour-baited traps are considered as possible monitoring devices. However, despite the important role of this mosquito in malaria transmission, knowledge regarding host odour components (or kairomones) that bring about the attraction to humans is limited. For the development of odour-baited traps, attractive host odours need to be identified. In this thesis a behavioural ecological investigation to the source, identification and production of kairomones for An. gambiae (henceforth simply termed 'malaria mosquitoes') is described.Source of kairomones for malaria mosquitoesSince the beginning of the century it has been recognised that malaria mosquitoes utilise host odours in their host-seeking behaviour. The source of these olfactory stimuli is expired air, the skin or both. Carbon dioxide present in expired air is an important kairomone for many haematophagous insects. For this reason carbon dioxide is often used in odour-baited traps. From field research (Chapter 2) we learnt that malaria mosquitoes can find their host in the absence of breath, and, thus, the presence of carbon dioxide is not compulsory for finding a host. This suggests that volatiles from the skin of the host also play a role in the attraction of malaria mosquitoes. The addition of skin volatiles to a carbon dioxide baited trap will probably bring about higher trap catches. Moreover, for logistic reasons, an odour-baited trap without presence of carbon dioxide is preferable. Carbon dioxide is highly volatile and can be delivered only by gas cylinders or dry ice (= frozen carbon dioxide), which is impractical in the African field situation. The composition of body odour is complex: more than 300 components have been identified. However, a synthetic blend of the complete human odour has not yet been synthesised. For this reason, the identification of some important components that attract malaria mosquitoes was initiated. A prerequisite for the identification was the entrapment of natural skin emanations separate from the skin. Sweat appeared to be an attractive complex olfactory stimulus since it is not artificial but rather true to nature (Chapter 4 and 5) in the bioassays in the laboratory and it forms the 'heart' of the thesis.The identification of kairomones for malaria mosquitoesSweat was collected from the foreheads of a number of volunteers, who performed exercises on a hometrainer in a warm and humid room. The behavioural response of the malaria mosquitoes to this fresh sweat was rather variable; they were attracted to some fresh sweat samples (Chapter 8 and 9) but not to others (Chapter 5 and 7). However, the response of the mosquitoes to sweat that had been incubated for two days at body temperature was stable, and all incubated sweat samples were attractive to the mosquitoes. It appeared that the incubation released volatile components that were attractive to mosquitoes. Sweat is basically a watery solution of lactic acid, urea and ammonia. After incubation the lactic acid and urea concentration had decreased and the ammonia concentration showed a distinct increase (Chapter 8 and 9). For this reason ammonia was tested in the bioassay. For the first time, malaria mosquitoes were attracted to a single component other than carbon dioxide, namely ammonia. Lactic acid is an essential kairomone for another mosquito species, the yellow fever mosquito Aedes aegypti . However, the selective removal of lactic acid from the sweat did not affect the reaction of malaria mosquitoes. Therefore, we conclude that lactic acid is not an essential component of attractive odour blends for malaria mosquitoes. Urea was not tested, as it is not volatile. The fact that attraction was sometimes found to the fresh sweat with a rather low concentration of ammonia indicates that components other than ammonia also play a role in the host-seeking behaviour of malaria mosquitoes. The identity of these components needs further exploration.The production of kairomones for malaria mosquitoesThe skin of humans (and other animals) forms a good habitat for some microorganisms (bacteria and fungi), together called the skin microflora. During the collection of sweat samples, microorganisms are taken up with the sweat. An exponential growth of microorganisms in the sweat samples is found during incubation (Chapter 4, 6 and 7). Sweat constituents are broken down into more volatile components by the growing microorganismal population and this appears to bring about the enhancement of the attractiveness of sweat to malaria mosquitoes (Chapter 9). Such processes also probably play a role in the production of kairomones on the skin. However, this needs further exploration.ConclusionsKairomones for malaria mosquitoes originate from the human skin, in addition to carbon dioxide from exhaled air. Microorganisms of the skin flora play an important role in the production of kairomones for malaria mosquitoes An. gambiae s.s.. Ammonia is one of the components responsible for the attraction of malaria mosquitoes to sweat.