The Culicidae, called mosquitoes, or mosquitoes, form a family of insects called Culicidae. Classified in the order Diptera and the suborder Nematocera, they are characterized by long, slender, multi-article antennae, scaly wings, and females with long mouthparts in the shape of a rigid biting-sucking proboscis. To date, 3,546 species of mosquitoes in 111 genera have been inventoried worldwide, but far fewer bite humans.
Mosquitoes have a role in ecosystems but primarily in human and animal epidemiology, as they are not only a nuisance through their bites but also the most important vector group for pathogens transmissible to humans, including zoonoses. They are vectors of three groups of human pathogens: Plasmodium, filaria and many arboviruses.
They are present on all the planet’s land surfaces (with the exception of Antarctica and Iceland), in forest, savannah or urban environments, as soon as a fresh or brackish water surface, even a small or temporary one, is available.
The scientific name (see Binominal name) of the Culicidae family comes from its genus type Culex given by Linné in 1758. Culex comes from the Latin aculeus (“sting”), itself derived from the proto-Indo-European *ḱuH-ló- of the same meaning, and refers to the biting-sucking apparatus of these insects with which the females feed on blood.
The vernacular name mosquito is borrowed from the Spanish mosquito (literally “little fly”), derived from the Latin mŭsca, “fly”.
Table of Contents
- 1 Morphology of the various stages of development
- 2 Mosquito bioecology
- 3 Development cycle
- 4 Feeding
- 5 Predators
- 6 Biological disease vector
- 7 Mosquito control
Morphology of the various stages of development
Mosquitoes are holometabolic insects passing through 4 stages of development; egg, larva (4 larval stages), pupa and adult. The first three are aquatic, the last one aerial. The total duration of this development, strongly influenced by temperature, is 10 to 15 days for tropical areas of the world with the highest densities of species.
This stadium is aquatic. From the egg, a first instar larva (L1) of reduced size will, by a succession of three moults, increase its size, giving in a few days a larva of stage IV (L4), of a size, variable according to the species and the conditions of development, between 4 and 10 mm. It is on this stage IV that taxonomic identifications are made.
The larvae consist of three parts:
- a head provided with a pair of antennae, a pair of mandibles armed with teeth on their distal edge and which together with the mentum form the masticatory apparatus, the whole flanked by a pair of mouth brushes which carry the food towards this apparatus. There are two pairs of pigmented eyes: a larger, non-functional pair that are the future eyes of the adult, and a smaller posterior pair that are the true eyes of the larva. The number, shape, size and arrangement of the various cephalic and antennal bristles provide information for species identification.
- a thorax wider than the head with the prothorax, mesothorax and metathorax, all three provided with setae. Culicidae larvae are apodic.
- an abdomen provided at the eighth segment with a respiratory siphon for the subfamily Culicinae. Species of the subfamily Anophelinae lack this, breathing directly from posterior anal papillae. The eighth segment with its siphon, and the X-segment with its usually comb and ventral brush, are also very valuable for genus and species identification.
Aquatic, the nymph has a strongly sclerified and swollen cephalothorax with two respiratory trumpets, fairly close together. The compound eyes of the future adult are visible laterally through the integument. At the level of the cephalothorax, the outlines of various organs of the future adult can be distinguished: proboscis, legs, wings.
The abdomen consists of nine segments, the last one smaller than the others, and carries at its apical part a pair of swimming paddles (fins), each held rigid by a median rib. At the end of the rib, the blade carries a terminal tang and an accessory tang on the ventral side. The outer edge of the fins bears teeth, variable in size and extension, which constitute a good diagnostic feature. The characters of the bristles of the posterolateral angle of the eighth segment, as well as the accessory bristle, are particularly used; each of the eight abdominal segments carries several pairs of different bristles dorsally. Each of the eight abdominal segments carries several pairs of different bristles dorsally. The first segment also carries a pair of webbed bristles which help to ensure the nymph’s equilibrium by adhering to the water surface by capillary action. The nymph, also aquatic, does not feed but, during this stage (1 to 5 days), the mosquito undergoes profound morphological and physiological changes in preparation for the adult stage. When the adult exudes, internal pressure causes the teguments of the cephalothorax to rupture along a mid-dorsal line. The edges of the slit diverge to allow the adult to emerge at the water surface.
In the adult stage, their size varies according to genus and species from 3 to 40 mm but rarely exceeds 10 mm, with the exception of the mosquitoes of the Toxorhynchitini tribe.
In the adult stage, mosquitoes have, like all Diptera, a single pair of long, narrow membranous wings, folded horizontally at rest. Culicidae have a slender body and long, slender legs. They are easily recognized by the presence of scales on most of their body. Females also have long mouthparts, characteristic of the family, of the stinging-sucking type: the proboscis, called rostrum, which inflicts the dreaded sting. Their head has two faceted eyes, but Culicidae do not have eye-spots.
On the head, this family is a member of the sub-order Nematocera with long, slender antennae with numerous articles (15 articles in the male and 14 in the female), without style or arista. Females are easily distinguished from males, which are the only ones with feathery antennae.
The thorax of mosquitoes consists of 3 segments, with an enlarged middle segment containing the wing muscles. This segment carries the long, narrow wings. The wings have six longitudinal ribs, with the third, fourth and fifth ribs being forked. The presence of scales on the wings is a diagnostic characteristic of the family. These tinted scales may form spots along the veins or along the fore or hindwing edge (Anopheles, Orthopodomyia). These wings allow them to fly at an average speed of 3 km/hour.
Each segment is provided with a pair of long, slender legs made up of 5 parts (coxa, trochanter, femur, tibia and tarsus made up of 5 tarsomers) often provided with scales whose ornamentation (ring, band, spot) is an identifying feature. The distribution of bristles and scales on the thorax is of great importance in determining the different genera and species of Culicidae. These include: acrostic setae (on the ‘back’ of the thorax), pre- or postspiracular setae (before or after the spiracle), lower and upper mesepimeral setae.
The abdomen of mosquitoes is made up of ten segments, the last two of which are telescoped inside the 8th segment: they are modified into reproductive organs. The first segments form interlocking rings that are joined together by a flexible membrane. The dorsal part (tergite) and ventral part (sternite) of each ring are joined laterally by flexible membranes that allow the abdomen to dilate strongly when blood is fed. This capacity also ensures the mosquito’s breathing through the large amplitude dilation and contraction movements of the abdomen, allowing air to circulate at the level of its spiracles.
In males, the 9th and 10th segments that form the genitalia have a fairly wide variety of structures. Their morphological characteristics are widely used for species determination, for example in Culex, Eretmapodites and Aedes of the subgenus Aedimorphus.
The mosquito therefore plays a role in many food chains. Adult males and females feed on flower nectar and therefore participate in the pollination of plants in the same way as other diptera, butterflies or hymenoptera.
Although a source of serious public health problems, mosquitoes (sometimes favoured by human development or behaviour) are part of the biological and functional diversity of wetlands, where they are important for the carbon cycle, especially nitrogen, and even have a bio-indicator value according to biologists such as Martina Schäfer (2004) and Willott (2004). They are among the species found in biodiversity hotspots, including in Europe.
Researchers are interested in their ecological characteristics and life history traits, in particular to clarify their role in the ecological niches they occupy, or even to highlight ecosystem services, or to understand in retrospect how human practices may have unintentionally favoured mosquitoes and the pathogens they carry (such as Plasmodium, formerly the cause of malaria in the Alpine valleys and (more broadly) in south-eastern France).
Their larvae are naturally part of the zooplankton assemblages of many “non-tidal” wetlands, but with different characteristics from those of other Diptera (a significant part of their life cycle is fixed).
Mosquitoes (larvae and adults) are a source of food for many predators (insects, lizards, amphibians, birds…), transferring from water to land a large amount of energy and biomass, a service provided by only a few groups of insects and seabirds or waterfowl. Some larvae, sometimes representing an important part of the biomass of aquatic ecosystems, filter up to two litres per day by feeding on micro-organisms and organic waste. They therefore participate in the bio-purification of swampy waters and, through their corpses or droppings, make elements such as nitrogen essential for plant growth.
In equatorial zones, they are present all year round as larvae or adults, and the closer one gets to the poles, the more the mosquitoes develop seasonally and with a marked time lag between egg laying and the emergence of larvae and adults (which feed different groups of insectivores; aquatic or terrestrial and aerial respectively). In cold and temperate zones, mosquito predators are mainly species that overwinter and eat mosquitoes as they develop.
Several vector species develop easily in urban areas where light can also attract them (phototactism).
Some animals have developed avoidance behaviours: in the Arctic, caribou seem to take the wind into account to escape swarms of mosquitoes.
For mating purposes, males and females form a swarm shortly after sunset, a few metres above the ground. This phenomenon can be observed for An. gambiae and An. funestus and is likely to occur in other species and genera as well. Mating takes place shortly after the emergence of adults, with each female being fertilised once for her entire life. The characteristic buzzing sound of mosquitoes is emitted only by females. It allows males to spot them, each species having its own characteristic frequency.
Most mosquitoes have haematophagous females, the blood meal being essential for egg-laying. They are called “endophagous” when they bite inside houses, “exophagous” when they bite outside (whereas entophilia and exophilia refer respectively to adult mosquitoes whose females spend the time of their blood meal digestion respectively inside and outside houses; endophagy does not imply entophilia and vice versa. It is more difficult to bring exophilic species into contact with pesticides, unless very large quantities are used) and a parade of some mosquitoes (e.g. Anopheles gambiae) to pesticide treatments could be to become exophilic,. However, females also feed – like the males – by eating sugar water and plant juices (nectar, sap), and can live for several months (some anthropophilic species spend the winter in diapause in caves, caves, stables; others in shelters in the undergrowth), but then they build up fat reserves instead of laying eggs.
Mosquitoes often have good dispersal strategies, to be taken into account in epidemiological and eco-epidemiological studies, as they partly explain the dispersal of human arbovirosis (dengue, chikungunya, yellow fever…).
Aquatic phase: larval deposits
Forty-eight hours after taking the blood meal, fertilized females lay their eggs, depending on the species: on the surface of permanent or temporary, stagnant or flowing water, in natural or artificial receptacles or on floodable land (swamp, rice field, etc.). Some species lay eggs capable of withstanding a drought of several months, and the eggs can be left in this way for months before being re-watered. These eggs are laid either singly (Toxorhynchites, Aedes, Anopheles), in clusters (Culex, Culiseta, Coquillettidia, Uranotaenia) or attached to a submerged plant support (Mansonia, Coquillettidia). The total fecundity of a female varies according to the species from 500 to 2,000 eggs (20 to 200 per clutch depending on the amount of blood available), several clutchings possible, generally one to four) These eggs develop in one to two days (depending on weather conditions) and hatch, giving birth to first stage aquatic larvae which have (except Anophelinae) at the end of the abdomen a respiratory siphon in contact with the air.
The larval deposits are very diverse according to genus and species and include all possible water sources except seas and oceans: running water (mountain torrents, rivers or streams) or stagnant water (pond, pond, rice field, swamp, riverside, ditch, puddle), sunny (path) or shaded (in the forest), large (lake, river) or small (dead leaf), with a high content of mineral salts (brackish water): mangroves, salt marshes) or loaded with organic matter (tree hole), natural deposits formed by plants (phytotelmes): leaf axils (banana tree, Bromeliacae. …), split bamboo, tree hole, carnivorous plant urn (Nepenthes), hollow fungus, leaf on the ground, hollow fruit), minerals: puddles, ruts, brick quarry, cattle footprints, crab hole, snail shell, rock hole, or artificial: cistern, latrine, sewage discharge, drinking trough, gutter, tire, car body, canister, tarpaulin, tin can, flowerpot… In certain genera (Aedes, Haemagogus, Psorophora), the eggs are resistant to desiccation, while waiting for their laying site to be put back in water.
The larvae feed and rest below the water surface, breathing through their spiracles which are flush with the surface and are located either directly at the level of the 8th abdominal segment for Anopheles (which must therefore remain parallel to the water surface in order to breathe) or directly at the level of the 8th abdominal segment for Anopheles, The Culicinae have the same type of body as the Culicinae (which must therefore keep their body oblique to the surface to breathe), either at the end of the 8th segment respiratory siphon.) Finally, some genera of Culicinae have their larvae immersed, breathing through the stem of a plant in which they insert their siphon (Coquillettidia, Mansonia, some species of the genus Mymomyia). The larvae go through four larval stages resulting in an increase in size, and metamorphose into a pupa.
The pupa is aquatic and breathes atmospheric air through these two respiratory trumpets. The abdominal end of the nymph is flattened into paddles or fins. The nymph does not feed. This is a transition stage to the adult during which the insect undergoes profound physiological and morphological changes.
The flying adult emerges from the nymph after two to five days.
Most species are nocturnal (genus Culex, Anopheles, Mansonia) or mainly diurnal (Toxorhynchites, Tripteroides) to twilight (genus Aedes). In Afrotropical regions, the majority of mosquitoes feed at night or at dusk, at least in savannah areas and at low altitude; in the mountains, where it is very cold at night, and in dense forests, where it is always semi-darkness, a number of species that are otherwise nocturnal or twilight, commonly attack during the day. Each species of mosquito seems to have its own cycle of activity under specific climatic conditions. In the genus Anopheles, the duration of the larval stage is about seven days (if external conditions are favourable: mainly water quality, temperature and food). Adults live from 15 to 40 days depending on conditions and species, except for some species where females may overwinter.
The males move relatively little from their original home range and have a relatively short life span. Females can migrate up to 100 km from their birthplace (passive transport by wind). In temperate zones, with the onset of winter, some species may overwinter in the adult stage, while others leave their larvae alone to perpetuate the species when spring arrives. Life expectancy can vary from two to three weeks for some species to several months for others. In a diapause state, the life expectancy of some mosquitoes can reach several months (depending on the species).
Blood sampling per bite
For haematophagous species, blood supply is necessary for egg-laying. The sequence (blood meal, egg maturation and oviposition) is repeated several times during the mosquito’s life and is called the gonotrophic cycle. The duration of this cycle depends on the species, but especially on the external temperature (for example, in A. gambiae, the cycle lasts 48 hours when the average day/night temperature is 23° C). The bite, most often nocturnal (especially at dawn or dusk), lasts two to three seconds if the mosquito is not disturbed.
The adult female mosquito bites the animals for reproduction to draw their blood, which contains the proteins necessary for the maturation of the eggs (in particular the yolk used to feed the egg germ). It is called anautogenic female, as opposed to autogenous females (which can do without blood for the maturation of their eggs).
During the sting, the female injects anticoagulant saliva which, in humans, causes an allergic inflammatory reaction that varies in severity depending on the individual: it is the formation of an itchy “pimple”. When the mosquito has finished collecting the blood, it uses its wings to take off, and not its legs like most other insects: thus, the take-off is almost imperceptible for the individual bite.
Female hunting techniques
Role of phototactics
All mosquitoes (larvae and adults) have eyes and can orient themselves according to light and in low light.All fasting mosquitoes show phototacticism in low light.
The female searching for blood temporarily loses this sensitivity to light and becomes primarily sensitive to the odours emitted by her target. Once she is gorged with blood, she regains her phototactic skill, which allows her to leave the room, stable or cave where she has bitten her host.
Some species have a very photosensitive retina and can immediately after their meal orient themselves towards the ambient light outside a starry or moonlit sky (a frequent case in tropical areas according to Muirhead-Thomson, 1951 cited by Beklemišev). Females of other species (e.g. A hyrcanus, A. bifurcatus and many others) will in the same circumstances (after the blood meal) only be attracted by the light of the rising sun to reach a diurnal refuge (rock cracks, ground crevices, dense vegetation…), which explains why females that have sucked blood are only very rarely found in the morning. Species showing this behaviour are typically exophilic (in this case the females never hibernate in houses), e.g. A. hyrcanus). The females of some species only slowly regain their phototacticism and finish their digestion in the house, as long as they are not scared away (A. maculipennis, A. superpictus, A. gambiae) or other highly endophilic species.
During the day, some species of mosquitoes are also attracted by darkness. Females are immediately attracted to these sources while they are repellent to males. It can also happen that females that have just gorged themselves with blood outside a house use it to protect themselves from the light the following day (until the blood has been digested, before they are ready to look for a place to lay their eggs); this is quite common in A. M. sacharovi, A. pulcherrimus and A. superpictus.
Role of chemotacticism
Just like ticks, mosquitoes locate their target by smell: during a journey of up to 2 km, the latter first reveals traces of carbon dioxide (emitted by breathing and perspiration) up to 30 m, then fatty acids such as butyric acid or lactic acid, 4 methyl phenol and substances with an ammonia-like smell, emitted by perspiration of the skin and its degradation by the skin microflora. Thermo-receptors will then allow the female to find the veinula where to prick. The visual system is sensitive to light, movement and colour but it is not very efficient, and would only intervene at less than 1.5 m). It is not the light but the smell that attracts the female biter.
Anthropophilic species are especially sensitive to kairomones such as lactic acid or sebum, or to the many odours emitted by sweat or breath (such as ammonia, lactic acid, aminobutane), the clean smell of the skin, urine, alcohol or perfume vapours and many others (e.g. the smell of someone who has drunk beer or cheese). Mosquitoes are also sensitive to heat (15 to 30° C) and humidity (in practice rather in summer and in stormy weather, therefore), and will be more attracted to a person with a high temperature. Attractive or repellent substances may vary from one species to another. Mosquitoes are still sensitive to many other parameters (e.g. the height at which the odour is perceived, in the case of An. gambiae, which flies close to the ground and preferably bites the feet and ankles). Beliefs that mosquitoes are sensitive to the amount of blood sugar in their blood and that lights should be turned off to avoid attracting female mosquitoes are unfounded.
Adult Diet: Adults, both male and female, are primarily nectarivores, feeding on nectar and the sweet juice of ripe fruit to cover their energy needs. In breeding (in medical entomology laboratories), they are thus provided with cotton swabs soaked in sugar water, which are sufficient for their survival, without having to resort to blood feeding.
In addition, females (with the exception of species of the genus Toxorhynchites), for the sole purpose of ensuring the development of their eggs, have recourse to blood meals on various warm-blooded vertebrates (birds, mammals including humans) or cold-blooded vertebrates such as amphibians (frogs, toads), reptiles (snakes, turtles) or even other insects (Lepidoptera larvae, leafhopper nymphs, mantises). Crossing the skin to a vessel, they take a blood sample. Each species has its own more or less assertive specificity in the choice of host for this blood meal. Thus, Culex hortensis and Culex impudicus preferentially bite amphibians, Cusileta longiareolata and the genus Aedeomyia birds, while Anopheles gambiae, Aedes albopictus, Aedes caspius, Aedes vexans, Culex pipiens and Culex quinquefasciatus prefer humans. We speak of anthropophilic mosquitoes if they preferentially bite humans or zoophilic mosquitoes if they preferentially bite other vertebrates.
Larval feeding: Mosquito larvae are mostly fed with phytoplankton, bacterioplankton and particles of organic matter suspended in the water of the lodge. The larvae feed by the beating of their mouth bristles, which creates a current sufficient to suck up food.
Other species are predatory in the larval stage, feeding primarily on the larvae of various Culicidae. This type of feeding behaviour is quite rare among the Culicidae, occurring only in all species of the genera Toxorhynchites and Lutzia, in species of the subgenus Psorophora, in Aedes of the subgenus Mucidus, in Tripteroides of the subgenus Rachisoura and in species of the genera Sabethes, Eretmapodites (Er. dracaenae, a predator of larvae of Aedes simpsoni [Pajot 1975]) and Culiseta (Cs. longiareolata), most of which are recognizable by their mouth brush often modified into strong, downwardly curved, prehensile spines.
Mosquito larvae and nymphs are eaten by water birds, amphibians (newts, frogs, toads, salamanders), fish (e.g. gambusia), insects (Chaoboridae, Notonectes, beetles, dragonflies…), crustaceans (Copepoda Cyclopoida such as Mesocyclops aspericornis), or the nematode Romanomermis culicivorax, etc.
Other species feed on adult mosquitoes: spiders, certain species of fish such as stickleback, dragonflies, bats or birds such as swallows or nightjars.
Epidemiological and medical importance
The sting and its treatment
The female’s proboscis (proboscis) is composed of vulnar mouthparts or styluses (maxilla, labia, hypopharynx) which are enveloped by the flexible labium (i) which folds up at the time of the bite.
The mosquito pushes the stylets into the epidermis to a blood capillary through the jaws that perforate the skin and allow the tube to stay in place during blood collection.
The stylets delimit two channels: one (salivary channel), formed by the hypopharynx, through which an anticoagulant saliva is injected, and the other (alimentary channel), at the level of the labra, through which blood is drawn which, if infected, will contaminate the mosquito.
The amount of blood collected varies from 4 to 10 mm3 in 1 to 2 minutes. According to the American Mosquito Control Association website, the average blood collection is 5 millionths of a litre; the insect ingests 5 mg of blood, which is twice its own mass because it weighs an average of 2.5 mg.
The bites can be completely painless or cause very unpleasant pruritus or more serious allergies, exceptionally up to anaphylactic shock. Hypersensitivity has an immune origin, which is an extreme reaction of our antibodies to antigens in the mosquito’s saliva.
Some of these sensitizing antigens exist in all mosquitoes, while others are species-specific. The hypersensitivity reaction can be immediate (types I and III) or delayed (type IV).
Various remedies are more or less effective depending on the individual and the time of application. Apart from zinc peroxide vinegar, whose calming effect is not medically proven, and products prohibited because of their toxicity, a few drugs exist; oral or topical antihistamines applied and diphenhydramine (Benadryl ointment), which would relieve itching. Topical corticosteroids such as hydrocortisone and triamcinolone may provide relief for inappropriately placed bites. Marseille soap has a calming effect (rubbing at the site of the sting). You can also place a hot object (a cup of hot tea, for example) on the bite for a few seconds, or dab it with an ice cube, or deodorant rollon and anti-hemorrhoid cream.
Direct application of a cloth soaked in very hot but not boiling water can block the release of histamine around the bite for a few hours. Finally, any cortisone cream is effective because of its anti-inflammatory effect [ref. required].
Biological disease vector
The Culicidae are the very first group of insects of medical and veterinary interest in terms of disease transmission.
About the animal
Along with ticks, mosquitoes are the first vectors of diseases that can be transmitted between animals (e.g. Myxomatosis) or zoonotic diseases that can also be transmitted to humans.
The mosquito is the animal that causes the most deaths in humans (an average of 725,000 deaths per year).
According to sources, the mosquitoes that collect human blood are only the females of about 80 of the approximately 2,600 species of mosquitoes described in 2010, or 3%, or only about 100 of the 3,500 species, or 6%. They do this to ensure the development of their eggs.
Mosquitoes are vectors of three groups of human pathogens: Plasmodium, filariae of the genera Wuchereria and Brugia, as well as numerous arboviruses.
More than 150 species of Culicidae in 14 genera have been observed to carry viruses involved in human disease (Mattingly, 1971). The mosquito transmits pathogens to humans or animals through its biting proboscis.
Mosquitoes are responsible for the transmission of malaria, one of the leading causes of human mortality (each year, between 250 and 600 million people are affected worldwide, and more than a million die,,) and of many viral diseases (arbovirosis) such as dengue fever, yellow fever, Rift Valley Fever, West Nile Virus, chikungunya, various viral encephalitises and filariasis and as such constitute one of the major subjects of studies in medical entomology.
When a mosquito bites a host carrying a parasite, it sucks in the pathogenic parasite (except filaria, dengue, yellow fever, West Nile or chikungunya viruses, etc.) together with the blood, which then reaches the mosquito’s stomach and passes through the stomach wall. Once multiplied, it ends up in the salivary glands of the mosquito, which inoculates it to its host during the bite, through the infected saliva, via the hypopharynx.
The genera Anopheles (malaria), Aedes (dengue and yellow fever, chikungunya), Culex (West Nile fever and various encephalitises) as well as Eretmapodites (Rift Valley fever) and Mansonia (filariasis) contain the majority of vector species that contaminate humans.
Mosquitoes carrying serious diseases are mainly present in southern countries (especially Africa, South Asia, Latin America). However, the movement of people and goods, combined with climate change, allows the species in question (e.g. the tiger mosquito and Aedes japonicus) to extend their territory further and further north, bringing with them diseases that were previously absent or extinct (malaria having been eradicated from Europe in the 20th century). Thus, many cases of chikungunya, a virus carried by certain Aedes, and in particular the tiger mosquito, appeared in 2007 in the Veneto region. The tiger mosquito, already present in Italy or southern France in 2010, could have colonized the whole of Europe by 2030.
Main diseases transmitted to humans by mosquitoes
- Rift Valley Fever
- Yellow fever: the species involved are Aedes simpsoni, Aedes africanus, and Aedes aegypti in Africa, as well as Haemagogus and Sabethes in South America.
- Chikungunya: The species involved are mainly Aedes albopictus and Aedes aegypti.
- West Nile virus: in Europe, this virus has been isolated from Culex pipiens, Cx. modestus, Mansonia richiardii, Aedes cantans, Ae. caspius, Ae. excrucians, Ae. vexans, and from an anopheles species of the maculipennis complex (Hubálek, 2007).
- Dengue: Aedes aegypti
- It is important to note that AIDS is not one of these mosquito-borne diseases, for several reasons, including the fact that the AIDS virus is not able to reproduce in the mosquito and reach its salivary glands. The AIDS virus, digested with the blood in less than 24 hours and destroyed, does not survive on the mosquito.
- Lymphatic filariasis
More than 40 species of Culicidae, belonging to 4 genera, are involved in the transmission of lymphatic filariasis. These are parasitic infections caused by three species of filariae: Wuchereria bancrofti, the most frequent and its pacifica variety, Brugia malayi and Brugia timori.
Bancroft’s Wuchereria bancrofti filariasis occurs throughout the intertropical zone (Caribbean, Latin America, Africa, India, Southeast Asia and the Pacific Islands). The pacifica variety occurs in Oceania.
Malaysian filariasis (or eastern lymphatic filariasis) due to Brugia malayi, is exclusively Asian (Southeast Asia, India, Sri Lanka, Korea and China). Brugia timori or Timor filaria is found in the southeast islands of Indonesia (Timor).
Mosquitoes of the genera Culex (in particular Culex quinquefasciatus), Anopheles (Anopheles gambiae, An. funestus) and Aedes (Aedes polynesiensis) are vectors of both types of filariasis.
In Africa, W. bancrofti is transmitted by Cx. quinquefasciatus and, in central and western Africa, only by Anopheles: An. funestus, An. gambiae complex.
In addition, species of the genus Mansonia transmit Malaysian filariasis (Brugia malayi). Species living in open swamps (Mansonia uniformis, Ma. annulifera, Ma. indiana) are vectors from India to East Asia. Zoophilous and rural species, Ma. bonneae, Ma. dives and Ma. uniformis are vectors in Thailand, Malaysia and the Philippines, and species of the genus Coquillettidia are reported to be vectors in Indonesia.
Wuchereria bancrofti pacifica present in the South Pacific islands is transmitted mainly by Aedes (Stegomyia) polynesiensis, Ae. (Stegomyia) pseudoscutellaris, Ae. (Stegomyia) tongae, Ae. (Stegomyia) hebridea as well as Ae. (Ochlerotatus) vigilax, a very aggressive mango species, Brugia timori is transmitted by Anopheles barbirostris.
The cycle is indirect and involves humans as the final host and a mosquito as an intermediate host. The microfilariae (1st instar larvae) are absorbed by the mosquito during a blood meal in an infested host. Within 12 hours, they pass through the stomach wall and reach the mosquito’s thoracic musculature. There, after two moults, they transform into infective forms in about 10 days. Finally, the third instar larvae migrate to the labium and are inoculated to the host during a new blood meal of the mosquito, actively penetrating through the wound created by the bite. The parasite does not multiply in the vector.
The high presence of microfilariae in the thoracic muscles of the Culicidae leads to a reduction in the Culicidae’s ability to fly.
Lymphatic filariasis affects 120 million people in 83 countries in Africa, Latin America and Asia and 40 million of them suffer from severe deformities and disabilities. Nearly a third of the carriers of the disease live in India, another third in Africa, while the remaining third are in South-East Asia, the Western Pacific and Latin America.
Encephalitis of Saint Louis: This encephalitis owes its name to the major epidemic that broke out in 1933 in the city of Saint-Louis (United States) during an exceptional drought. These climatic conditions associated with a strong insalubrity favoured the development of Culex quinquefasciatus, mosquito vector of this encephalitis due to an arbovirus (Flavivirus). Less than 1% of infections are symptomatic, with a mortality rate varying from 5 to 20% affecting, above all, the elderly. This encephalitis is present on the American continent, from Canada to southern Argentina. In 2005, an epidemic was observed in Argentina with 47 cases including 9 deaths, and 40 cases in 2010.
Murray Valley Encephalitis: Sometimes referred to as Australian encephalitis, it was first reported in 1951 in the Murray River Valley in Australia. It has since been found in parts of Australia (Province of Victoria in 2008) and New Guinea. It is caused by a virus of the family Flaviviridae, transmitted mainly by species of the genus Culex, in particular Culex (Culex) annulirostris and by Aedes (Stegomyia) aegypti (transovarial transmission). The main reservoir is constituted by aquatic birds of the order Ciconiiformes (herons and cormorants). There is no effective treatment or vaccine, but it is not transmitted from human to human. Although mortality reaches 25% of the symptomatic forms, this encephalitis has only resulted in 32 deaths since 1951.
Japanese encephalitis: Culexes of the Vishnui subgroup, mainly Culex (Culex) tritaeniorhynchus Cx. pseudovishnui, Cx. vishnui, and to a lesser extent Culex annulus, Culex gelidus, Culex fuscocephala are the vectors of Japanese encephalitis. These species are normally zoophilic, but attack humans during heavy outbreaks. Cx. quinquefasciatus has been found infected with this virus in Vietnam and Cx. bitaeniorhynchus and Cx. infula in India. Aedes japonicus is also cited as a vector species and can transmit the virus to its progeny (transovarial transmission) (Takashima & Rosen, 1989).
Vertical (transovarial) transmission of Japanese encephalitis and St. Louis encephalitis viruses by Aedes albopictus is possible (Rosen, 1988). This viral disease (Flavivirus) is endemic in southeast India and southeast Asia (Malaysia, Thailand, Vietnam, Philippines, Indonesia). It is epidemic in China (part), Korea and parts of Oceania, northern Australia and Japan. Japanese encephalitis is a major cause of viral encephalitis with 30,000 to 50,000 clinical cases reported each year, causing 15,000 deaths.
Starting in 2016, an epidemic of encephalitis is raging in South America, particularly in Brazil, due to bites from pregnant women. The mosquito transmits the Zika virus infection to them.
The affected areas are mainly rural, as mosquitoes are abundant in rice fields and flooded areas, with high twilight and night-time activity, causing painful bites to humans and domestic animals. Humans are only an accidental host for the virus, favoured by the creation of rice fields and pigpens near human habitations. The basic epidemiological reservoir of the virus is constituted by Ardeidae birds (herons and egrets) and ducks living in wetlands, with domestic animals (mainly pigs) acting as a relay reservoir. Horses, bats and reptiles are also cited as hosts. There is no human-to-human transmission and there is an effective vaccine against the disease.
Humans have long sought to control mosquitoes, which are vectors of disease, using passive or active, biological or chemical methods that are sometimes adapted to the stage of development of these insects.
In France, Act No. 64-1246 of 16 December 1964 “on the control of mosquitoes” was originally intended to promote the development of tourism on the coast and was later extended to other fields such as public health.
In the egg, larva and pupa stages, mosquitoes develop in stagnant (and sometimes slowly flowing), temporary or permanent water. Water is vital for at least one of the mosquito’s developmental stages (mud, sand or wetland will not do).
Large-scale chemical control
Since the 1950s, in inhabited areas or areas close to inhabited and heavily infested areas, such as swampy areas, larvicides have been used on a large scale to limit the proliferation of mosquitoes.
After a few generations, the larvae frequently become resistant to a product, so researchers must constantly develop new pesticide or biopesticide formulations.
Large-scale non-chemical control
Some forms of land and wetland planning (including and including water bodies and artificial wetlands,) allow :
Limit egg-laying sites: sewage collection, tarring of roads, disposal of rubbish dumps and open-air storage (this is the method recommended to avoid the spread of species such as Aedes albopictus, responsible for chikungunya, transported from one country to another in stocks of old tyres), management of water levels and possibly drainage, at the cost of disturbing wetlands;
Disadvantaging troublesome mosquitoes (those that bite) and favouring their predators in the water (dragonfly larvae, beetles, amphibians…) and in the air (swallows, swifts and bats in particular) by maintaining or restoring functional wetlands;
not attracting them to human settlements.
In their range, biological control is practised by releasing Toxorhynchite larvae, large mosquitoes that do not bite vertebrates but whose larvae feed, among others, on Culicidae larvae, into the wild. The success of this method varies according to the country or species targeted.
In Canada, the United States and France, Bacillus thuringiensis is used as a biological larvicide with low direct impact on the environment, although some studies point to significant indirect effects.
Protecting or restoring populations of predators of mosquito larvae, such as newts, frogs, toads, salamanders, swallows, swifts, bats, etc. also helps control their proliferation.
Fighting through the destruction of domestic and urban lodgings
Eliminating as much as possible any potential reservoirs of standing water where mosquitoes could lay eggs and develop larvae, even small volumes, reduces the risk of mosquitoes in urban areas. The health authorities therefore recommend monitoring the environment close to homes and removing containers where water may stagnate permanently (flowerpot saucers, vases, jerrycans, tarpaulins, gutters, open-air bins, wheelbarrows, etc.). Flowerpot saucers can be filled with sand or gravel.
A complementary technique consists, after having removed all other nearby water points, in offering “trap” lodgings (containers of stagnant rainwater) where the laying of females can be controlled: as soon as the larvae are large enough and visible, well before they pupate (i.e. about every five days), the water is filtered or emptied into the ground (making sure that it is completely absorbed). The larvae, deprived of water, die.
Containers that cannot be emptied (cesspools, wells, latrines, open rainwater collectors…), can be hermetically covered with mosquito netting or, failing that, with a thin layer of oil: the culex larvae can no longer breathe and die, but those of the Culicidae survive because they take their oxygen from the vessels of helophytes.
Control in the adult stage
Many methods are known to avoid being bitten by mosquitoes. Many are ineffective, not very effective or without proven effectiveness. Some effective methods have long-term negative effects. To prevent bites in heavily infested areas, a combination of protection and sometimes control methods is needed.
The female is attracted by the CO2 emitted by the host and to a lesser extent by a temperature between 18° and 30° as well as perspiration: humidity as well as its smell, accentuated by certain foods (beer, cheese…). Some medicines such as steroids or anti-cholesterol drugs also attract mosquitoes, as well as perfumes.
It is advisable to take into account the activity schedules of mosquitoes so as not to be exposed to them. It is recommended above all to wear long clothing covering the whole body; loose fitting as mosquitoes can bite through tight clothing. Fluid clothing will also allow the skin to breathe. However, be careful to pay attention to wrists, ankles and neck, which are risk areas. Colours are important: avoid dark colours simply because they increase the heat and therefore the CO2,,,.
Impregnated mosquito net
Mosquito netting can be fitted to doors and windows, surround cribs, cradles or strollers and even protect the face in heavily infested areas. It is also used to prevent females from laying eggs in water supplies.
In 1983, in Burkina Faso, the first insecticide-net combination was set up in the city of Bobo-Dioulasso, using insecticide-impregnated mosquito nets. These nets proved to be particularly effective against Anopheles in terms of mosquito mortality and reduced bite rates. Overall, the impregnated net reduces the mosquito bite rate by 36% compared to an untreated net and kills around 37% of the mosquitoes present. Widespread use could reduce the impact of malaria by about half and child mortality by 20 per cent.
In infested areas, not only the skin, but also clothing may be impregnated with insect repellent. Depending on the type of skin, the pharmacist may recommend a particular repellent. When travelling, it is best to buy locally, as the products will be more suitable for local mosquitoes.
The World Health Organization (WHO) mainly recommends those containing DEET (N,N-diethyl-3-methylbenzamide, formerly known as N, N-diethylm-toluamide), IR3535 (ethyl butylacetylaminopropionate) or icaridine (1-piperidinecarboxylic acid, 2-(2-hydroxyethyl)- 1-methylpropylester).
The most effective repellent is DEET, but recent studies show possible toxicity in humans, especially for pregnant women and children. Twelve cases of convulsions in children have been reported worldwide since the use of this product, without the origin of these convulsions being attributable to the product; this is therefore a precautionary principle that some consider abusive [ref. necessary].
Repellents based on soybean oil and IR3535 have a shorter duration of protection [ref. required].
Other plant-based repellents, including citronella oil, have a very short duration of effect and are therefore considered ineffective outdoors. According to the WHO, sprays, such as citronella spray, “can also reduce stings inside buildings”.
The cultivation, for example on window sills, of certain plants (lemongrass, lamiaceae (labiaceae) such as lemon balm, thyme, lemon thyme, rosemary, lavender, basil, small-leaved basil, geraniaceae such as geranium, especially lemon geranium, and pelargonium, pyrethrum, tomato plants, nasturtiums) would have a repellent effect.
According to some accounts of life away from civilization, saliva mixed with tobacco could be effective. Nicotine is indeed an excellent natural insecticide. Fire and smoke would also repel mosquitoes, but not without consequences for the health of humans who breathe in this smoke [ref. necessary].
Mosquito wristbands are practically inoperative,. Similarly, electronic mosquito repellent devices, which are supposed to repel mosquitoes by emitting ultrasounds, are actually ineffective, as the female is insensitive to these vibrations,,,.
Chemical control and resistance
Insecticide aerosols and dispensers are commercially available, but they are only of interest in a closed room, where they pose other known or potential health risks to the occupants who breathe them in, especially children. In addition, it has been observed (at least since the 1960s) that insecticides rapidly induce resistance to their efficacy in most targeted species of mosquitoes.
According to the WHO, mosquito coils and other sprays “can also reduce bites inside buildings.
The resistance of many strains of mosquitoes to certain pesticides has rapidly and sharply increased (much faster than for plant resistance to weed killers). For example, resistance (genetically heritable for progeny) to DDT was found in mosquitoes as early as 1947 in Florida, only one year after the first uses of DDT (Hemingway and Ranson 2000).
There is evidence to suggest that the presence of insecticides in the aquatic environment where the larvae develop accumulates in the larval tissues and thus in the adult, possibly resulting in “the maintenance of the induction of certain detoxification enzymes and thus the maintenance of the increase in tolerance to the insecticide”. In any case, it can be observed that “mosquitoes from agricultural areas or more generally areas polluted by organic compounds are more tolerant of insecticides”,,,,), which does not exclude phenomena of cross-resistance with various pesticides used in agriculture, veterinary medicine, or having been used, but persistent and therefore still present in the environment of the larvae.
These adaptations pose problems in the fight against mosquito-borne diseases (malaria, etc.), and could continue to increase, while populations of undesirable mosquitoes could expand as a result of global warming and the globalisation of trade.
To respond to these adaptations, in addition to the use of insecticide cocktails and the regular changing of molecules, another strategy is to avoid encouraging environments favourable to biting mosquitoes (stagnant water) and to encourage the development of natural predators of mosquitoes, for example by protecting fish and aquatic insects that eat mosquito larvae and by providing nesting boxes for bats and swallows to control the common mosquito. In Polynesia the breadfruit tree serves as a natural repellent against mosquitoes and other insects by burning the male flower of the tree . However, these strategies are not sufficiently effective.
Night light attracts mosquitoes in general except for females looking for blood (light is used to attract mosquitoes in traps used to count them, in combination with a piece of dry ice that emits CO2 that will also attract females ready to bite because when the female is looking for her blood meal, it is only by the smell of her target and before that by the CO2 emitted by that target that she is attracted). This is why insect electrocuctors using white or ultraviolet light to attract insects have a very low efficiency on female mosquitoes (they make up 0.2% of the insects trapped). The latter – prior to egg-laying – seem to be mainly attracted by the carbon dioxide emitted by respiration and then by certain molecules emitted by human skin (or other mammals), temperature may also play a role,,. According to the American Mosquito Control Association, UV is ineffective against female mosquitoes, but a combination of high-brightness LEDs in blue, green, red and infrared tones in certain wavelength ranges would be able to attract a wide spectrum of mosquito species into traps, far better than expensive, bulky and inefficient carbon dioxide traps. However, on a community scale, CO2 and “odour” mosquito traps can provide an effective mosquito barrier around homes.
Tiger Mosquito Control : Research advances
Carriers of many diseases such as dengue fever, researchers are developing solutions to irradiate the species. The goal will be to sterilize the females and infect the males.
As human populations spread and gain ground in forests and wetlands, some natural insectivores of mosquitoes (insectivorous reptiles, amphibians, bats… ) are in sharp decline in all or part of their range, and that some species of mosquitoes have adapted to most insecticides, The management of mosquito populations and natural environments and the chemical and technical means used for the management of eco-epidemiological risks posed by certain species of mosquitoes (such as ticks or other species that are vectors of disease or disturbance to agriculture) raise complex bioethical and environmental ethics issues such as the balance between the protection of human health and preservation of the environment or the programmed extinction of species.
Last update 2021-03-06