Anopheles () is a genus of mosquito first described by the German entomologist J. W. Meigen in 1818, and are known as nail mosquitoes and marsh mosquitoes. 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 (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

Fossil history

Fossils of the genus Anopheles are rare; only two had been found by 2015. and Anopheles rottensis <small>Statz</small> in German amber from the Late Oligocene ( to ).

Phylogeny

The ancestors of all flies including mosquitoes appeared . The culicine and Anopheles clades of mosquitoes diverged between and . The Old and New World Anopheles species subsequently diverged between and . The cladogram is based on an analysis of mosquito genomes by Heafsey and colleagues in 2015: The name comes from the Ancient Greek word 'useless', derived from , 'not', 'un-' and 'profit'.

The taxonomy of the genus was greatly advanced in 1901 when the English entomologist Frederick Vincent Theobald described 39 Anopheles species in his 5-volume monograph on the Culicidae. He was provided with mosquito specimens sent in to the British Museum (Natural History) from around the world, on the 1898 instruction of the Secretary of State for the Colonies, Joseph Chamberlain. Classification into species is based on morphological characteristics – wing spots, head anatomy, larval and pupal anatomy, chromosome structure, and more recently, on DNA sequences. In the taxonomy published by Harbach and Kitching in 2016, it was shown that three species of Bironella (B. confusa, B. gracilis, and B. hollandi) are phylogenetically more similar to A. kyondawensis than other Bironella species. That phylogeny argues that, based on genetic similarity, A. implexus is divergent from the common ancestor of Anopheles.

Life cycle

Like all mosquitoes, anophelines go through four stages in their life cycles: egg, larva, pupa, and adult. The first three stages are aquatic and together last 5–14 days, depending on the species and the ambient temperature. The adult stage is when the female Anopheles acts as malaria vector. The adult females can live up to a month (or more in captivity), but most probably do not live more than two weeks in nature.

Eggs

thumb|upright|Anopheles eggs with their distinctive side floats

Adult females lay 50–200 eggs per oviposition. The eggs are quite small (about × ). Eggs are laid singly and directly on to water. They are unique in that they have floats on either side. Eggs are not resistant to drying and hatch within 2–3 days, although hatching may take up to 2–3 weeks in colder climates.

The head has an elongated, forward-projecting proboscis used for feeding, and two maxillary palps. These palps carry the receptors for carbon dioxide, a major attractant that enables the mosquito to locate its host. The thorax is specialized for locomotion. Three pairs of legs and a pair of wings are attached to the thorax. The abdomen is specialized for food digestion and egg development. This segmented body part expands considerably when a female takes a blood meal. The blood is digested over time, serving as a source of protein for the production of eggs, which gradually fill the abdomen. Females feed on sugar sources for energy, but usually require a blood meal for the development of eggs. After obtaining a full blood meal, the female rests for a few days while the blood is digested and eggs are developed. This process depends on the temperature, but usually takes 2–3 days in tropical conditions. Once the eggs are fully developed, the female lays them and resumes host-seeking. The cycle repeats itself until the female dies. While females can live longer than a month in captivity, most do not live longer than one to two weeks in nature. Their lifespans depend on temperature, humidity, and their ability to successfully obtain a blood meal while avoiding host defenses. 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.

thumb|center|upright=3.5|Global distribution of Anopheles species covers both the tropics with numerous malarias, and colder areas not subject to malaria at the start of the 21st century. The adults can however live in dry regions such as Africa's [[savanna and Sahel. They can travel far from water, and are sometimes blown hundreds of kilometres by suitable winds. Adults can aestivate for months at a time, becoming dormant in hot dry weather, allowing them to persist through the African dry season. Further, Anopheles have been documented travelling in baggage, such as on aircraft.

Parasites

Parasites of Anopheles include Microsporidia of the genera Amblyospora, Crepidulospora, Senoma and Parathelohania. Two distinct life cycles are found in the Microsporidia. In the first type, the parasite is transmitted by the oral route and is relatively species nonspecific. In the second, while again the oral route is the usual route of infection, the parasite is ingested within an already infected intermediate host. Infection of the insect larval form is frequently tissue-specific, and commonly involves the fat body. Vertical (transovarial) transmission also occurs.

The parasitic Wolbachia bacteria have been studied for use as control agents.

Predators

The jumping spider Evarcha culicivora indirectly feeds on vertebrate blood by preying on female Anopheles. Juvenile spiders choose the Anopheles over all other prey regardless of whether it actually is carrying blood. Juvenile spiders have adopted an Anopheles-specific prey-capture behavior, using the posture of Anopheles as a primary cue to identify them.

Malaria vectors

Preferred sources for blood meals

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. However, the primary malaria vectors in Africa, A. gambiae and A. funestus, are strongly anthropophilic and are consequently major vectors of human malaria. 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.

Control

alt=Person wearing personal protective equipment performing indoor residual spraying on a house to prevent mosquitoes.|thumb|Person wearing personal protective equipment performing indoor residual spraying on a house to prevent malaria.

Insecticide control and resistance

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. 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.

Eradication

In 2016, a CRISPR-Cas9 gene drive system was proposed to eradicate Anopheles gambiae, by deleting the dsx gene, causing female sterility. Such a gene drive system has been shown to suppress an entire caged A. gambiae population 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. 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. 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.

See also

  • Tropical disease

References

  • Anopheles Database (archived)
  • Anopheles gambiae Genome and Related Data
  • CDC – National Center for Infectious Diseases, Division of Parasitic Diseases; Malaria
  • CDC – World map showing distribution of various Anopheles species (archived)
  • Walter Reed Biosystematics Unit. () – Links to the online mosquito catalog, keys for mosquito identification, images and information on medically important species and much more.
  • Malaria Atlas Project (archived)
  • Anopheles gambiae taxonomy, facts and life cycle (archived)
  • Anopheles quadrimaculatus, common malaria mosquito (archived) on the University of Florida / Institute of Food and Agricultural Sciences Featured Creatures website
  • Anopheles