Although a few species of mosquitoes are harmless or even useful to humanity, most are a nuisance because they consume blood from living vertebrates, including humans. The females of many species of mosquitoes are blood eating pests. In feeding on blood, some of them transmit extremely harmful human and livestock diseases, such as malaria.
The study of vector population behaviour - why some species are highly antropophilic? How mosquitoes find their blood-host? Why some people attract mosquitoes more then others? - is crucial in managing public health and highly beneficial in selecting the most appropriate vector control strategy for maximum cost-benefit.
Who are mosquitoes? Diptera Culicidae
| Scientific Classification |
| Kingdom: Animalia |
| Phylum: Arthropoda |
| Class: Insecta |
| Order: Diptera |
| Suborder: Nematocera |
| Infraorder: Culicomorpha |
| Superfamily: Culicoidea |
| Family: Culicidae Meigen, 1818 |
| Subfamilies: Anophelinae, Culicinae, Toxorhynchitinae |
The mosquitoes are a family of small, midge-like flies: the Culicidae. Mosquitoes are very widespread, occurring in all regions of the world except for Antarctica. Over 3500 species of mosquitoes have already been described from various parts of the world. Some mosquitoes that bite humans routinely act as vectors for a number of infectious diseases affecting millions of people per year (Global Health Observatory (GHO) - World Health Statistics ; International travel and health - 2012 Edition). Others that do not routinely bite humans, but are the vectors for animal diseases, may become disastrous agents for zoonosis of new diseases when their habitat is disturbed, for instance by sudden deforestation. In Europe are present over a hundred of species. In Italy there are about 60 species even if some of them are periodically reintroduced. The most represented are Culex, Anopheles, Ochelorotatus and Aedes.
Many species of mosquitoes are not blood eaters, and many of those that do it do not transmit disease. Also, in the bloodsucking species, only the females suck blood. Furthermore, even among mosquitoes that do carry important diseases, neither all species of mosquitoes, nor all strains of a given species transmit the same kinds of diseases, nor do they all transmit the diseases under the same circumstances; their habits differ. Some species feed preferencially on humans some of them attack people in houses while others prefer to attack people walking in forests. Accordingly, in managing public health, knowing which species, even which strains, of mosquitoes with which one is dealing is important. For example, in western Thailand Anopheles minimus is found in abundance and regarded as the most predominant malaria vector species. An. minimus shows a propensity to feed on humans and indoors; this would support use of long-lasting insecticide-treated materials to protect people from malaria inside the household. (Host feeding patterns and preference of Anopheles minimus (Diptera: Culicidae) in a malaria endemic area of western Thailand: baseline site description -2012).
Like all flies, mosquitoes go through four stages in their life cycle: egg, larva, pupa, and adult or imago. The period of development from egg to adult varies among species and is strongly influenced by ambient temperature. Adult males typically live for about a week, feeding on nectar and other sources of sugar. After obtaining a full blood meal, the female will rest for a few days while the blood is digested and eggs are developed. This process depends on the temperature, but usually takes two to three days in tropical conditions. Once the eggs are fully developed, the female lays them and resumes host-seeking. Typically, both male and female mosquitoes feed on nectar and plant juices, but in many species the mouthparts of the females are adapted for piercing the skin of animal hosts and sucking their blood as ectoparasites. In many species, the female needs to obtain nutrients from a blood meal before she can produce eggs, whereas in many other species, she can produce more eggs after a blood meal. Both plant materials and blood are useful sources of energy in the form of sugars, and blood also supplies more concentrated nutrients, such as lipids, but the most important function of blood meals is to obtain proteins as materials for egg production. When a female reproduces without such parasitic meals, she is said to practise autogenous reproduction, as in Toxorhynchites (Toxorhynchites never drink blood. This genus includes the largest extant mosquitoes, the larvae of which prey on the larvae of other mosquitoes; these mosquito eaters have been used in the past as mosquito control agents, with varying success); otherwise, the reproduction may be termed anautogenous, as occurs in mosquito species that serve as disease vectors, particularly Anopheles and some of the most important disease vectors in the genus Aedes. In contrast, some mosquitoes, for example, many Culex, are partially anautogenous; they do not need a blood meal for their first cycle of egg production, which they produce autogenously; however, subsequent clutches of eggs are produced anautogenously, at which point their disease vectoring activity becomes operative.
Antropophily and eggs deveopment
What kind of beneficial effects do feeding on human blood carries to mosquitoes' egg production? Does human blood contains "the secret ingredient" for mosquito eggs? Some female mosquitoes need to drink blood from vertebrate animals for their eggs to develop. Some of them are highly antropophilic. Adult female Aedes aegypti have an affinity for feeding on human blood and a tendency to forego feeding on sugar. This observation challenges two tenets of mosquito biology: (1) mosquitoes imbibe plant carbohydrates for synthesis of energy reserves and blood for reproduction and (2) egg production is reduced (sub-optimal amounts of the amino acid isoleucine in human blood, particularly free isoleucine in plasma, are thought to be responsible for lowered egg production) when mosquitoes feed on human blood compared with blood from other species. Probably feeding on human blood is associated with a selective advantage for Ae. aegypti. A study shows that high isoleucine concentration enhances egg production only when females also feed on sugar, an unusual behaviour for most domestic Ae. aegypti populations. When mosquitoes imbibe blood meals and do not feed on sugar, increased isoleucine concentration decrease energy reserves and did not increase egg production. Females fed human blood and water have greater age-specific survival, reproductive output, and cumulative net replacement than cohorts fed human blood plus sugar or isoleucine-rich mouse blood with or without access to sugar. The unique isoleucine concentration of human blood increases this mosquito's fitness, synthesis of energy reserves, and contact with human hosts. (Why do female Aedes aegypti (Diptera: Culicidae) feed preferentially and frequently on human blood? - 2001).
New research from scientists at the University of Queensland in Australia has found that when Aedes aegypti are infected with a bacteria called Wolbachia, they can produce eggs only after drinking human blood. No one knows for sure why this happens, but there have been some guesses made. One clue is that human blood is rich in an amino acid called threonine. Threonine is a key amino acid needed for the production of mosquito egg proteins. Threonine is also a nutrient that Wolbachia can’t make by itself–it has to get it from its host. Wolbachia may be depriving the mosquito’s eggs of threonine, leading to the loss in fertility when fed on non-human blood, which is lower in the amino acid. Another possibility is cholesterol. Human blood tends to have more of it than mice or chickens. Wolbachia needs cholesterol to make part of its cell wall, and the mosquito needs it to replenish its fat stores, creating another potential conflict between Wolbachia and the host mosquito. Even though Wolbachia infection in A. aegypti is not exactly natural, it may be a way for scientists to unmask the key nutrients mosquitoes use to make eggs with blood. (Human blood contains the secret ingredient for mosquito eggs - 2011).
Host-seeking process
Mosquitoes mainly use three sensory inputs to find their targets: they locate blood-hosts by scent, sight and heat. Attraction is primarily chemotropic: because of the high degree of anthropophily of some species, it is probable that several of these chemical cues are similar for these species. There is indeed a clear overlap among the kairomones to which a lot of mosquito species respond: carbon dioxide (CO2), ammonia, lactic acid, and other aliphatic carboxylic acids play a very important role in the host-seeking process.
It is now well recognized that mosquitoes release on the host body pheromones which attract other females.
Even if the odor is narrower, when clusters of hosts are more tightly associated on smaller patches, the biting rate per host is decreased. For two host groups of unequal number but equal spatial density, the biting rate per host is lower in the group with more individuals, indicative of an attack abatement effect of host aggregation. (A spatial model of mosquito host-seeking behavior - 2012)
Scent
Mosquitoes’ olfactory system consists of a pair of long, many-segmented antennae (15 segments). The antennae are important for detecting host odors, as well as odors of breeding sites where females lay eggs. Seven different types of setae have been found on the mosquito antenna: type A1 setae may mediate attractive odors such as that of the human hand, while type A2 setae may mediate repellent odors.
With regard to host location, female mosquitoes hunt their blood host by detecting organic substances produced from the host. They can sense carbon dioxide and lactic acid up to 100 feet (36 meters) away. Mammals and birds gives off CO2 as part of their normal breathing. Because CO2 is present in the atmosphere, mosquitoes respond to higher-than-normal concentrations, especially when the CO2 is mixed with host-odor.
Some studies indeed shows that an odorous blend consisting of ammonia, lactic acid, and two other carboxylic acids is almost as attractive to Ae. aegypti as an extract of human skin residues. A more complex blend of CO2, ammonia, lactic acid, and seven other carboxylic acids attracts three to five times more An. gambiae sensu lato females into experimental huts than sleeping humans.
Peristimulus-time histograms: each panel reports a peristimulus-time histogram illustrating the temporal dynamics of the responses to each odor of table1. In each panel, point 0 indicates the onset of the 0.5 sec odorant stimulus. The olfactory responses were assayed as firing rates in consecutive 50 ms intervals beginning 1 sec before the odor stimulus and continuing for another 1 sec. Each panel is representative of just one AalOR2 OSN.
In mosquitoes, the olfactory system plays a crucial role in many types of behaviour, including nectar feeding, host preference selection and oviposition. Aedes albopictus, known also as the tiger mosquito, is an anthropophilic species, which in the last few years, due to its strong ecological plasticity, has spread throughout the world. Although long considered only a secondary vector of viruses, the potential of its vector capacity may constitute a threat to public health. AalOR2 ortholog, represents the first candidate member of the odorant receptor (OR) family of proteins from A. albopictus. AalOR2 is expressed in the larval heads and antennae of adults. A. albopictus OR2 (AalOR2) shares a high degree of identity with other mosquito OR2 orthologs characterized to date, confirming that OR2 is one of the most conserved mosquito ORs. AalOR2 is narrowly tuned to indole and inhibited by -menthone. Comprehensive behavioral studies have indicated that the most crucial cues regulating many activities of mosquitoes, such as host-seeking, searching for oviposition sites and feeding, are volatiles emitted from hosts or plants. The ability of mosquitoes to identify a host for a blood meal or a correct site where to lay eggs via olfactory cues is conferred by a rich repertoire of Odorant Receptors (ORs) expressed in olfactory sensory neurons (OSNs) housed in the olfactory sensilla. Insect ORs belong to the 7-transmembrane type, but show no homology to any other ORs identified in vertebrates or nematodes. They also display an inverted insertion into the membrane. It has been shown that insect ORs function as heteromeric ligand-gated ion channels consisting of an olfactory receptor and a highly conserved member of this family (the olfactory co-receptor Orco), which is not directly involved in the recognition of odor molecules but in the transduction cascade.
Signal transduction is initiated when odorants (either alone or in complexes with Odorant Binding Proteins) bind to members of seven transmembrane containing odorant receptors (ORs). Originally thought to be G-protein coupled, insect ORs are now almost universally recognized to be signaling complexes that combine ligand specificity subunit (conventional ORs) with a widely-expressed, non-conventional OR subunit (called OR7) that probably acts as a direct ionotropic channel on ORN dendrites.
In the last decade genomes of many species have been released, allowing the identification of large OR families. In mosquitoes these genomic analyses have led to the identification of 79 ORs in the malaria vector mosquito A. gambiae, 131 in the yellow fever and dengue virus vector A. aegypti and 180 in C. pipiens quienquiefasciatus. A further significant insight into the mosquito sense of smell has recently been obtained by the functional characterization of fifty ORs in A. gambiae. The deorphanization was performed using both two-electrode, voltage-clamp electrophysiology in Xenopus oocytes and the D. melanogaster “empty neuron” system.
A. gambiae uses a combination of both narrowly tuned (specialist) and broad spectrum (generalist) ORs, and each AgOR has a distinct odor-response profile and tuning breadth; moreover, certain odors activate some OSN types while they inhibit others, suggesting that responses to odors could be integrated already at the antennal level. In mosquitoes, one narrowly tuned receptor that has been studied very well is OR2. A. gambiae OR2 (AgOr2) is tuned to a small set of aromatics including indole, which is an oviposition attractant for C. pipiens and has been found to constitute nearly 30% of the volatile headspace of human sweat. AgOR2 is expressed also in larvae where it is involved in the detection of 2-methylphenol, benzaldehyde, indole, and 3-methylindole. Further functional characterizations of the OR2 orthologs in C. pipiens and A. aegypti have shown that also in these mosquito species OR2 is narrowly tuned to indole. This structural and functional conservation suggests that OR2 represents an OR sub-family that may play an important role in the life of mosquitoes. As is the case for other members of the OR2 group, AalOR2 shares a great similarity with its orthologs from other mosquito species, and is highly related to its relative in A. aegypti (AaeOR2). Also AalOR2 responds to a small set of aromatic compounds including indole. Furthermore, AalOR2 expressed in the Drosophila empty neuron is inhibited by (–)-menthone. (Molecular and Functional Characterization of the Odorant Receptor2 (OR2) in the Tiger Mosquito Aedes albopictus - 2012).
Dose dependent response of AalOR2 to indole. A) The firing rates of the AalOR2-“empty neuron” in response to increasing concentrations of indole ranging from 10−7 M to 2 ×10−2 M. Indole led to an increase in spike frequency directly proportional to its concentration, suggesting its specificity of action. B) The same responses as in A) are reported in a graph. The red bars represent the means ± S.E.M. of three separate experiments (three sensilla per fly; n = 3).
Female Anopheles mosquitoes, the world's most important vector of Plasmodium falciparum malaria, locate their human hosts primarily through olfactory cues. Anopheles gambiae protein AgOr1, a female-specific member of a family of putative odorant receptors responds to a component of human sweat. AgOr1 confers a strong response to the odorant 4-methylphenol. As this compound is a component of human sweat that elicits an electrophysiological response from the antenna of female A. gambiae, it may contribute to the anthropophilic host-seeking behaviour of this mosquito. This idea is supported by the fact that AgOr1 is expressed specifically in the olfactory tissue of only female mosquitoes, and its expression is downregulated after a blood meal (the host-seeking behaviour of these mosquitoes is specific to the female and is reduced after blood-feeding). Furthermore, 4-methylphenol increases the effectiveness of traps for the tsetse fly Glossina morsitans morsitans (Olfaction: Mosquito receptor for human-sweat odorant - 2004.).
The dominant odor naturally produced in humans and birds that attracts the blood-feeding Culex mosquitoes, which transmits West Nile virus and other life-threatening diseases is nonanal. It is the powerful semiochemical that triggers the mosquitoes’ keen sense of smell, directing them toward a blood meal. A semiochemical is a chemical substance or mixture that carries a message. Antennae of the Culex quinquefasciatus are highly developed to detect even extremely low concentrations of nonanal. Mosquitoes detect smells with the olfactory receptor neurons of their antennae. Nonanal acts synergistically with carbon dioxide, a known mosquito attractant (Scientists identify key smell that attracts mosquitoes to humans - 2009).
Skin Microbiota
Composition of human skin microbiota affects attractiveness to malaria mosquitoes. Microbial communities on the skin play key roles in the production of human body odour and the composition of the skin microbiota affects the degree of attractiveness of human beings to this mosquito species. Interestingly, the two closely-related sibling species An. quadriannulatus and An. arabiensis, have a wider host range that is more zoophilic or opportunistic respectively. Bacterial communities of other vertebrate species are likely to differ from those on human beings and may play an important role in determining the host range of mosquitoes. Individuals that are highly attractive to An. gambiae s.s. have a significantly higher abundance, but lower diversity of bacteria on their skin than individuals that are poorly attractive. Without bacteria, human sweat is odourless to the human nose. Skin bacteria convert non-volatile compounds into volatile compounds having characteristic smells. The body odour of individual human beings correlates with the presence of specific microorganisms. Human eccrine sweat is more attractive to An. gambiae after incubation with skin bacteria for one or two days, and washing the feet with a bactericidal soap significantly alters the selection of biting sites by An. gambiae. Recently, it was shown that volatiles produced by human skin bacteria, grown in vitro, are attractive to female An. gambiae when tested in an olfactometer or with mosquito traps. Feet produce volatiles that are attractive to An. gambiae and there is evidence that this body part produces volatiles that influence the selection of biting sites by this mosquito species. On average, 5.8×10^5 culturable bacteria are present per cm2 on the sole of a human foot. The average number of bacteria per cm2 on the sole of a foot positively correlates with the relative attractiveness of the individuals to An. gambiae. The abundance of Staphylococcus spp. also correlates with the relative attractiveness of the individuals to An. gambiae. The abundance of Corynebacteria spp., Micrococcus spp. and Propionibacteria spp. does not seem to be in correlation with the relative attractiveness of the individuals.
Skin bacterial abundance and relative attractiveness to An. gambiae.
Correlation between the number of bacteria (log), determined by counts of colony forming units (CFUs) on non-selective plates and the relative attractiveness of the individuals.
The phylogenetic diversity of bacteria on the highly attractive individuals and on the poorly attractive individuals are significantly different: bacterial communities on the poorly attractive individuals are higher. The abundance of Staphylococcus spp. is higher in the more attractive individuals while Pseudomonas spp. are higher in the poorly attractive ones. The abundance of Brevibacterium spp. and Corynebacterium spp. is not different. Variovorax spp. and Pseudomonas spp. are correlated with poorly attractive individuals. Leptotrichia spp., Delftia spp. and Actinobacteria Gp3 spp. are significantly correlated with highly attractive individuals. Some in vitro studies demonstrates that volatiles released by Staphylococcus epidermidis are attractive to An. gambiae females and volatiles from Pseudomonas aeruginosa are unattractive, in contrast to the volatiles produced by four other bacterial species, all commonly found on human skin. This studies suggest that Pseudomonas spp. and possibly Variovorax spp. a) convert some of the attractive compounds produced by other bacteria, b) signal to other bacteria in ways that prevent them from emitting these attractive compounds, c) produce compounds that repel An. gambiae, or d) mask the effect of the attractive volatiles emanating from the human skin. More heterogeneous microbiotas may include more bacterial species that produce volatiles attenuating the attractiveness of individuals to mosquitoes, and may explain the interference effect described for the yellow fever mosquito Aedes aegypti: higher levels of specific volatile compounds were found to be responsible for decreased attractiveness of individuals to Ae. aegypti.
Rarefaction curves showing average bacterial diversity from poorly attractive (PA=red) and highly attractive (HA=blue) individuals.
It is possible that lower attractiveness to mosquitoes is caused by a selective group of skin microbiota that emanates compounds that interfere with the attraction of mosquitoes to their human hosts and thus function as an in-built defence system. Genes of the Major Histocompatibility Complex have been shown to influence body odour and may exert this influence by changing the skin microbiota composition and hence the volatiles produced by these bacteria and/or the human host. Skin microbiota could play an important role in this built-in defence system and may, therefore, affect transmission of malaria parasites. Individuals with a higher microbial diversity and a higher abundance of Pseudomonas spp. or Variovorax spp. are less attractive to mosquitoes and may therefore receive fewer bites. The discovery of the connection between skin microbial populations and attractiveness to mosquitoes may lead to the development of new mosquito attractants to be used in traps for monitoring malaria mosquito populations or lure-and-kill strategies and will lead to the development of personalized methods for protection against vectors of malaria and other infectious diseases. Compounds that inhibit microbial production of human odour, or manipulation of the composition of the skin microbiota may indeed reduce a person's attractiveness to mosquitoes. (Composition of Human Skin Microbiota Affects Attractiveness to Malaria Mosquitoes - 2011).
Sweat
Variation in sweat composition causes differential attractiveness to mosquitoes between humans and other mammals and between individals. Characteristics of skin glands and skin microbiota define the odorous organic compounds emitted by sweat, thereby the degree of attractiveness of the host to mosquitoes. Carboxylic acids in particular appear to characterize humans. What is known about sweat produced by humans compared to sweat of other mammals?
Eccrine glands are the best developed and most abundant glands in humans and are widely distributed over the general body surface. By contrast, in most mammalian groups eccrine glands are limited to the friction surfaces of the hands, feet and tail.
Apocrine glands, which play an important role in chemical communication, have a restricted distribution in most mammals including humans. The microbiota composition on human skin is determined mainly by the number and density of apocrine, eccrine, and
sebaceous glands at a specific site and whether or not the site is shielded from direct contact with the surrounding air. Skin lipids present in human sebum are catabolized by propionibacteria, corynebacteria, and staphylococci into long-chain carboxylic acids, which are subsequently converted by corynebacteria into short-chain carboxylic acids (C1–C12). Metabolism of branched aliphatic amino acids, present in eccrine sweat, by staphylococci also results in short-chain carboxylic acids. Many of them increase the attractiveness of ammonia and/or lactic acid, present in eccrine sweat, to mosquitoes. Because eccrine glands are distributed over the entire body, although in different densities, ammonia and lactic acid can be emitted from the entire body. As several skin bacterial species convert skin lipids into short-chain carboxylic acids, it is probable that these
VOCs (volatile organic compounds) are not only emitted from the axillae, but also from the head and upper trunk where many sebaceous glands are also present. However, because amino acids, which are present in eccrine sweat, are metabolized into short-chain carboxylic acids as well, it is probable that these compounds can be found also on other body parts. Depending on the local bacterial composition and density, different body locations will differ qualitatively and quantitatively in
VOC production; this could have an effect on biting site selection of mosquitoes.
Interpersonal variability in skin microbiota composition is high and the correlation between the microbial and chemical odour profile is disturbed by personal habits and living conditions. Most information on the composition of and odour production by sweat has been retrieved from humans. Axillary odour especially has received much attention, not only because humans prefer to mask this odour, but also because the armpits are considered prime candidates for the release of human
pheromones. Remarkably,
more volatile organic compounds (VOCs) are present in human axillary sweat than in human urine or saliva. Besides axillary odour, malodour produced by human feet is of interest both in research and pharmaceuticals. Although only eccrine glands are present on the feet, feet provide an excellent, moist, environment for bacterial species such as staphylococci.
Human skin lipid composition differs remarkably from other mammals, consisting mainly of triglycerides and their breakdown products such as free carboxylic acids. Mammalian skin surface lipids, including primates, contain little or no triglycerides or free carboxylic acids: anthropophilic mosquitoes might locate humans using carboxylic acids, which humans uniquely emit. At first glance, two of the other compounds that are attractive to mosquito species,
CO2 and ammonia, do not seem to be obvious cues to differentiate between possible blood hosts. Carbon dioxide is a major component of vertebrate breath whereas ammonia is mainly produced from animal urine and faeces. Vertebrates, however, release
CO2 and ammonia at various rates, depending on body size, and the concentration exhaled depends on the metabolic rate of an animal.
Because the bacterial composition on human skin depends largely on the number and density of the three types of skin glands, it is clear that these glands play an important role in the attractiveness of human sweat to mosquito species.
Even though anthropophilic mosquitoes can distinguish humans from other animals based on body odour,
human individuals are not all equally attractive.
Infants and children have been reported to be bitten less frequently by An. gambiae mosquitoes than adolescents and adults, which might be caused by differences in body mass or physiological differences. The most obvious change in relation to human sweat and body odor production is that apocrine and sebaceous glands mature during
puberty, and are then colonized by bacteria. Moreover, children produce sweat at a lower rate compared to adults. Both Ae. aegypti and An. gambiae s.s. do not exhibit an age preference when considering adult persons. Men have been found to be more attractive to Ae. aegypti mosquitoes than women, whereas An. gambiae s.s. did not discriminate between human individuals based on gender. No significant differences relating to gender in
VOCs of non-axillary human skin exist. Gender-specific axillary sweat components have been identified, but these compounds are not unique for either sex. In general, individuals vary in the relative abundance of
VOCs emitted. Because the behavioural effect of semiochemicals on mosquitoes depends largely on their concentration, variations in body odour intensities could contribute to differential attractiveness of individuals to mosquitoes. Men produce more sweat than women during exercise and more sebum is produced per cm2 male skin, presumably because of differing hormonal levels between the sexes. Nevertheless, no differences in concentrations of volatile carboxylic acids related to subjects’ gender or age have been found. Sweat appears to have a characteristic individual signature, which is relatively consistent over time, and axillary sweat samples from identical twins are more similar than those of unrelated persons. Individuals can even be distinguished based on human hand odours alone. An interesting point in this context is that
the composition of microbiota on human skin, which is strongly correlated with human body odour, is unique for each person and stays relatively stable over time. The density and variety of microorganisms colonizing the human skin is higher for men than women, influenced by pH, sweat and sebum. (
Sweaty skin: an invitation to bite? - 2011)
Sight and Heat
Mosquitoes use sight as their second method of detection. They have compound eyes distinctly separated from one another. Eyes have many separate visual units called ommatidia. Each ommatidium contains a lens and a light-sensitive cell. Eyes of this design allow for the detection of small movements, but are weak in distinguishing detail (mosquitoes pick up movement which indicates a likely living target).
So, mosquitoes don't see very well, but they zoom in like a heat-seeking missile. In the spherical arrangement of their compound eyes, blind spots separate each eye from the next one. As a result, they can't see a human being until they are 30 feet (10 meters) away. Even then, they have trouble distinguishing objects of similar size and shape. When they are 10 feet (3 meters) away they use extremely sensitive thermal receptors on the tip of their antennae to locate blood near the surface of the skin. The range of these receptors increases threefold when the humidity is high (in mosquito habitat such as forests or wetlands).
A mosquito’s sight is also based on color. A mosquito sees the contrast between the color of clothing and that of a person's skin. If you are wearing clothing that contrasts with the background, and especially if you move while wearing that clothing, mosquitoes can see you and zero in on you.
Heat sensitivity is a sensory modality that plays a critical role in close-range host-seeking behaviors of adult female. An essential step in this activity is the ability to discriminate and respond to increases in environmental temperature gradients through the process of peripheral thermoreception which is provided by a set of distal antennal sensory structures. Recent behavioral studies, however, have indicated that heat alone has no effect on the landing response, but significantly synergizes the attraction to the host odor. (Anopheles gambiae TRPA1 is a heat-activated channel expressed in thermosensitive sensilla of female antennae - 2009)
"Why Always Me?"
- Perfumes, hairstyling products, deodorants and other strong smelling products can confuse the mosquitoes, but not necessarily in a repellent way. Floral scents are especially attractive to mosquitoes. Many skin care products contain lactic acid and so attract mosquitoes. Lotions and creams labeled "alpha hydroxy" provide the most lactic acid.
- Alcohol makes the skin warmer and mosquitoes find that very appealing. Thus it is a good idea to stay away from drinking any alcoholic drinks if you would like to stay clear of mosquito bites.
- Physical activity makes people more "visible" to mosquitoes because of a lot of cues: increased secretion of sweat, warmer skin, movement, more production of lactic acid and CO2 (mosquitoes detect carbon dioxide in the air, so the more you breathe, the more likely you are to become a blood meal).
- Folic acid also seems to be something they find tasty. Folic acid is a very important nutrient and most pregnant women take folic acid supplements. Not surprisingly women often find they are bitten more frequently when pregnant than when not.
- Pheromones do not only attract the opposite sex. Recent research shows that mosquitoes are experts on recognizing pheromones and that they are very much attracted to it. So, if you are in love, you better watch out for those nasty little mosquitoes.
- Diseases can influence attractiveness in positive or negative way. Hyperhidrosis, for example, is a medical condition in which a person sweats excessively and unpredictably. People with hyperhidrosis may sweat even when the temperature is cool or when they are at rest. This would attract more mosquitoes. Seborrhea (a pathologic overproduction of sebum) and lactic acidosis as well. Endocrine diseases and (sex) hormones imbalances too are correlated.
Simona Grosso
