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Anopheles gambiae is a night-biting mosquito of the family Culicidae (subfamily Anophelinae, genus Anopheles, subgenus Cellia, series Pyretophorus). It was described by the British entomologist George Michael James Giles in 1902 from specimens collected in the Gambia River valley of West Africa. The name is used in two senses: Anopheles gambiae sensu stricto, a single species, and the Anopheles gambiae complex (An. gambiae sensu lato), a group of at least nine closely related species that cannot be told apart by their appearance and that occur across Sub-Saharan Africa and nearby islands.

Anopheles gambiae sensu stricto is among the most efficient vectors of human malaria caused by Plasmodium falciparum. Together with the related species Anopheles coluzzii and Anopheles arabiensis, and the more distantly related Anopheles funestus, it sustains most malaria transmission in Sub-Saharan Africa. The region accounts for about 94% of the world's estimated 250 to 263 million annual malaria cases and about 95% of the roughly 600,000 annual deaths. An. gambiae also transmits Plasmodium ovale and Plasmodium malariae, is an important rural African vector of the filarial worm Wuchereria bancrofti, which causes lymphatic filariasis, and is the main vector of O'nyong'nyong virus, the only human alphavirus known to be transmitted by Anopheles mosquitoes.

Four features explain why this species is such an effective vector:

1. Strong preference for humans and for feeding and resting indoors. An. gambiae sensu stricto and An. coluzzii take 90 to 99% of their blood meals from humans, bite mainly indoors late at night, and often rest indoors afterwards. Because the human biting rate enters the standard transmission model squared, this specialization greatly increases transmission.
2. Chromosomal inversions and ecological adaptation. Large inversions on chromosome 2, particularly the inversion known as 2La, hold together sets of genes affecting resistance to drying, heat tolerance, and choice of larval habitat. This has allowed the complex to occupy a wide range of aquatic habitats.
3. A moderate temperature optimum. Transmission by An. gambiae peaks near 25 °C, about 6 °C cooler than older models assumed. As a result, warming is expected to extend transmission into cooler highlands while reducing suitability in the hottest lowlands.
4. Widespread insecticide resistance. Decades of agricultural pesticide use and the distribution of more than two billion insecticide-treated nets since 2000 have selected for resistance through several mechanisms at once, prompting the development of new insecticides and of genetic control methods.

Morphological characters and sibling species

Anopheles gambiae belongs to the subgenus Cellia, whose females have a well-developed set of chitinous teeth in the foregut (the cibarial armature) and several pale scale patches along the leading edge of the wing. Mosquitoes of the complex are easily distinguished from Aedes and Culex at all life stages, but the sibling species within the complex cannot be separated by appearance.

• Eggs: laid singly on the water surface, about 0.47 mm long, boat-shaped, with a pair of air-filled floats along the sides. Unlike Aedes eggs, they cannot survive drying for more than a few days.
• Larvae: lack the breathing siphon found in Culex and Aedes. The spiracles open directly on the upper surface of the eighth abdominal segment, and water-repellent palmate hairs along the back allow the larva to lie horizontally just beneath the water surface, where it filter-feeds using rotating head brushes.
• Adults: slender mosquitoes 3.5 to 5 mm long that rest with the body held in a straight line at an angle of about 30 to 45° to the surface. In females the maxillary palps are about as long as the proboscis and bear pale rings. The wings have alternating blocks of dark and pale scales, and the legs are speckled.

Until the mid-20th century, Anopheles gambiae was regarded as a single widespread African species, despite puzzling variation in behaviour: populations in some villages fed almost exclusively on humans, while others fed on cattle or bred in brackish water. Crossing experiments that produced sterile male hybrids, carried out by George Davidson, Hugh Paterson, and Mario Coluzzi between the 1950s and 1970s, together with chromosome studies and later DNA-based tests, resolved the group into nine species.

Species of the Anopheles gambiae complex

Polytene chromosome inversions and ecological divergence

Work by the Italian geneticist Mario Coluzzi and colleagues, using the large polytene chromosomes of ovarian cells, showed that ecological adaptation within Anopheles gambiae is closely linked to inversions on chromosome 2. Because recombination is suppressed in individuals carrying one inverted and one standard chromosome, an inversion keeps many co-adapted genes together, so that the whole block behaves like a single gene.

The 2La inversion. This inversion spans about 22 megabases on the left arm of chromosome 2, roughly 8% of the genome and several hundred genes. Its frequency follows a steep gradient of aridity:

• The standard (uninverted) arrangement predominates in humid equatorial rainforest, where rainfall exceeds about 1,500 mm per year.
• The inverted arrangement, which is fixed in the arid-adapted An. arabiensis and An. merus and reached An. gambiae by hybridization, approaches fixation in the dry Sahel. Its frequency also rises within a single village during the dry season.
• Mosquitoes homozygous for the inverted form lose water more slowly, tolerate heat better, and are more likely to rest outdoors. Several heat-shock protein genes lie within the inversion.

Inversions on the right arm (2Rj, 2Rb, 2Rc, 2Rd, and 2Ru). Different combinations of these inversions define five chromosomal forms in West Africa: Forest, Savanna, Bamako, Bissau, and Mopti, each associated with particular habitats, from humid forest to the rock pools of the upper Niger and the irrigated inner Niger Delta.

Divergence of An. gambiae and An. coluzzii. Analysis of ribosomal DNA on the X chromosome, and of a short interspersed element insertion known as SINE200 X6.1, showed that the Mopti form corresponded to a distinct incipient species. It was formally named Anopheles coluzzii in 2013, while the Savanna and Bamako forms remained An. gambiae sensu stricto. Whole-genome sequencing shows that the two species remain similar across most of their genomes because of continued hybridization, but differ at several regions near the centromeres of the X, 2L, and 3L chromosomes. Reproductive isolation is reinforced by mating behaviour, since males of the two species tend to form separate swarms, and by differences in larval ecology: An. gambiae larvae move and feed actively and can complete development in about a week in temporary pools that lack predators, whereas An. coluzzii larvae move less and dive when disturbed, which helps them survive among the predators of permanent water bodies.

Genome architecture and genetic diversity

In 2002, an international consortium published the reference genome of the Anopheles gambiae PEST laboratory strain, with about 278 Mb of assembled sequence. The mosquito has two autosomes (chromosomes 2 and 3) and an X chromosome, with males carrying a Y chromosome; much of the repetitive heterochromatin remains unassembled. The male-determining gene Yob was identified on the Y chromosome in 2016.

Sequencing of thousands of wild mosquitoes by the Anopheles gambiae 1000 Genomes Consortium revealed exceptionally high genetic diversity:

• Nucleotide diversity is about 1.5%, meaning that two randomly chosen chromosomes differ at roughly one in 70 base pairs. This is among the highest recorded for any animal and implies an effective population size of millions.
• Beneficial mutations can spread between species. When the kdr insecticide-resistance mutation arose in An. gambiae, occasional hybridization carried the surrounding resistant chromosome segment into An. coluzzii and An. arabiensis within a few years.

Evolution of anthropophily

Genetic and paleoclimatic evidence indicates that the strong preference of An. gambiae and An. coluzzii for human blood evolved recently, probably within the past few thousand years, in association with the spread of agriculture in West and Central Africa:

1. Before agriculture, closed rainforest canopy shaded the forest floor and excluded larvae that require sunlit water, while small, mobile hunter-gatherer groups could not support a mosquito specialized on humans.
2. Clearing forest for cultivation created both sunlit pools suitable for egg-laying, such as borrow pits, footprints, and furrows, and permanent villages that provided a reliable supply of blood at night.
3. The resulting combination of farming, human-biting mosquitoes, and Plasmodium falciparum, which appears to have entered humans from gorillas, produced some of the strongest known natural selection on the human genome. It favoured the spread of protective blood variants including sickle cell trait, haemoglobin C, the thalassemias, glucose-6-phosphate dehydrogenase deficiency, and Duffy-negative blood groups.

Olfactory and thermal cues in host location

To find a sleeping person inside a dark house, Anopheles gambiae females use three kinds of cue in sequence:

1. Carbon dioxide (from tens of metres). Plumes of exhaled CO₂ are detected by receptors on the maxillary palps. CO₂ triggers upwind flight and makes the mosquito more responsive to body odour and warmth.
2. Human body odour (from a few metres). Unlike cattle-feeding relatives, An. gambiae responds to a blend of compounds produced when skin bacteria, especially Corynebacterium, Staphylococcus epidermidis, and Brevibacterium, break down sweat. The blend includes lactic acid, ammonia, and carboxylic acids such as butyric, isovaleric, and hexanoic acid. These are detected by ionotropic receptors on the antennae. Because such bacteria are particularly abundant on the feet, the mosquito often bites the ankles and lower legs. A second family of odorant receptors, which detect compounds such as indole and 1-octen-3-ol, helps distinguish humans from cattle.
3. Heat and humidity (within about half a metre). Temperature-sensitive neurons on the antennae detect the warm, humid air rising from skin and guide the mosquito to a landing site.

There is evidence that the parasite influences this process. People carrying P. falciparum gametocytes produce altered skin volatiles, including aldehydes, and some studies have found that infected people are more attractive to mosquitoes than uninfected ones.

Nocturnal biting and mating biology

Adult Anopheles gambiae exhibit specialized circadian feeding and reproductive behaviors:

• Nocturnal biting. Host-seeking is controlled by circadian clock genes. It begins after dusk, increases after 22:00, and peaks between midnight and 04:00, when people are asleep indoors. This behaviour is what makes bednets so effective.
• Mating swarms. Shortly after sunset, males gather in swarms one to three metres above visual markers on the ground, such as a patch of bare soil or a woodpile, for about twenty minutes. Where two species occur together, their swarms are usually separated in space, which limits hybridization. When a female enters a swarm, males detect her wingbeat, near 400 Hz, using their feathery antennae and Johnston's organ. A male and female then adjust their wingbeat frequencies so that particular harmonics match before they mate in flight.
• The mating plug. Male An. gambiae transfer a gelatinous plug into the female during mating. It contains the steroid hormone 20-hydroxyecdysone, which makes the female unwilling to mate again, switches her behaviour from swarming to host-seeking, and increases egg production. A single mating therefore fertilizes all the eggs she lays in her life.
• The pre-gravid stage. Because larvae often develop in nutrient-poor temporary pools, newly emerged females may have insufficient reserves to produce eggs after a single blood meal. Many require two blood meals in their first few days. This increases the chance that a young female becomes infected early in life, and so the chance that she survives long enough to transmit the parasite.

Aestivation and wind-borne migration

In the Sahel and Sudanian savanna, no rain falls for five to seven months, all surface water dries up, and daytime temperatures exceed 40 °C. Yet adult Anopheles gambiae sensu lato reappear in villages within a few days of the first rains, sooner than eggs laid after that rain could produce adults. Research led by Tovi Lehmann and colleagues, using mark-release-recapture, stable isotopes, and aerial sampling, showed that different species solve this problem in different ways:

1. Dormancy in An. coluzzii. Mated females survive locally through the dry season in a dormant state, sheltering in cool, humid places such as abandoned buildings, animal burrows, and rock crevices. They greatly reduce their activity and egg production, change the waxy compounds in their cuticle to reduce water loss, and can live for several months instead of the usual few weeks.
2. Long-distance wind-borne migration. An. gambiae sensu stricto and An. arabiensis largely disappear from Sahelian villages in the dry season. Aerial sampling using tethered balloons carrying sticky nets at heights of 40 to 290 m showed that large numbers of females, many of them blood-fed or carrying eggs, fly at these heights at night. Carried by seasonal winds, they can travel more than 100 km in a single night, recolonizing the Sahel each rainy season. This also helps explain how insecticide-resistance genes spread rapidly across large regions.

Sporogonic development of Plasmodium

Anopheles gambiae is the definitive host of the malaria parasite, in which sexual reproduction occurs. It transmits P. falciparum, P. ovale, and P. malariae. Plasmodium vivax is transmitted in parts of the Horn of Africa and Madagascar, but is rare across much of West and Central Africa because many people there lack the Duffy blood-group antigen the parasite uses to enter red blood cells.

When a female takes a blood meal containing mature gametocytes, the parasite must complete development in the mosquito, a process whose duration depends strongly on temperature:

1. Fertilization (within an hour). A drop in temperature and exposure to xanthurenic acid in the mosquito gut trigger the male gametocyte to produce eight motile gametes within about fifteen minutes. These fertilize female gametes to form zygotes.
2. Crossing the gut wall (18 to 36 hours). The zygote develops into a motile, banana-shaped ookinete, which digests its way through the chitinous membrane lining the gut, crosses an epithelial cell, and comes to rest beneath the basal lamina on the outer surface of the gut.
3. Oocyst development (about 9 to 14 days at 25 to 27 °C). The ookinete forms a rounded oocyst that grows and divides repeatedly, producing thousands of thread-like sporozoites, which are released into the body cavity when it bursts.
4. Invasion of the salivary glands. A minority of sporozoites reach and invade the salivary glands. Once there, the mosquito remains infectious for the rest of her life, injecting sporozoites into the skin each time she probes. The number injected per bite varies widely but is usually fewer than a hundred.

Complement-like immunity and Pfs47 evasion

Even in a susceptible Anopheles gambiae mosquito, the great majority of parasites die before forming oocysts, so that of perhaps a thousand gametocytes ingested, only a few oocysts develop. The mosquito's main defence, described by Elena Levashina, Carolina Barillas-Mury, and others, resembles the vertebrate complement system:

• Marking the parasite. When an ookinete crosses a gut cell, the damaged cell produces nitric oxide and related enzymes, which chemically modify (nitrate) proteins on the parasite surface, marking it for destruction.
• The TEP1 complex. Thioester-containing protein 1 (TEP1) circulates in the mosquito's blood and is structurally similar to vertebrate complement component 3. It is held in a stable form by two partner proteins, LRIM1 and APL1C. When it encounters a marked ookinete, TEP1 binds covalently to the parasite surface, leading either to lysis or to the parasite being encased in melanin.
• Evasion by African parasites. African P. falciparum strains carry a particular variant of the surface protein Pfs47, which interacts with a receptor on the mosquito gut and suppresses the nitration response. As a result, TEP1 fails to recognize the parasite. Strains from South America or Asia carry different Pfs47 variants adapted to other mosquito species, and are efficiently killed when fed to African An. gambiae of a resistant genetic type.

Lymphatic filariasis and O'nyong'nyong virus

In rural Sub-Saharan Africa, members of the An. gambiae complex are the main vectors of Wuchereria bancrofti, while Culex quinquefasciatus predominates in polluted urban areas.

• Nocturnal periodicity. The microfilariae produced by adult worms remain in the lungs during the day and enter the peripheral blood between about 22:00 and 04:00, matching the biting period of An. gambiae. Ingested microfilariae cross the gut wall, develop in the flight muscles over about two weeks, and then move to the proboscis, from which they enter the skin when the mosquito next feeds.
• Facilitation. The well-developed cibarial teeth of Anopheles damage and kill many of the microfilariae as blood is pumped into the gut. Culex has a less developed armature and transmits efficiently even at low microfilarial densities, a pattern called limitation. In Anopheles, by contrast, transmission becomes disproportionately less efficient as densities fall, a pattern called facilitation. This gives filariasis transmitted by Anopheles a relatively high threshold below which it cannot be sustained, which is one reason why mass drug administration combined with bednets has interrupted transmission in several African countries.

Anopheles mosquitoes do not transmit most arboviruses, including dengue, yellow fever, Zika, and chikungunya. The exception is O'nyong'nyong virus, for which An. gambiae and An. funestus are the main vectors.

• Relationship to chikungunya. The virus, whose name comes from an Acholi phrase describing severe joint pain, is an alphavirus closely related to chikungunya virus, with which it shares about 85% of its genome sequence and the same human cell receptor.
• Epidemic history. First isolated at Gulu in northern Uganda in 1959, it spread across East, Central, and West Africa between 1959 and 1962 in one of the largest recorded arbovirus epidemics, with an estimated two million infections. Symptoms include fever, joint pain, rash, and swollen lymph nodes; no deaths have been reported. The virus re-emerged in southern Uganda in 1996 and 1997, when surveys found that a large proportion of people in affected villages had been infected, and in Côte d'Ivoire in 2003. Serological surveys indicate continuing circulation.
• Why it infects Anopheles. Experiments exchanging genes between chikungunya and O'nyong'nyong viruses indicate that the ability to infect An. gambiae depends on parts of the nsP3 protein and the structural genes.

Vectorial capacity and the Ross-Macdonald framework

The mathematical framework for malaria transmission by Anopheles gambiae was developed by Ronald Ross and George Macdonald. Two quantities are central. The vectorial capacity V is the number of infectious bites that would eventually arise from all the mosquitoes feeding on a single infectious person in one day. The basic reproduction number R₀ is the number of new human infections arising from one infected person in a fully susceptible population:

where:

• m is the number of female mosquitoes per person;
• a = Q_h f is the number of human bites per mosquito per day, the product of the proportion of blood meals taken from humans (the human blood index Q_h) and the feeding frequency f, usually one meal every two to three days. It appears squared because a mosquito must bite once to become infected and again to transmit;
• p = e^−_μ_ is the probability of surviving one day, where μ is the daily mortality rate and 1/_μ_ the average adult lifespan;
• n is the extrinsic incubation period, the days required for the parasite to reach the salivary glands;
• b and c are the probabilities of transmission from mosquito to human and from human to mosquito, and 1/_r_ is the duration of human infectiousness, which for untreated P. falciparum is several months.

From this, Macdonald derived two quantities that explain why An. gambiae is so effective:

• Stability index. The number of human bites a female delivers in her lifetime is S = _a_/_μ_. For a mosquito that feeds mainly on cattle (Q_h ≈ 0.05, f = 0.33 per day, μ = 0.22), S is less than 0.1, giving unstable malaria that is sensitive to control. For An. gambiae and An. coluzzii (Q_h ≈ 0.95, f ≈ 0.33 to 0.5 per day, μ ≈ 0.11 to 0.13), S is about 2.5 to 4.5, giving stable, year-round transmission. Because a enters R₀ squared, a twenty-fold difference in the human blood index translates into a several-hundred-fold difference in transmission. Combined with the long duration of untreated human infection, this can push R₀ in rural African villages into the hundreds.
• Entomological inoculation rate. Transmission intensity is measured in the field as the number of infectious bites a person receives per year:

EIR = 365 × HBR × SR where HBR is the nightly human biting rate, measured by landing catches or light traps, and SR is the proportion of captured mosquitoes with sporozoites in their salivary glands. Annual values across endemic Africa range from about 5 to more than 500 infectious bites per person. Because infection in humans saturates at high exposure, substantial reductions in prevalence require reducing the annual rate to below about one infectious bite per person.

Macdonald also showed that the two main approaches to control differ greatly in effect. Mosquito density m enters R₀ in direct proportion, so halving larval habitat halves R₀. Adult survival, by contrast, enters both as 1/_μ_ and in the exponential term e^−_μn_, because a mosquito must survive the whole incubation period to transmit. Measures that kill adult females, such as indoor spraying and insecticide-treated nets, therefore reduce transmission far more than their effect on mosquito numbers alone would suggest.

Nonlinear thermal responses

For decades, malaria models for Anopheles gambiae used a simple relation for the extrinsic incubation period derived by Detinova and Moshkovsky:

This predicts an incubation period of about 28 days at 20 °C, 11 days at 26 °C, and 7 days at 32 °C. Because it shortens continuously as temperature rises, models based on it suggested that transmission would be greatest at 31 to 33 °C.

In 2013, Erin Mordecai, Krijn Paaijmans, Leah Johnson, Matthew Thomas, Sadie Ryan, and colleagues replaced this assumption with measured, hump-shaped temperature responses for each trait of both mosquito and parasite. Combining them gives transmission potential as:

where EFD is eggs laid per female per day, p_EA is survival from egg to adult, MDR is the mosquito development rate, and PDR is the parasite development rate (1/_n_).

Temperature responses of An. gambiae and P. falciparum

Daily temperature swings. Work by Paaijmans, Thomas, and colleagues showed that daily fluctuations of 5 to 12 °C around the mean have opposite effects at the two ends of the curve. In cool highlands with mean temperatures of 16 to 19 °C, afternoon warmth lifts the mosquito above the threshold for several hours a day, allowing transmission at lower mean temperatures than constant-temperature experiments predict. In hot lowlands with means of 27 to 30 °C, afternoon peaks push the mosquito past its upper limit, slowing parasite development and increasing mortality. Mosquitoes resting inside mud-walled houses, which are cooler by day and warmer by night than the outside air, are partly buffered from these extremes.

Climate change, deforestation, and elevation shifts

Because transmission by Anopheles gambiae peaks near 25 °C, warming is expected to have opposite effects in different parts of Africa. Historical records indicate that African Anopheles vectors have been moving to higher elevations and higher latitudes over the past century.

• Highlands. More than a hundred million people live in the cool highlands of Ethiopia, Kenya, Uganda, Rwanda, Burundi, Tanzania, eastern DR Congo, and Madagascar, much of which lay below the temperature threshold for transmission. Because temperature falls by about 6.5 °C per kilometre of altitude, warming of 1.5 to 2.5 °C moves that threshold several hundred metres upslope. Work by Mercedes Pascual, Amir Siraj, and colleagues showed that in warmer years, malaria extends to higher altitudes in the Ethiopian and Colombian highlands, where populations have little acquired immunity.
• Deforestation. Clearing highland forest and swamp vegetation for farming raises water temperatures in pools by several degrees and indoor temperatures by one to two degrees. Studies in western Kenya found that this local warming shortens larval development from about three weeks to about ten days, increases larval survival several-fold, and roughly halves the parasite's incubation period, greatly increasing transmission.
• Hot lowlands. In lowland West Africa and the Sahel, where mean temperatures are already 27 to 30 °C, further warming moves conditions past the optimum. Suitability for An. gambiae is expected to decline, with a shift toward the more heat- and drought-tolerant An. arabiensis. In cities, the invasive Asian species Anopheles stephensi, which has spread through the Horn of Africa since 2012 and reached West Africa, breeds in water tanks and wells and has a somewhat higher temperature optimum.
• Floods and cyclones. In southeastern Africa, intense tropical cyclones such as Idai in 2019 and Freddy in 2023 leave extensive standing water as floods recede, producing surges in mosquito numbers and malaria epidemics.

Entomological surveillance and molecular identification

Because the species of the Anopheles gambiae complex differ in behaviour and insecticide resistance but look identical, surveillance combines field collection with laboratory identification.

• Collecting adults. The traditional method for measuring biting rates is the human landing catch, in which collectors catch mosquitoes as they land. To avoid exposing collectors, programmes increasingly use light traps placed beside an occupied bednet, double-net traps, odour-baited traps, and aspiration of resting mosquitoes from houses in the morning.
• Identifying species and estimating age. DNA from a single leg is used in a standard PCR test that separates the main species, together with a second test that distinguishes An. coluzzii from An. gambiae. Dissecting the ovaries shows whether a female has laid eggs before, which allows daily survival to be estimated. Infrared spectroscopy combined with machine learning is increasingly used to identify species and estimate age rapidly.
• Detecting infection. The head and thorax are tested separately from the abdomen, so that parasites still in the gut are not counted, using an antibody test for circumsporozoite protein or PCR. This gives the sporozoite rate, from which the entomological inoculation rate is calculated.

Mechanisms of insecticide resistance

Between 2000 and 2015, the scale-up of insecticide-treated nets and indoor residual spraying against Anopheles gambiae is estimated to have prevented hundreds of millions of malaria cases, with nets accounting for most of the effect. Because nets relied almost entirely on a single class of insecticide, the pyrethroids, and because the same compounds are used in agriculture, resistance has become widespread. Work by Hilary Ranson, Janet Hemingway, Charles Wondji, Martin Donnelly, Victoria Ingham, and others has identified five mechanisms:

Mechanisms of insecticide resistance in the An. gambiae complex

Dual-active nets and chemical interventions

To address resistance and outdoor transmission in Anopheles gambiae, several new approaches have been introduced since about 2018.

1. Nets with two active ingredients.

• Pyrethroid–PBO nets combine a pyrethroid with piperonyl butoxide, which blocks the detoxifying enzymes and restores the insecticide's effect.
• Pyrethroid–chlorfenapyr nets combine a pyrethroid with chlorfenapyr, which is inactive until the mosquito's own detoxifying enzymes convert it into a compound that disrupts energy production in the flight muscles. Mosquitoes with high levels of those enzymes therefore activate it more readily, so resistance to pyrethroids does not protect against it. In trials in Tanzania and Benin, these nets reduced malaria in children by around 40 to 50% compared with standard nets, and the World Health Organization now recommends them.
• Pyrethroid–pyriproxyfen nets combine a pyrethroid with an insect growth regulator that sterilizes surviving females.

1. New insecticides for indoor spraying. Programmes have moved away from pyrethroids toward the organophosphate pirimiphos-methyl, the neonicotinoid clothianidin, and broflanilide, which acts on the insect GABA receptor at a site different from that affected by existing resistance mutations.
2. Drugs and baits for outdoor transmission.

• Ivermectin. When people or cattle take ivermectin, mosquitoes that feed on them are killed, regardless of where or when they bite. This has been tested in several large trials.
• Attractive targeted sugar baits. Because mosquitoes of both sexes feed on plant nectar, bait stations containing sugar solution mixed with an insecticide can kill mosquitoes outdoors. They are designed to exclude bees and other pollinators.

1. Larval control and house improvement. Bacterial larvicides such as Bacillus thuringiensis israelensis can be applied to identified breeding sites. Screening the open gaps under roofs, through which most mosquitoes enter traditional houses, is also effective, as are tubes fitted into those gaps and treated with insecticide or with fungi that kill mosquitoes.

Gene drives and microbial symbionts

Genetic and biological strategies targeting Anopheles gambiae are advancing in laboratory and semi-field evaluations:

1. Gene drives. The idea of using a selfish genetic element to suppress a mosquito population was proposed by Austin Burt in 2003 and developed in An. gambiae by Andrea Crisanti, Tony Nolan, and the Target Malaria consortium. Early versions targeting fertility genes failed because mutations arose that blocked the drive while preserving gene function. In 2018, Kyrou and colleagues instead targeted a sequence at a splice junction of the sex-determination gene doublesex. Because any change to this sequence itself causes sterility in females, resistant variants cannot spread. Females carrying two copies of the modified gene develop as sterile intersex individuals that cannot bite or lay eggs, while males and females with one copy are fertile and pass it to nearly all their offspring instead of half. In laboratory cages, introducing the drive at a modest frequency eliminated the population within about ten generations. Whether this would occur in the field, where populations are vastly larger and more structured, remains to be tested, and some later experiments have produced resistant variants.
2. Modifying rather than suppressing populations. An alternative approach leaves mosquito numbers unchanged but spreads genes that block the parasite, such as antibodies against parasite surface proteins or antimicrobial peptides expressed in the gut and salivary glands.
3. Microbial symbionts. Two naturally occurring microbes have attracted interest because they do not involve genetic modification.

• Microsporidia MB is a symbiont found in wild Anopheles near Lake Victoria. It is passed from mother to offspring and during mating, appears to impose little cost on the mosquito, and in initial studies no infected mosquito carried the malaria parasite, though the numbers examined were small.
• Delftia tsuruhatensis strain TC1 is a harmless gut bacterium that colonizes mosquitoes after a single exposure. It produces the compound harmane, which blocks parasite development in the gut, reducing infection substantially in laboratory experiments.

How does the 2La chromosomal inversion help *Anopheles gambiae* adapt to arid environments?

The 2La paracentric inversion spans approximately 22 megabases on the left arm of chromosome 2 in *Anopheles gambiae* and suppresses meiotic recombination in heterokaryotypes, locking hundreds of co-adapted genes into a single inherited supergene. Homozygotes for the inverted 2La arrangement exhibit higher expression of heat-shock proteins, thicker epicuticular hydrocarbon layers that slow transcuticular water loss, and increased exophilic resting behavior, allowing persistence across the arid Sahel.

How does African *Plasmodium falciparum* evade the TEP1 complement-like immune system of *Anopheles gambiae*?

When a *Plasmodium* ookinete traverses the *Anopheles gambiae* midgut epithelium, damaged host cells normally upregulate nitric oxide synthase and peroxidases that nitrate parasite surface proteins, targeting the ookinete for covalent binding and lysis by thioester-containing protein 1 (TEP1). However, sympatric African *Plasmodium falciparum* strains express specific alleles of the 6-cysteine surface protein Pfs47 that bind a mosquito midgut receptor (P47Rec) and suppress epithelial nitration, rendering the parasite invisible to TEP1.

How do CRISPR-Cas9 gene drives targeting *doublesex* suppress *Anopheles gambiae* populations?

Engineered homing endonuclease gene drives in *Anopheles gambiae* express Cas9 and a guide RNA targeting the highly conserved intron 4–exon 5 splice junction of the sex-determination gene *doublesex* (*dsx*). During germline meiosis, Cas9 cleaves the wild-type allele and copies the drive cassette via homology-directed repair, converting heterozygotes to homozygotes in over 95% of gametes; homozygous females fail to splice the female-specific *dsxF* isoform and develop as sterile intersex mosquitoes unable to bite or oviposit.

• Malaria
• Mosquito-borne disease
• Gene drive
• Insecticide-treated net
• Vector control