Anopheles
Anopheles (/əˈnɒfɪliːz/) is a genus of mosquito first described by J. W. Meigen in 1818, and are known as nail mosquitoes and marsh mosquitoes.[1] Many such mosquitoes are vectors of the parasite Plasmodium, a genus of protozoans that cause malaria in birds, reptiles, and mammals, including humans. The Anopheles gambiae mosquito is the best-known species of marsh mosquito that transmits the Plasmodium falciparum, which is a malarial parasite deadly to human beings; no other mosquito genus is a vector of human malaria.
The genus Anopheles diverged from other mosquitoes approximately 100 million years ago (mya), and, like other mosquitoes, the eggs, larvae, and pupae are aquatic. The Anopheles larva has no respiratory siphon through which to breathe, so it breathes and feeds with its body horizontal to the surface of the water. The adult mosquito hatches from the surface and feeds on the nectar of flowers; the female mosquito also feeds on blood, which animal diet allows them to carry and transmit parasites between hosts. The adult's feeding position is head-down, unlike the horizontal stance of the culicines. Anopheles are distributed almost worldwide, throughout the tropics, the subtropics, and the temperate regions of planet Earth. In hot weather, adult Anopheles aestivate, which is a state of dormancy that enables the mosquito to survive in hot dry regions, such as the Sahel.
Evolution[edit]
Fossil history[edit]
Fossils of the genus Anopheles are rare; only two had been found by 2015.[2] They are Anopheles (Nyssorhynchus) dominicanus Zavortink & Poinar in Dominican Republic amber from the Late Eocene (40.4 million years ago to 33.9 million years ago),[3] and Anopheles rottensis Statz in German amber from the Late Oligocene (28.4 million years ago to 23 million years ago).[2]
Phylogeny[edit]
The ancestors of all flies including mosquitoes appeared 260 million years ago.[4] The culicine and Anopheles clades of mosquitoes diverged between 120 million years ago and 150 million years ago.[4][5] The Old and New World Anopheles species subsequently diverged between 80 million years ago and 95 million years ago.[4][5] Anopheles darlingi diverged from the African and Asian malaria vectors ~100 million years ago.[6] The cladogram is based on an analysis of mosquito genomes by Heafsey and colleagues in 2015:[6]
Anopheles larva
Feeding position of an Anopheles larva (A), culicine larva with its siphon (B)
Ecology[edit]
Distribution[edit]
Anopheles species live both in tropical areas known for malaria such as sub-Saharan Africa, and in colder latitudes. Malaria outbreaks have in the past occurred in colder climates, for example during the construction of the Rideau Canal in Canada during the 1820s.[19] Anopheles species that can transmit malaria are not limited to malaria-endemic areas, so areas where they have been eliminated are constantly at risk of reintroduction of the disease.[20]
Malaria vectors[edit]
Preferred sources for blood meals[edit]
Since the genus Anopheles is the sole vector for malaria, it has been studied intensively in the search for effective control methods. One important behavioral factor is the degree to which an Anopheles species prefers to feed on humans (anthropophily) or animals such as cattle or birds (zoophily). Anthropophilic Anopheles are more likely to transmit the malaria parasites from one person to another. Most Anopheles are not exclusively anthropophilic or zoophilic, including the primary malaria vector in the western United States, A. freeborni.[30][31] However, the primary malaria vectors in Africa, A. gambiae and A. funestus, are strongly anthropophilic and are consequently major vectors of human malaria.[16]
Probability of transmitting malaria[edit]
Once ingested by a mosquito, malaria parasites must undergo development within the mosquito before they are infectious to humans. The time required for the parasite to develop in the mosquito (the extrinsic incubation period) ranges from 10–21 days, depending on the parasite species and the temperature. If a mosquito does not survive long enough for the parasite to develop, then she transmits no parasites.[16]
It is not possible to measure directly the lifespans of mosquitoes in nature, but indirect estimates of daily survivorship have been made for several Anopheles species. Estimates of daily survivorship in Tanzania of A. gambiae, the vector of the dangerous Plasmodium falciparum parasite, ranged from 0.77 to 0.84, meaning that after one day, between 77% and 84% have survived.[32] Assuming this survivorship is constant through the adult life of a mosquito, less than 10% of female A. gambiae would survive longer than a 14-day extrinsic incubation period. If daily survivorship increased to 0.9, over 20% of mosquitoes would survive longer than the same period. Control measures that rely on insecticides (e.g. indoor residual spraying) may actually impact malaria transmission more through their effect on adult longevity than through their effect on the population of adult mosquitoes.[16]
Patterns of feeding and resting[edit]
Most Anopheles are crepuscular (active at dusk or dawn) or nocturnal (active at night). Some feed indoors (endophagic), while others feed outdoors (exophagic). After feeding, some blood mosquitoes prefer to rest indoors (endophilic), while others prefer to rest outdoors (exophilic). Biting by nocturnal, endophagic Anopheles can be markedly reduced through the use of insecticide-treated bed nets or through improved housing construction to prevent mosquito entry (e.g. window screens). Endophilic mosquitoes are readily controlled by indoor spraying of residual insecticides. In contrast, exophagic/exophilic vectors are best controlled by destroying breeding sites, such as by filling in ponds.[16]
Gut flora[edit]
Because transmission of disease by the mosquito requires ingestion of blood, the gut flora may have a bearing on the success of infection of the mosquito host. The larval and pupal gut is largely colonized by photosynthetic cyanobacteria, while in the adult, gram-negative bacteria in the Pseudomonadota and Bacteroidota phyla predominate. Blood meals drastically reduce the diversity of microorganisms in the gut, favouring bacteria.[33]
Control[edit]
Insecticide control and resistance[edit]
Insecticides have offered a first line of approach to ridding areas of malarial mosquitoes. However, mosquitoes, with a short generation time, may rapidly evolve resistance, as experienced during the Global Malaria Eradication Campaign of the 1950s.[34] The use of insecticides in agriculture has resulted in resistance in mosquito populations, implying that an effective control program must monitor for resistance and switch to other means if resistance is detected.[35]
Eradication[edit]
In 2016, a CRISPR-Cas9 gene drive system was proposed to eradicate Anopheles gambiae,[36] by deleting the dsx gene, causing female sterility. Such a gene drive system has been shown to suppress an entire caged A. gambiae population[37] within 7–11 generations, typically less than a year. This has raised concerns with both the efficiency of a gene drive system as well as the ethical and ecological impact of such an eradication program.[38] Therefore, there have been efforts to use the gene drive system to more efficiently introduce genes of Plasmodium resistance into the species, such as targeting and knocking out the FREP1 gene in Anopheles gambiae.[39] Researchers in Burkina Faso have created a strain of the fungus Metarhizium pinghaense that is genetically engineered to produce the venom of an Australian funnel-web spider; exposure to the fungus caused populations of Anopheles to crash by 99% in a controlled trial.[40]