ABSTRACT

Research continues to ask why mosquitoes are attracted to certain hosts and what attractants are responsible for the mosquito’s odor mediated behavior. There is a plethora of evidence to suggest that host seeking in mosquitoes is mediated by info chemicals emanating from the host and visual cues that the mosquitoes recognize. Info chemicals are synonymous with semiochemicals. This student review paper discusses the following attractants: (1) human body odor; (2) long-range olfactory responses; (3) short-range olfactory and visual responses; and (4) human skin microflora. Mosquitoes have evolved a wide range of host-oriented responses. The four above are some of the latest research areas this review paper addresses regarding mosquito host attractants. As Gibson & Torr (1999) reported, "carbon dioxide appears to be universally attractive to mosquitoes, and is probably the most understood of the volatile host cues" (p. 2). Mosquito eyesight is poor, but their sensitivity to light is high. This enhanced light sensitivity allows mosquitoes to follow host-odor plumes even at low light intensities by using optomotor anemotaxis (a mechanism first demonstrated by Kennedy (1940) in Ae. Aegypti). Anemotaxis is a movement in response to a learned behavior. Human skin microflora may play a role in the production of odorous compounds that might function as kairomones for mosquitoes. The degree of association between mosquitoes and their hosts varies. Host preference could play a more important role in host-oriented behavior than previously recognized. The range of attraction to host cues, the relative importance of carbon dioxide and other components in host odor, and the visual and short-range behavior near hosts are all likely to be "species specific" (Gillies, 1972, p. 3).

INTRODUCTION

James (1992) introduces mosquitoes as blood-feeders, and blood is a requirement for egg production. Only females feed on blood, while males feed upon plant juices and nectar from flowers. Mosquitoes are nuisance pests to both livestock and humans. In addition to causing annoyance, mosquitoes are vectors of diseases that cause malaria, viral encephalitis, dengue fever, filariasis and dog heart worm . Kline (1994) has conducted numerous studies on mosquitoes. Kline states "two common approaches to controlling mosquito disease transmission are to employ the use of chemical insecticides for area-wide mosquito abatement and to use repellents for personal protection". Because of insecticide resistance in mosquitoes and concern about environmental pollution when using pesticides, there has been an increased emphasis on the development of alternative mosquito control technologies (Kline, 1994). Host-attractant baited traps and targets are promising technologies currently being considered for this purpose. A widely used trap for the surveillance of mosquitoes is the Centers for Disease Control (CDC) trap. The CDC trap uses carbon dioxide, and a light source to attract mosquitoes. Mosquitoes use attractants to locate their mates and find hosts for consuming blood meals. Host attractants appear to offer potential application for using traps and targets in the effort to minimize the human disease threat caused by mosquitoes.

Human Body Odor

People often enjoy camping in the summertime but they wonder why they get eaten alive by mosquitoes in the evening. The answer may lie in body odor. Support for this idea is provided by a recent study in Tanzania by Knols, DeJong & Takken (1995), in which three volunteers slept for nine nights in separate tents outfitted with a mosquito entrance and two exit traps. Analysis of the mosquito catches showed that one of the volunteers was significantly less attractive to two Anopheles and one Culex species than were the other two volunteers. Research concluded that three of the mosquito species must have selected their hosts based on a blend of human odors. Carbon dioxide was discounted as a selection factor because the three volunteers were of about the same weight, size, and age. Van den Hurk et al. (1997) demonstrated that it is possible to sample an appropriate fraction of the mosquito population with a nonsuction trap, baited with human odors. In a different study by McCall et al. (1996), extracts of whole mouse odor, which did not include carbon dioxide or other highly volatile chemicals, elicited highly significant responses in another study of Ae. aegypti. This followed their demonstration that showed how An. gambiae s.s. fed predominantly on the feet of a human volunteer. Knols & DeJong (1996) and Knols et al. (1997) have gone on to show that this species is attracted by the volatile odors from Limburger cheese, used as an analogy to human foot odor. These studies were followed up by Andreasen (1997) who found that placing recently worn socks next to a membrane blood-feeding device enhanced feeding and fecundity in laboratory colonies of An. gambiae s.s. and An. stephensi. What remains is to identify a suitable artificial human odor bait. From a practical standpoint one should not forget to cover one’s feet when sleeping at night in the great outdoors. Another body odor is lactic acid. Lactic acid is a by product of muscle metabolism. Though humans can not smell this odor, mosquitoes are attracted to some degree to this compound. Geier et al, (1996) searched for components that would attract the Aedes aegypti mosquito. Geier and his collegues used behavioral response as a bioassay to identify any attractants in liquid chromatography fractions of human skin extracts. A "Y" shaped wind tunnel was used to test fractions of L-lactic acid. Geir found that L-lactic acid was only slightly effective in attracting the Aedes aegypti mosquito.

Mboera et al, (1997) conducted field studies to determine if mosquitoes were more attracted to tents baited with human odor or just carbon dioxide. The study was set up in which mosquitoes had a choice of entering two tents. One tent had carbon dioxide pumped into it and the other tent had human odor pumped into it. The human odor bioassay was set up simply by digging a hole under the tent and placing a human volunteer in to the hole over night. Significantly fewer mosquitoes were caught in the tent into which carbon dioxide was pumped than the human odor baited tent. Even when Mboera and his colleagues increased the carbon dioxide, their were still more mosquitoes attracted to the human odor baited tent.

Long-range Olfactory Responses

In West Africa, Constantini et al. (1998) researched long-range olfactory responses. Carbon dioxide is the best understood of the mosquito host attractants. Mosquitoes are not only attracted to host carbon dioxide, but when one places carbon dioxide in a wind tunnel alone, with the absence of all host odors, mosquitoes still become activated towards the carbon dioxide. It enhances the catch size of virtually any type of trap when used as a bait; it enhances the attractiveness of whole host odor and some components of host odor; it induces attraction to some chemicals which are not attractive on their own; the range of attraction increases with dose; and removal of carbon dioxide from human breath decreases attraction to the host (Constantini et al, 1998). Carbon dioxide is considered a kairomone. A kairomone is a semiochemical or infochemical that acts between individuals of different species to the benefit of the receiver, which is the mosquito in this paper. Some other karimones or long-range olfactory responses are lactic acid, 1-octen-3-ol, acetone and carboxylic fatty acids (Takken and Knols, 1999). Most research in this field concentrates on the identification of kairomones produced by humans, because it is the mosquito-human interaction that attracts most interest, and human odors have been studied in much more detail than the odors of other vertebrates. So far, behavioral responses to lactic acid, prominently present in human skin emanations were found only with Aedes aegypti, and evidence for attraction to the compound in the field is scarce. One octen-3-ol, a compound present in bovine emanations, has a behavioral effect on many mosquito species and is now routinely used for collection of mosquitoes in the field (Kline, 1994). The compound acts synergistically with carbon dioxide. However, results with acetone and fatty acids were obtained in the laboratory and need to be verified in the field (Mboera, L.E.G, et al 1998). Van den Hurk et al, (1997) studied if mosquitoes of the Anopheles farauti complex. They wanted to find out if mosquitoes were attracted to CDC traps baited with four attractants. The attractants were: (1) octenol, carbon dioxide and light, (2) octenol and light; (3) carbon dioxide and light or (4) carbon dioxide and octenol without light. "The combination of light, octenol and carbon dioxide caught significantly more An. Farauti s.l. when compared to carbon dioxide and light alone. Only small numbers of the An. Farauti complex were captured when CDC traps were baited with octenol alone, i.e. no light or carbon dioxide" p. 177. More research is expected on these long-range cues and will contribute to the understanding to mosquito host attractants.

Short-range Olfactory and Visual Responses

According to Muir et al. (1992b) "the eyes of adult mosquitoes have relatively poor resolution for Aedes aegypti but overall high sensitivity" (p. 278). Bidlingmayer (1994) reported that "this extreme sensitivity of mosquito eyes has long been observed in their ability to maneuver in starlight conditions" (p. 273), even in far-red light (Gibson, 1995), wavelengths to which they are apparently only minimally sensitive (Muir et al., 1992b). This enhanced light sensitivity allows mosquitoes to follow host-odour plumes even at low light intensities by optomotor anemotaxis (a mechanism first demonstrated by Kennedy (1940) in Ae. Aegypti).

Allan et al. (1987) found that diurnal species respond to visual characteristics of hosts, such as colour/brightness, pattern and movement (p. 297). Muir et al. (1992a) showed in Aedes aegypti, that the attractiveness of colored targets is probably related to the mosquito’s spectral sensitivity.

Bidlingmayer, (1994) showed that nocturnally-active mosquitoes have specific responses to conspicuous objects and barriers. Conspicuous objects are such things as poles, sheds, houses or lawn mowers in a field. Even in the absence of hosts, the females of some species were attracted to conspicuous objects from as far as 15-20 meters away. Interpretation of the behavior of a species near an odor source must take into account whether or not that species typically feeds indoors or outdoors. Snow (1987) studied house entry and mosquitoes. House entry was controlled by the readiness with which a species would fly up a vertical barrier and also by its tendency to enter narrow openings. Once mosquitoes have entered a house, the short-range stimuli of the host itself can be detected (Snow, 1987).

As discussed earlier, carbon dioxide, a long-range chemical cue, is the best understood mosquito host attractant. The least understood cues are those used by mosquitoes to land on or near hosts at short-range distances. Mosquitoes need to control their behavior near odor sources and trap entrances at short-range distances. Since such an enormous amount of cues are present, a mosquito needs to control its flight behavior since so many sensations are being produced in the adjacent proximity of a potential host. Early studies showed that mosquitoes respond to humidity and temperature gradients associated with convection currents (Takken, 1991). Eiras & Jepson (1994) confirmed, with Aedes aegypti, that convection currents alone were attractive, and both lactic acid and sweat enhanced the number of landings in the presence of carbon dioxide, and that a human hand was significantly more attractive than the best combination of the components tested.

Comparative studies on the landing-site preferences of a range of mosquito species, from generalist to specialist in their host preference, have shown that olfactory cues emanating from particular areas of the human body enhance landing on those areas for the specialist species, whereas generalists feeders land at random (De Jong & Knols, 1996). De Jong and Knols also learned that out of the eight species of mosquitoes they studied, five of them prefer receiving blood meals at the head and upper torso. Of these five, three preferred biting the face directly. Their study suggests that carbon dioxide is a very powerful compound and it tends to attract mosquitoes to the area of the body that emits the largest amount of compound.

Abiotic factors such as relative humidity (RH) may play a role in short term responses to mosquitoes. Takken et al (1997) found that if a mosquitoes "were more successful in entering either of two odorless entry ports during periods of rising RH than when RH was stable or falling" p 119.

Human Skin Microflora

Some of the most recent literature on host attractants deals with human skin microflora and skin emanations. Researchers are just beginning to try to capture emanations or volatiles that could be host attractants. Braks, Anderson, & Knols (1999) find that human skin is inhospitable to most microorganisms; only limited species can survive. The composition and growth of skin microflora depends on skin temperature, humidity, PH, the concentration of inhibitors and the availability of nutrients. Skin microbe distribution is determined by species-specific nutrient demands, as well as the presence of cutaneous glands and the physical characteristics of skin sites. Human skin microflora is divided into roughly three groups: gram positive coci, diphtheroids-like species, and fungi. Volatiles arise from the skin flora. When oxidation of nutrients by skin microflora occurs, it results in the production of water and carbon dioxide. In general, incomplete oxidation produces other small volatile breakdowns and excretory molecules. The origin of these volatiles is know only in limited cases. Humans are unique in having a high level of triglycerides, which are broken down principally by Propionibacterium spp and which give rise to a large number of long and short chain free fatty acids. Corynebacterium spp are responsible for the modification of the initially odorless apocrine secretions including androsterone sulphate and dehydroepiandosterone, into the typical axillary smell of 5-androst-16-en-3-one and short chain fatty acids. Brevibacterium epidermidis is responsible for production of methanethiol and isovaleric acid and components of pungent foot odor. An. gambiae responds positively to volatiles produced by a related species, Brevibacterium linens (Braks, Anderson & Knols 1999). Meijerink & Van Loon (1999) found that limburger cheese, the odor of which is similar to that produced by feet, obtains its flavor by microbial action of B. linens and raises the number of mosquitoes in olfactometer traps by 2-3 times. Similar responses to the acid fraction of this cheese and to artificial mixture of short-chain fatty acids occurring in the cheese and foot odor were also found. In addition, significant electrophysiological responses of An. Gambiae s.s. were observed towards short chain fatty acids (Meijerink, J. and Van Loon, J.J. A. 1999). Similarly, Schreck & James (1968) found that Ae. aegypti was attracted by air led through broth cultures of transient skin bacteria, Bacillus cereus.

Bernier, et al. (2000) analyzed human skin emanations by gas chromatography mass spectrometry (GC/MS). The purpose of the study was to identify compounds that may be human-produced kairomones, which are used for host location by the mosquito, Aedes aegypti. There were 346 peaks detected by the GC/MS. Peaks are fractions of the suspected kairomones which are detected by the GC/MS. Of the peaks detected, 43 were unidentified while 303 were identified. There were 26 compounds confirmed to be of background origin, leaving 277 compounds as candidate attractants for Ae. aegypti. The number of compounds observed makes for a difficult and tedious bioassay process. Even with such a large number, there is no guarantee that all of the important compounds have been detected, and it is strongly believed that they have not been.

CONCLUSION

Mosquito host attractants along with mosquito behaviors are mediated by a wide range of semiochemicals and the chemical ecology of these remains poorly understood. With improved knowledge of these chemical cues and behaviours, new methods of mosquito control could be implemented. Currently, Integrated Pest Management (IPM) is used to control mosquitoes, and when pesticides are used, they are used conservatively due to environmental contamination and pesticide worker safety issues. Many cities and counties in the United States are pressuring mosquito abatement organizations to use biorational pesticides. Mosquitoes are nuisance pests not only because they bite, but also because of the diseases that they transmit. The United States received an outbreak of west nile encephalitis in New York City in the summer of 1999. This disease was unheard of in North America until this date. Will this arboviral disease reemerge and will the Culex species that the virus is transported in become resistant to pesticides? We don’t know. Mosquitoes do evolve and become resistant to pesticides in the short-term or the long-term. For lack of better tools, the control of malaria, yellow fever, dengue fever and numerous arboviral diseases are still being conducted with pesticides. Many entomologists believe that global warming will increase mosquito habitat, therefore increasing the medical threat of arboviral diseases. Reduction of the human-mosquito contact by behavioral manipulation with infochemicals would be far more acceptable than the widely applied chemical control methods.
 
 

REFERENCES

Allen, S.A., et al. (1987). Visual ecology of biting flies. Annual Review of Entomology, 32, 297-316.

Andreasen, M.H. (1997). Sweaty socks are an aid to membrane feeding of mosquitoes. Transactions of the Royal Society of Tropical Medicine and Hygiene, 91, 250.

Bidlingmayer, W.L. (1994). How mosquitoes see traps: role of visual responses. Journal of the American Mosquito Control Association, 10, 272-279.

Braks, M.A.H., Anderson, R.A., Knols, B.G.J. (1999). Infochemicals in Mosquito Host Selection: Human Skin Microflora and Plasmodium Parasites. Parasitology Today, 15(10), 409-413.

Bernier U.R., Kline D.L., Barnard D.R., Schreck C.E., & Yost, R.A. (2000). Analysis of human skin emanations by gas chromatography/mass spectrometry. 2. Identification of volatile compounds that are candidate attractants for the yellow fever mosquito (Aedes aegypti). Analytical Chemistry, 72 (4), 747-756.

Constantini, C. (1997). Behavioral studies on West African Malaria Vectors in the Field. PhD Thesis, University of London.

Constatini, C., Sagnon, N.F., Della Torre, A., Diallo, M., Brady, J., Gibson, G. & Coluzzi, M. (1998). Odor mediated host preferences of West African mosquitoes, with particular reference to malaria vectors. American Journal of Tropical Medicine and Hygiene, 58, 56-63.

Cork, A., (1996). Olfactory basis of host location by mosquitoes and other haematophagous Diptera. In G.R. Bock and G. Cardew (Eds.), Olfaction in Mosquito-Host Interactions, Ciba Foundation Symposium 200 (pp. 71-80). Chichester, England: John Wiley & Sons.

De Jong, R., & Knols, B.G. J. (1996). Selection of biting sites mosquitoes. In G.R. Bock and G. Cardew (Eds.), Olfaction in Mosquito-Host Interactions, Ciba Foundation Symposium 200 (pp. 89-103). Chichester, England: John Wiley & Sons.

Eiras, A.E. & Jepson, P.C. (1994). Responses of female Aedes aegypti (Diptera:Culicidae): to host odours and convection currents using an olfactometer bioassay. Bulletin of Entomological Research, 84, 207-211.

Geier, et al. (1996). A search for components in human body odor that attract females of Aedes aegypti. Olfaction I Mosquito-Host Interactions, Ciba Foundation Symposium 200 (pp. 132-148). Chichester, England: John Wiley & Sons.

Gibson, G. (1995). A behavioral test of the sensitivity of a nocturnal mosquito. Anopheles gambiae, to dim white, red and infra-red light. Physiological Entomology, 20, 224-228.

Gibson, G. & Torr, S.J. (1999). Visual and olfactory responses of haematophagous Diptera to host stimuli. Medical and Veterinary Entomology, 13, 2-13.

Gillies, M.T. (1972). Some aspects of mosquito behaviour in relation to the transmission of parasites. In E.U. Canning & C.A. Wright (Eds.), Behavioral Aspects of Parasite Transmission (pp. 69-810). London, England: Academic Press.

Gillies, M.T. (1980). The role of carbon dioxide in host finding by mosquitoes (Diptera; Culicidae): a review. Bulletin of Entomological Research, 70, 525-532.

James A.J. (1992). Science (Washington, D.C.), 257, 37-8.

Kennedy, J.S. (1940).Visual responses of flying mosquitoes. Proceedings of the Zoological Society of London, 109, 221-242.

Kline, D.L. (1994). Olfactory attractants for mosquito surveillance and control: 1-octen-3-ol. Journal of the American Mosquito Control Association, 10, 253-287.

Knols, B.G. J., De Jong, R. and Takken, W. (1995). Differential attractiveness of isolated humans to mosquitoes in Tanzania. Transactions of the Royal Society of Tropical Medicine and Hygiene, 89, 604-606.

Knols, B.G.J., & De Jong, R. (1996). Limburger Cheese as an Attractant for the Malaria Mosquito Anopheles gambiae s.s. Parasitology Today,12(4),159-161.

Knols, B.G.J., et al. (1997). Behavioral and electrophysiological responses of the female malaria mosquito Anopheles gambiae (Diptera: Culicidae) to Limburger cheese volatiles. Bulletin of Entomological Research, 87, 151-159.

Mboera, L.E.G. & Takken, W. (1997). Carbon dioxide chemotropism in mosquitoes (Diptera: Culicidae) and its potential in vector surveillance and management
programmes. Review of Medical and Veterinary Entomology, 85, 355-368.

Mboera, L.E.G, et al (1997). The response of Anopheles gambiae s.i. and A. Funestus (Diptera: Culicidae) to tents baited with human odor and carbon dioxide in Tanzania. Bulletin of Entomological Research, 87, 173-178.

Mboera, L.E.G, et al. (1998). The Olfactory responses of female Culex quinquefasciatus Say (Diptera: Culicidae) in a dual choice olfactometer. Journal of Vector Ecology, 23, 107-113.

McCall, P.J., Harding, G., Roberts, J. & Auty, B. (1996). Attraction and trapping of Aedes aegypti (Diptera: Tabanidae). Journal of the American Mosquito Control Association, 3, 655-656.

Meijerink, J. and Van Loon, J.J.A. (1999). Sensitivities of antennal olfactory neurons of the malaria mosquito, Anopheles gambiae, to carboxylic acids. Journal of Insect Physiology, 45, 365-373.

Muir, et al (1992a) Aedes Aegypti (Diptera: Culicidae) Vision: Response to Stimuli From the Optical Environment Journal of Medical Entomology, 29, 278-281.

Muir, et al (1992b). Aedes aegypti (Diptera:Culicidae) vision: spectral sensitivity and other perceptual parameters of the female eye. Journal of Medical Entomology, 29, 278-281.

Schreck, C.E. and James, J. (1968). Broth cultures of bacteria that attract female mosquitoes. Mosquito News, 28, 33-38.

Snow, W.F. (1987). Studies of house-entering habits of mosquitoes in The Gambia, West Africa: experiments with prefabricated huts with varied wall apertures. Medical and Veterinary Entomology, 1, 9-21.

Takken, W. (1991). The role of olfaction in host seeking of mosquitoes: a review. Insect Science and its Application, 12, 287-295.

Takken, et al, (1997). Interactions between physical and olfactory cues in the host seeking behavior of mosquitoes: the role of relative humidity. Annals of Tropical  Medicine and Parasitology, 91, 1, 119-120.

Takken W., and Knols B. G. J. (1999). Odor mediated behavior of Afrotropical malaria mosquitoes. Annual Review of Entomology,44, 131-157.

Van den Hurk, A.F., Beebe, N.W. & Ritchie, S.A. (1997). Responses of mosquitoes of the Anopheles farauti complex to 1-octen-3-ol and light in combination with carbon dioxide in northern Queensland, Australia. Medical and Veterinary Entomology, 11, 117-180.