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Anopheles

Anopheles

Overview
Anopheles is a genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of mosquito
Mosquito
Mosquitoes are members of a family of nematocerid flies: the Culicidae . The word Mosquito is from the Spanish and Portuguese for little fly...

. There are approximately 460 recognized species: while over 100 can transmit human malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

, only 30–40 commonly transmit parasites of the genus Plasmodium
Plasmodium
Plasmodium is a genus of parasitic protists. Infection by these organisms is known as malaria. The genus Plasmodium was described in 1885 by Ettore Marchiafava and Angelo Celli. Currently over 200 species of this genus are recognized and new species continue to be described.Of the over 200 known...

, which cause malaria in humans in endemic
Endemic (ecology)
Endemism is the ecological state of being unique to a defined geographic location, such as an island, nation or other defined zone, or habitat type; organisms that are indigenous to a place are not endemic to it if they are also found elsewhere. For example, all species of lemur are endemic to the...

 areas. Anopheles gambiae
Anopheles gambiae
Anopheles gambiae is a complex of at least seven morphologically distinguishable species of mosquitoes in the genus Anopheles. This complex was recognised in the 1960s and includes the most important vectors of malaria in sub-Saharan Africa and the most efficient malaria vectors known.This species...

 is one of the best known, because of its predominant role in the transmission of the most dangerous malaria parasite species – Plasmodium falciparum
Plasmodium falciparum
Plasmodium falciparum is a protozoan parasite, one of the species of Plasmodium that cause malaria in humans. It is transmitted by the female Anopheles mosquito. Malaria caused by this species is the most dangerous form of malaria, with the highest rates of complications and mortality...

.

The name comes from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 , , meaning not, and , , meaning profit, and translates to useless.

Some species of Anopheles also can serve as the vectors for canine heartworm
Heartworm
Heartworm is a parasitic roundworm that is spread from host to host through the bites of mosquitoes. The heartworm is a type of filaria, a small thread-like worm. The definitive host is the dog but it can also infect cats, wolves, coyotes, foxes and other animals, such as ferrets, sea lions and...

 Dirofilaria immitis, the Filariidae Wuchereria bancrofti
Wuchereria bancrofti
Filaria, is a parasitic filarial nematode spread by a mosquito vector. It is one of the three parasites that cause lymphatic filariasis, an infection of the lymphatic system by filarial worms. It affects over 120 million people, primarily in Africa, South America, and other tropical and...

 and Brugia malayi
Brugia malayi
Brugia malayi is a nematode , one of the three causative agents of lymphatic filariasis in humans. Lymphatic filariasis, also known as elephantiasis, is a condition characterized by swelling of the lower limbs. The two other filarial causes of lymphatic filariasis are Wuchereria bancrofti and...

, and virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es such as one that causes O'nyong'nyong fever.
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Encyclopedia
Anopheles is a genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of mosquito
Mosquito
Mosquitoes are members of a family of nematocerid flies: the Culicidae . The word Mosquito is from the Spanish and Portuguese for little fly...

. There are approximately 460 recognized species: while over 100 can transmit human malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

, only 30–40 commonly transmit parasites of the genus Plasmodium
Plasmodium
Plasmodium is a genus of parasitic protists. Infection by these organisms is known as malaria. The genus Plasmodium was described in 1885 by Ettore Marchiafava and Angelo Celli. Currently over 200 species of this genus are recognized and new species continue to be described.Of the over 200 known...

, which cause malaria in humans in endemic
Endemic (ecology)
Endemism is the ecological state of being unique to a defined geographic location, such as an island, nation or other defined zone, or habitat type; organisms that are indigenous to a place are not endemic to it if they are also found elsewhere. For example, all species of lemur are endemic to the...

 areas. Anopheles gambiae
Anopheles gambiae
Anopheles gambiae is a complex of at least seven morphologically distinguishable species of mosquitoes in the genus Anopheles. This complex was recognised in the 1960s and includes the most important vectors of malaria in sub-Saharan Africa and the most efficient malaria vectors known.This species...

 is one of the best known, because of its predominant role in the transmission of the most dangerous malaria parasite species – Plasmodium falciparum
Plasmodium falciparum
Plasmodium falciparum is a protozoan parasite, one of the species of Plasmodium that cause malaria in humans. It is transmitted by the female Anopheles mosquito. Malaria caused by this species is the most dangerous form of malaria, with the highest rates of complications and mortality...

.

The name comes from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 , , meaning not, and , , meaning profit, and translates to useless.

Some species of Anopheles also can serve as the vectors for canine heartworm
Heartworm
Heartworm is a parasitic roundworm that is spread from host to host through the bites of mosquitoes. The heartworm is a type of filaria, a small thread-like worm. The definitive host is the dog but it can also infect cats, wolves, coyotes, foxes and other animals, such as ferrets, sea lions and...

 Dirofilaria immitis, the Filariidae Wuchereria bancrofti
Wuchereria bancrofti
Filaria, is a parasitic filarial nematode spread by a mosquito vector. It is one of the three parasites that cause lymphatic filariasis, an infection of the lymphatic system by filarial worms. It affects over 120 million people, primarily in Africa, South America, and other tropical and...

 and Brugia malayi
Brugia malayi
Brugia malayi is a nematode , one of the three causative agents of lymphatic filariasis in humans. Lymphatic filariasis, also known as elephantiasis, is a condition characterized by swelling of the lower limbs. The two other filarial causes of lymphatic filariasis are Wuchereria bancrofti and...

, and virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es such as one that causes O'nyong'nyong fever. There is an association of brain tumor
Brain tumor
A brain tumor is an intracranial solid neoplasm, a tumor within the brain or the central spinal canal.Brain tumors include all tumors inside the cranium or in the central spinal canal...

 incidence and malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

, suggesting that the Anopheles might transmit a virus or other agent that could cause a brain tumor.

Mosquitoes in other genera (Aedes
Aedes
Aedes is a genus of mosquito originally found in tropical and subtropical zones, but now found on all continents excluding Antarctica. Some species have been spread by human activity. Aedes albopictus, a most invasive species was recently spread to the New World, including the U.S., by the used...

, Culex
Culex
Culex is a genus of mosquito, and is important in that several species serve as vectors of important diseases, such as West Nile virus, filariasis, Japanese encephalitis, St. Louis encephalitis and avian malaria....

) can also serve as vectors of disease agents.

Evolution


The ancestors of Drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...

 and Anopheles diverged ago. The culicine
Culicinae
Culicinae is the most extensive subfamily of mosquitos divided into two tribes: Culicini and Sabethini.-External links:*...

 and Anopheles clades diverged between and . The Old and New World Anopheles species subsequently diverged between and . The Anopheles gambiae
Anopheles gambiae
Anopheles gambiae is a complex of at least seven morphologically distinguishable species of mosquitoes in the genus Anopheles. This complex was recognised in the 1960s and includes the most important vectors of malaria in sub-Saharan Africa and the most efficient malaria vectors known.This species...

 and Anopheles funestus clades diverged . A molecular study of several genes in seven species has provided additional support for an expansion of this genus during the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 period.

The Anopheles genome (230–284 Mb) is comparable in size to that of Drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...

 but considerably smaller than those found in other culicine genomes (528 Mb–1.9 Gb). Like most culicine species, the genome is diploid with six chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

s.

The only known fossils of this genus are those of Anopheles (?Nyssorhynchus) dominicanus Zavortink & Poinar contained in Dominican amber from the Late Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

  and Anopheles rottensis Statz contained in German amber from the Late Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...

 .

Systematics



The genus Anopheles belongs to a subfamily Anophelinae with three genera: Anopheles Meigen (nearly worldwide distribution), Bironella Theobald (Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 only) and Chagasia Cruz (Neotropics). Bironella appears to be the sister taxon to the Anopheles with Chagasia forming the outgroup in this subfamily.

The classification of this genus began in 1901 with Theobald. Despite the passage of time the taxonomy remains incompletely settled. Classification into species is based on morphological characteristics - wing spots, head anatomy, larval and pupal anatomy, chromosome structure - and more recently on DNA sequences.

The genus itself has been subdivided into seven subgenera based primarily on the number and positions of specialized setae on the gonocoxites of the male
Male
Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...

 genitalia. The system of subgenera originated with the work of Christophers who in 1915 described three subgenera: Anopheles (widely distributed), Myzomyia (later renamed Cellia) (Old World
Old World
The Old World consists of those parts of the world known to classical antiquity and the European Middle Ages. It is used in the context of, and contrast with, the "New World" ....

) and Nyssorhynchus
Nyssorhynchus
Nyssorhynchus is an extinct subgenus of the genus Plasmodium.There is one known species in this genus - Plasmodium dominicana. It was identified from Dominican amber....

 (Neotropical). Nyssorhynchus was first described as Lavernia by Theobald. Edwards in 1932 added the subgenus Stethomyia
Stethomyia
Stethomyia is a subgenus of the mosquito genus Anopheles. The type species is Anopheles nimbus Theobald 1902. There are five species in this subgenus.-Phylogeny:...

 (Neotropical distribution). Kerteszia was also described by Edwards in 1932 but then recognised as a subgrouping of Nyssorhynchus. It was elevated to subgenus status by Komp in 1937 and it is also found in the Neotropics. Two additional subgenera have since been recognised: Baimaia (Southeast Asia only) by Harbach et al. in 2005 and Lophopodomyia (Neotropical) by Antunes in 1937.

Within the genus Anopheles there are two main groupings: one formed by the Cellia and Anopheles subgenera and a second by Kerteszia, Lophopodomyia and Nyssorhynchus. Subgenus Stethomyia is an outlier with respect to these two taxa. Within the second group Kerteszia and Nyssorhynchus appear to be sister taxa.

The number of species currently recognised within the subgenera is given here in parentheses: Anopheles (206 species), Baimaia (1), Cellia (216), Kerteszia (12), Lophopodomyia (6), Nyssorhynchus (34) and Stethomyia (5).

Taxonomic units between subgenus and species are not currently recognised as official zoological names. In practice a number of taxonomic levels have been introduced. The larger subgenera (Anopheles, Cellia and Nyssorhynchus) have been subdivided into sections and series which in turn have been divided into groups and subgroups. Below subgroup but above species level is the species complex. Taxonomic levels above species complex can be distinguished on morphological grounds. Species within a species complex are either morphologically identical or extremely similar and can only be reliably separated by microscopic examination of the chromosomes or DNA sequencing. The classification continues to be revised.

Subgenus Nyssorhynchus has been divided in three sections: Albimanus (19 species), Argyritarsis (11 species) and Myzorhynchella (4 species). The Argyritarsis section has been sub divided into Albitarsis and Argyritarsis groups.

The Anopheles Group was divided by Edwards into four series: Anopheles (worldwide), Myzorhynchus (Palearctic, Oriental, Australasian and Afrotropical), Cycloleppteron (Neotropical) and Lophoscelomyia (Oriental); and two groups, Arribalzagia (Neotropical) and Christya (Afrotropical). Reid and Knight (1961) modified this classification and consequently subdivided the subgenus Anopheles into two sections, Angusticorn and Laticorn and six series. The Arribalzagia and Christya Groups were considered to be series. The Laticorn Section includes the Arribalzagia (24 species), Christya and Myzorhynchus Series. The Angusticorn Section includes members of the Anopheles, Cycloleppteron and Lophoscelomyia Series.

All species known to carry human malaria lie within either the Myzorhynchus or the Anopheles Series.

Life stages


Like all mosquitoes, anophelines go through four stages in their life cycle: egg
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...

, larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

, pupa
Pupa
A pupa is the life stage of some insects undergoing transformation. The pupal stage is found only in holometabolous insects, those that undergo a complete metamorphosis, going through four life stages; embryo, larva, pupa and imago...

, and imago
Imago
In biology, the imago is the last stage of development of an insect, after the last ecdysis of an incomplete metamorphosis, or after emergence from the pupa where the metamorphosis is complete...

. The first three stages are aquatic and last 5–14 days, depending on the species and the ambient temperature. The adult stage is when the female Anopheles mosquito acts as malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

 vector. The adult females can live up to a month (or more in captivity) but most probably do not live more than 1–2 weeks in nature.

Eggs


Adult females lay 50–200 eggs per oviposition. The eggs are quite small (~0.5 × 0.2 mm). Eggs are laid singly and directly on 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.

Larvae



Mosquito larvae have a well-developed head with mouth brushes used for feeding, a large thorax
Thorax (insect anatomy)
The thorax is the mid section of the insect body. It holds the head, legs, wings and abdomen. It is also called mesosoma in other arthropods....

 and a nine segmented abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

. They have no legs. In contrast to other mosquitoes, Anopheles larvae lack a respiratory siphon and for this reason position themselves so that their body is parallel to the surface of the water.

Larvae breathe through spiracle
Spiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...

s located on the 8th abdominal segment and therefore must come to the surface frequently. The larvae spend most of their time feeding on algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, and other microorganisms in the surface microlayer. They dive below the surface only when disturbed. Larvae swim either by jerky movements of the entire body or through propulsion
Marine propulsion
Marine propulsion is the mechanism or system used to generate thrust to move a ship or boat across water. While paddles and sails are still used on some smaller boats, most modern ships are propelled by mechanical systems consisting a motor or engine turning a propeller, or less frequently, in jet...

 with the mouth brushes.

Larvae develop through 4 stages, or instar
Instar
An instar is a developmental stage of arthropods, such as insects, between each molt , until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, or...

s, after which they metamorphose
Metamorphosis
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...

 into pupae. At the end of each instar, the larvae molt, shedding their exoskeleton, or skin, to allow for further growth. 1st stage larvae are ~1 mm in length; 4th stage larvae are normally 5–8 mm in length.

The process from egg laying to emergence of the adult is temperature dependent, with a minimum time of 7 days.

The larvae occur in a wide range of habitats but most species prefer clean, unpolluted water. Larvae of Anopheles mosquitoes have been found in fresh- or salt-water marshes, mangrove swamps, rice fields, grassy ditches, the edges of streams and rivers, and small, temporary rain pools. Many species prefer habitats with vegetation. Others prefer habitats that have none. Some breed in open, sun-lit pools while others are found only in shaded breeding sites in forests. A few species breed in tree holes or the leaf axils of some plants.

--170.235.0.113 (talk) 17:51, 31 October 2011 (UTC)

Pupae


The pupa is comma-shaped when viewed from the side. The head and thorax
Thorax (insect anatomy)
The thorax is the mid section of the insect body. It holds the head, legs, wings and abdomen. It is also called mesosoma in other arthropods....

 are merged into a cephalothorax
Cephalothorax
The cephalothorax is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. The word cephalothorax is derived from the Greek words for head and thorax...

 with the abdomen curving around underneath. As with the larvae, pupae must come to the surface frequently to breathe, which they do through a pair of respiratory trumpets on the cephalothorax. After a few days as a pupa, the dorsal surface of the cephalothorax splits and the adult mosquito emerges.
brooke cherry-flint

Adults


The duration from egg to adult varies considerably among species and is strongly influenced by ambient temperature. Mosquitoes can develop from egg to adult in as little as 5 days but usually take 10–14 days in tropical conditions.

Like all mosquitoes, adult Anopheles have slender bodies with 3 sections: head, thorax and abdomen.

The head is specialized for acquiring sensory information and for feeding. The head contains the eyes and a pair of long, many-segmented antennae
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

. The antennae are important for detecting host odors as well as odors of breeding sites where females lay eggs. The head also has an elongated, forward-projecting proboscis
Proboscis
A proboscis is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In simpler terms, a proboscis is the straw-like mouth found in several varieties of species.-Etymology:...

 used for feeding, and two sensory palps.

The thorax
Thorax (insect anatomy)
The thorax is the mid section of the insect body. It holds the head, legs, wings and abdomen. It is also called mesosoma in other arthropods....

 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
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 for the production of eggs, which gradually fill the abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

.

Anopheles mosquitoes can be distinguished from other mosquitoes by the palps, which are as long as the proboscis, and by the presence of discrete blocks of black and white scales on the wings. Adult Anopheles can also be identified by their typical resting position: males and females rest with their abdomens sticking up in the air rather than parallel to the surface on which they are resting.

Adult mosquitoes usually mate within a few days after emerging from the pupal stage. In most species, the males form large swarm
Swarm
Swarm behaviour, or swarming, is a collective behaviour exhibited by animals of similar size which aggregate together, perhaps milling about the same spot or perhaps moving en masse or migrating in some direction. As a term, swarming is applied particularly to insects, but can also be applied to...

s, usually around dusk, and the females fly into the swarms to mate.

Males live for about a week, feeding on nectar and other sources of sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

. Females will also 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 will rest 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 1–2 weeks in nature. Their lifespan depends on temperature, humidity, and also their ability to successfully obtain a blood meal while avoiding host defenses.

Habitat


Although malaria is nowadays limited to tropical areas, most notoriously regions of sub-Saharan Africa, many Anopheles species live in colder latitudes (see this map from the CDC). Indeed, malaria outbreaks have, in the past, occurred in colder climates, for example during the construction of the Rideau Canal
Rideau Canal
The Rideau Canal , also known as the Rideau Waterway, connects the city of Ottawa, Ontario, Canada on the Ottawa River to the city of Kingston, Ontario on Lake Ontario. The canal was opened in 1832 as a precaution in case of war with the United States and is still in use today, with most of its...

 in Canada during the 1820s. Since then, the Plasmodium parasite (not the Anopheles mosquito) has been eliminated from first world countries.

The CDC warns, however, that "Anopheles that can transmit malaria are found not only in malaria-endemic areas, but also in areas where malaria has been eliminated. The latter areas are thus constantly at risk of re-introduction of the disease."

Susceptibility to become a vector of disease


Some species are poor vectors of malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

, as the parasites do not develop well (or at all) within them. There is also variation within species. In the laboratory, it has been possible to select strains of A. gambiae that are refractory to infection by malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

 parasites. These refractory strains have an immune response that encapsulates and kills the parasites after they have invaded the mosquito's stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

 wall. Scientists are studying the genetic
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

 mechanism for this response. It is hoped that some day, genetically modified mosquitoes that are refractory to malaria can replace wild mosquitoes, thereby limiting or eliminating malaria transmission.

Malaria transmission and control


Understanding the biology and behavior of Anopheles mosquitoes can help understand how malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

 is transmitted and can aid in designing appropriate control strategies. Factors that affect a mosquito's ability to transmit malaria include its innate susceptibility to Plasmodium
Plasmodium
Plasmodium is a genus of parasitic protists. Infection by these organisms is known as malaria. The genus Plasmodium was described in 1885 by Ettore Marchiafava and Angelo Celli. Currently over 200 species of this genus are recognized and new species continue to be described.Of the over 200 known...

, its host choice and its longevity. Factors that should be taken into consideration when designing a control program include the susceptibility of malaria vectors to insecticide
Insecticide
An insecticide is a pesticide used against insects. They include ovicides and larvicides used against the eggs and larvae of insects respectively. Insecticides are used in agriculture, medicine, industry and the household. The use of insecticides is believed to be one of the major factors behind...

s and the preferred feeding and resting location of adult mosquitoes.

On December 21, 2007, a study published in PLoS Pathogens
PLoS Pathogens
PLoS Pathogens is an open-access scientific journal published by the Public Library of Science. It publishes research and reviews on the biology of pathogens and host-pathogen interactions. The 2010 impact factor is 9.079....

 found that the hemolytic C-type lectin
Lectin
Lectins are sugar-binding proteins that are highly specific for their sugar moieties. They play a role in biological recognition phenomena involving cells and proteins. For example, some viruses use lectins to attach themselves to the cells of the host organism during infection...

 CEL-III from Cucumaria echinata, a sea cucumber found in the Bay of Bengal
Bay of Bengal
The Bay of Bengal , the largest bay in the world, forms the northeastern part of the Indian Ocean. It resembles a triangle in shape, and is bordered mostly by the Eastern Coast of India, southern coast of Bangladesh and Sri Lanka to the west and Burma and the Andaman and Nicobar Islands to the...

, impaired the development of the malaria parasite when produced by transgenic A. stephensi. This could potentially be used one day to control malaria by spreading genetically modified mosquitoes refractory to the parasites, although there are numerous scientific and ethical issues to be overcome before such a control strategy could be implemented.

Preferred sources for blood meals


One important behavioral factor is the degree to which an Anopheles species prefers to feed on humans (anthropophily) or animals such as cattle (zoophily). Anthropophilic Anopheles are more likely to transmit the malaria parasites from one person to another. Most Anopheles mosquitoes are not exclusively anthropophilic or zoophilic. However, the primary malaria vectors in Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...

, A. gambiae and A. funestus, are strongly anthropophilic and, consequently, are two of the most efficient malaria vectors in the world.

Once ingested by a mosquito, malaria parasites must undergo development within the mosquito before they are infectious to humans. The time required for development in the mosquito (the extrinsic incubation period
Incubation period
Incubation period is the time elapsed between exposure to a pathogenic organism, a chemical or radiation, and when symptoms and signs are first apparent...

) ranges from 10–21 days, depending on the parasite species and the temperature. If a mosquito does not survive longer than the extrinsic incubation period, then she will not be able to transmit any malaria parasites.

It is not possible to measure directly the life span of mosquitoes in nature. But indirect estimates of daily survivorship have been made for several Anopheles species. Estimates of daily survivorship of A. gambiae in Tanzania
Tanzania
The United Republic of Tanzania is a country in East Africa bordered by Kenya and Uganda to the north, Rwanda, Burundi, and the Democratic Republic of the Congo to the west, and Zambia, Malawi, and Mozambique to the south. The country's eastern borders lie on the Indian Ocean.Tanzania is a state...

 ranged from 0.77 to 0.84, meaning that at the end of one day between 77% and 84% will have survived.

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 a 14-day extrinsic incubation period. Control measures that rely on insecticide
Insecticide
An insecticide is a pesticide used against insects. They include ovicides and larvicides used against the eggs and larvae of insects respectively. Insecticides are used in agriculture, medicine, industry and the household. The use of insecticides is believed to be one of the major factors behind...

s (e.g. indoor residual spraying) may actually impact malaria transmission
Transmission (medicine)
In medicine and biology, transmission is the passing of a communicable disease from an infected host individual or group to a conspecific individual or group, regardless of whether the other individual was previously infected...

 more through their effect on adult longevity than through their effect on the population of adult mosquitoes.

Patterns of feeding and resting


Most Anopheles mosquito
Mosquito
Mosquitoes are members of a family of nematocerid flies: the Culicidae . The word Mosquito is from the Spanish and Portuguese for little fly...

es are crepuscular
Crepuscular
Crepuscular animals are those that are active primarily during twilight, that is during dawn and dusk. The word is derived from the Latin word crepusculum, meaning "twilight." Crepuscular is, thus, in contrast with diurnal and nocturnal behavior. Crepuscular animals may also be active on a bright...

 (active at dusk or dawn) or nocturnal (active at night). Some Anopheles mosquitoes 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), though this can differ regionally based on local vector ecotype, and vector chromosomal makeup, as well as housing type and local microclimatic conditions. Biting by nocturnal, endophagic Anopheles mosquitoes can be markedly reduced through the use of insecticide
Insecticide
An insecticide is a pesticide used against insects. They include ovicides and larvicides used against the eggs and larvae of insects respectively. Insecticides are used in agriculture, medicine, industry and the household. The use of insecticides is believed to be one of the major factors behind...

-treated bed nets (ITNs) 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 through source reduction (destruction of the breeding sites).

Gut flora


Because transmission of disease by the mosquito requires ingestion of blood it is reasonable to presuppose that the gut flora may have a bearing on the success of infection of the mosquito host. This aspect of disease transmission has not been investigated until recently. The larval and pupal gut is largely colonised by photosynthetic cyanobacteria while in the adult Proteobacteria
Proteobacteria
The Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....

 and Bacteroidetes
Bacteroidetes
The phylum Bacteroidetes is composed of three large classes of bacteria that are widely distributed in the environment, including in soil, in sediments, sea water and in the guts and on the skin of animals....

 predominate. Blood meals drastically reduce the diversity of organisms and favor enteric bacteria.

Insecticide resistance


Insecticide
Insecticide
An insecticide is a pesticide used against insects. They include ovicides and larvicides used against the eggs and larvae of insects respectively. Insecticides are used in agriculture, medicine, industry and the household. The use of insecticides is believed to be one of the major factors behind...

-based control measures (e.g. indoor spraying with insecticides, ITNs) are the principal way to kill mosquitoes that bite indoors. However, after prolonged exposure to an insecticide over several generation
Generation
Generation , also known as procreation in biological sciences, is the act of producing offspring....

s, mosquitoes, like other insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s, may develop resistance, a capacity to survive contact with an insecticide. Since mosquitoes can have many generations per year, high levels of resistance can arise very quickly. Resistance of mosquitoes to some insecticides has been documented with just within a few years after the insecticides were introduced. There are over 125 mosquito species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 with documented resistance to one or more insecticides. The development of resistance to insecticides used for indoor residual spraying was a major impediment during the Global Malaria Eradication Campaign. Judicious use of insecticides for mosquito control can limit the development and spread of resistance. However, use of insecticides in agriculture has often been implicated as contributing to resistance in mosquito populations. It is possible to detect developing resistance in mosquitoes and control programs are well advised to conduct surveillance for this potential problem.

Eradication


With substantial numbers of malaria cases affecting people around the globe, in tropical and subtropical regions, especially in sub-Saharan Africa
Sub-Saharan Africa
Sub-Saharan Africa as a geographical term refers to the area of the African continent which lies south of the Sahara. A political definition of Sub-Saharan Africa, instead, covers all African countries which are fully or partially located south of the Sahara...

, where millions of children are killed by this infectious disease, eradication is back on the global health agenda.

Although malaria has existed since old times, its eradication was possible in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

, the Caribbean
Caribbean
The Caribbean is a crescent-shaped group of islands more than 2,000 miles long separating the Gulf of Mexico and the Caribbean Sea, to the west and south, from the Atlantic Ocean, to the east and north...

 and parts of Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

 and southern Central America during the first regional elimination campaigns in the late 1940s. However, the same results were not achieved in sub-Saharan Africa.

Even though the World Health Organization
World Health Organization
The World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health. Established on 7 April 1948, with headquarters in Geneva, Switzerland, the agency inherited the mandate and resources of its predecessor, the Health...

 adopted a formal policy on the control and eradication of the malaria parasite since 1955, it was recently, after the Gates Malaria Forum in October 2007, that key organizations started the debate on the pros and cons of redefining eradication as a goal to control malaria.

Clearly, the cost of preventing malaria is much less than treating the disease, in the long run. However, eradication of mosquito is not an easy task. For effective prevention of malaria, some conditions should be met such as conducive conditions in the country, data collection
Data collection
Data collection is a term used to describe a process of preparing and collecting data, for example, as part of a process improvement or similar project. The purpose of data collection is to obtain information to keep on record, to make decisions about important issues, to pass information on to...

 about the disease, targeted technical approach to the problem, very active and committed leadership, government’s total support, monetary free hand, community involvement, skilled technicians from different fields as well as an adequate implementation.

There is a wide range of strategies to achieve malaria eradication that start from simple steps to complicated strategies which may not be possible to enforce with the current tools.

Although mosquito control is an important component of malaria control strategy, elimination of malaria in an area does not require the elimination of all Anopheles mosquitoes. For instance, in North America and Europe, although the vector Anopheles mosquitoes are still present, the parasite has been eliminated. There are also some socioeconomic improvements (e.g., houses with screened windows, air conditioning
Air conditioning
An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...

) that once combined with vector reduction efforts and effective treatment lead to the elimination of malaria without the complete elimination of the vectors. Some important measures in mosquito control to be followed are: discourage egg laying, prevent development of eggs into larvae and adults, kill the adult mosquitoes, do not allow adult mosquitoes into places of human dwelling, prevent mosquitoes from biting human beings and deny blood meal.

Research in this sense continues, and a study has suggested that sterile mosquitoes might be the answer to malaria elimination. This research suggests that using the sterile insect technique
Sterile insect technique
The sterile insect technique is a method of biological control, whereby overwhelming numbers of sterile insects are released. The released insects are normally male as it is the female that causes the damage, usually by laying eggs in the crop, or, in the case of mosquitoes, taking a bloodmeal from...

 (SIT), in which sexually sterile male insects are released to wipe out a pest population, could be a solution to the problem of malaria in Africa. This technique brings hope, as female mosquitoes only mate once during their lifetimes, and in doing so with sterile male mosquitoes, the insect population would decrease. This is another option to be considered by local and international authorities that may be combined with other methods and tools to achieve malaria eradication in sub-Saharan Africa.

Parasites


A number of parasites of this genus are known to exist including microsporidia
Microsporidia
The microsporidia constitute a phylum of spore-forming unicellular parasites. They were once thought to be protists but are now known to be fungi. Loosely 1500 of the probably more than one million species are named now. Microsporidia are restricted to animal hosts, and all major groups of animals...

 of the genera Amblyospora, Crepidulospora, Senoma and Parathelohania.

Microsporida infecting the aquatic stages of insects, a group that includes mosquitoes and black flies
Black fly
A black fly is any member of the family Simuliidae of the Culicomorpha infraorder. They are related to the Ceratopogonidae, Chironomidae, and Thaumaleidae. There are over 1,800 known species of black flies . Most species belong to the immense genus Simulium...

, and copepods appear to form a distinct clade from those infecting terrestrial insects and fish. There are two distinct life cycles in this group: in the first type the parasite is transmitted by the oral route and is relatively non species specific. 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 is also known to occur.

Few phylogenetic studies of these parasites have been done, and their the relationship to their mosquito hosts is still being determined. One study suggested Parathelohania is an early diverging genus within this group.

The parasite Wolbachia
Wolbachia
Wolbachia is a genus of bacteria which infects arthropod species, including a high proportion of insects , as well as some nematodes. It is one of the world's most common parasitic microbes and is possibly the most common reproductive parasite in the biosphere...

 has been studied for use as a control agent.

See also

  • O'nyong'nyong virus
    O'nyong'nyong virus
    The O'nyong'nyong virus or O'nyong-nyong virus is a virus first isolated by the Uganda Virus Research Institute in Entebbe, Uganda in 1959. It is a togavirus , genus Alphavirus and is closely related to the Chikungunya and Igbo Ora viruses. The name comes from the Nilotic language of Uganda and...

  • Taxonomy of Anopheles
    Taxonomy of Anopheles
    Anopheles is a genus of mosquito . There are approximately 484 recognised species: while over 100 can transmit human malaria, only 30-40 commonly transmit parasites of the genus Plasmodium that cause malaria which affects humans in endemic areas...

  • Tropical disease
    Tropical disease
    Tropical diseases are diseases that are prevalent in or unique to tropical and subtropical regions. The diseases are less prevalent in temperate climates, due in part to the occurrence of a cold season, which controls the insect population by forcing hibernation. Insects such as mosquitoes and...


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