Mosquito Life Cycle

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QUESTION

Please explain the life cycle of a mosquito.

ANSWER

Mosquitoes are insects which go through several life stages, starting out as eggs, then hatching into larvae before developing into adults. Mosquito eggs are laid in slow-moving or stagnant pools of freshwater, to provide suitable habitat for the larvae once they hatch. Different species of mosquito prefer different water conditions; some prefer shaded areas, whereas some prefer more direct sunlight, and some mosquitoes even lay their eggs in very specific habitats, such as brackish water in estuaries. The way the mosquitoes lay their eggs can aid in identifying the genus of mosquito; some genera, such as Culex, lay rafts of many eggs, whereas Anopheles mosquitoes (the ones which transmit malaria to humans) lay their eggs singly. Larvae usually hatch from the eggs after a couple of days. These larvae are predatory, feeding on other aquatic insects and organisms, but themselves can also be eaten by fish, copepods and other creatures. Most larvae lay at an angle to the water surface and breathe through a specialized tube-like organ, known as a siphon, but Anopheles larvae lack the siphon and so much lay parallel to the water’s surface in order to breathe. Each larva must shed its skin (molt) four times, before reaching the stage where it forms a pupa. These four molts take anywhere from 7 to 14 days, depending on the water temperature. The pupa is just like a butterfly pupa – the mosquito does not feed and lays still in a cocoon as it develops into a adult. This process usually takes 2 days, after which the pupa splits and the adult emerges. The length of the full cycle is dependent on whether the conditions were optimal for that species of mosquito, and specifically based on temperature. Male adult mosquitoes usually live for about a week, feeding on nectar – they also possess very bushy antennae for seeking out females to mate with. Female mosquitoes have specialized mouthparts that allow them to feed on blood; they require the extra nutrients that blood provides in order to lay their eggs. The lifespan of a female adult depends on a number of environmental factors, but also her ability to get sufficient blood meals; in nature, they usually live 1-2 weeks.

A schematic of the life cycle is provided below:

Mosquito lifecycle schematic

Schematic of the mosquito lifecycle. Courtesy of Purdue University (Scott Charlesworth): http://extension.entm.purdue.edu/publichealth/insects/mosquito.html

Malaria Mosquito Classification

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QUESTION

What is the classification of mosquito that cause malaria?

ANSWER

The mosquitoes that cause malaria in humans (and indeed also in all other mammals) all belong to the genus Anopheles. They belong to the family Culicidae, which also includes other disease vectors such as Culex and Aedes, which transmit other diseases such as dengue virus, lymphatic filariasis, West Nile virus and Japanese encephalitis, among many others. The Culicidae are part of the Diptera, or the “true flies” which possess a pair of wings and a pair of halteres. The Diptera are part of the class Insecta, which is found within the phylum Arthropoda, in the Kingdom Animalia.

Anopheles Mosquito

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QUESTION

What does Anopheles looks like?

ANSWER

Mosquitoes of the genus Anopheles are responsible for all transmission of malaria in mammals, including to humans. They are night-feeding mosquitoes, usually biting between dusk and dawn, though they may also be active during the day in heavily shaded environments.

Like all mosquitoes, Anopheles are usually found either as freshwater larvae, which prefer stagnant, still water, or flying adults, of which only the females feed on blood (the males exclusively feed on nectar).  Anopheles larvae lie parallel to the surface of the water where they live (in contrast to Aedes and Culex larvae which hang at an angle), whereas the adults rest with their bodies at a 45 degree angle upwards (again in contrast to Aedes and Culex adults, which rest parallel to their resting surface).

Anopheles Mosquito

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QUESTION

Why is it only the female anopheles mosquito alone can cause malaria but not the male anopheles mosquito or any other mosquitoes?

ANSWER

Malaria is actually caused by a single-celled parasite called Plasmodium—it is transmitted via the bite of a female mosquito, of the genus Anopheles, as she takes a blood meal from a human (or other mammal) host. Male mosquitoes do not feed on blood (they only feed on nectar), whereas females need the nutrients from blood in order to produce their eggs; as such, only female Anopheles transmit mosquito.

Why only Anopheles are able to transmit malaria to humans is interesting—birds and reptiles also can get Plasmodium (though different species than those that infect humans and other mammals), and these kinds of malaria can also be transmitted by other kinds of mosquitoes, such as Aedes and Culex. Other closely related blood parasites can even be transmitted by other flying insects, such as sand flies and black flies. However, it is true that only Anopheles can transmit human malaria.

Sterilizing Mosquitoes to Fight Malaria

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QUESTION

Is it possible to breed mosquitoes in the laboratory and then sterilize them and release them into the environment in order to reduce their rate of reproducing.

ANSWER

That is a very good question, and indeed efforts to genetically modify mosquitoes in order to control the various diseases they transmit are underway in many laboratories across the world.

For almost 15 years, scientists have had the ability to modify mosquitoes so that they are sterile. The aim, as you rightly describe, is then to release these sterile mosquitoes into the wild in order to reduce numbers. If the gene that causes sterility can be passed to future offspring, without any reduction in survival of the insect, then the eventual result will be a total population extinction.

To date, many of the major mosquito disease vector species have been successfully genetically modified, though there are many fewer instances of field testing of these modified insects. For example, in 2000/2001, a World Health Organisation-led project in India created sterile mosquitoes of one species of each of the three main disease vector genera: Culex, Aedes and Anopheles, the latter of which acts as vectors for malaria. However, the project did not, in the end, release any of the modified Anopheles vectors into the wild.

While many scientists applaud the benefits of this approach (such as being very species-specific and being more environmentally friendly than spraying), there are also causes for caution. For example, there are concerns that the loss of mosquitoes in the food chain will have a negative impact on animals that rely on them for food. Similarly, if mosquitoes vanish from an ecosystem, their “niche” may be filled by another organism that is equally or even more dangerous and destructive, such as a crop pest or another disease vector. There is also a worry that changing mosquitoes may have unexpected and dangerous effects on the disease itself, for example forcing it to evolve into a more severe disease or changing its epidemiological patterns in ways we cannot predict in advance.

Finally, not all scientists are convinced that the approach will work in the first place—the sterile mosquitoes will have to survive equally well or better than normal mosquitoes in order to establish in the population, and must be equally or more successful at reproducing. As such, while a lot of money is being poured into GM mosquitoes, it is still the center of vigorous debate.

Perhaps the best indication of this controversy came last year, when Oxitec, a British company, released sterile Aedes aegypti mosquitoes on the Cayman Islands. These mosquitoes are the vectors of dengue fever, and so all eyes are on this study to see whether indeed sterile mosquitoes can survive in a population, and if they do, what other effects they will have longer term on the population size of mosquitoes and the rest of the ecosystem. You can read more information about that here: Oxitec: GM Mosquito Factory.

Mosquito Types

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QUESTION

How many types of mosquito are there?

ANSWER

There are over 3,500 species of mosquito! However, most of these do not transmit any diseases to humans. Mosquitoes are usually divided into two sub-families, the Anophelinae and the Culicinae. The latter group consists of about 40 genera, including Culex and Aedes, which contain some species that transmit diseases to humans (such as yellow fever, dengue fever and West Nile). The former contains the genus Anopheles, which are the mosquitoes that transmit malaria. There are about 460 described species of Anopheles mosquito, of which about 100 can transmit malaria, though the vast bulk of transmission is usually limited to about 30 species.

Malaria and Ross River Fever

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QUESTION

Is malaria similar to Ross river Fever which is common in Australia? If you have Ross River and get bitten by a mosquito can the mosquito pass it on to some one else.

ANSWER

Ross River fever is similar to malaria in that both are transmitted by mosquitoes. However, Ross River fever is caused by a virus, whereas malaria is caused by a protozoan (single-celled) parasite. Also, the type of mosquitoes they use are different. Malaria can only be transmitted by mosquitoes of the genus Anopheles, whereas the Ross River fever virus is usually transmitted by Aedes or Culex mosquitoes.

Another difference is that Ross River virus also infects lots of other mammals, with kangaroos and wallabies key reservoir species in the wild, whereas the species of malaria which infect humans are more or less limited to us (though there have been cases of human malaria infecting closely related animals, such as gorillas and chimpanzees).

As for your other question, as far as I know there is no reason why a mosquito infected with Ross River virus couldn’t bite multiple humans or other mammals, and thus transmit the disease to several new hosts.

Evolution of Malaria

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QUESTION

how did malaria evolve?

ANSWER

Malaria in humans probably evolved independently several times, and both times likely due to a cross-over event from a closely related primate malaria. For example, Plasmodium vivax is evolutionarily closely related to several species of malaria found in macaque monkeys in south-east Asia, and so a cross-over of one of those species into human, with subsequent adaptation and speciation, is one hypothesis as to the origin of P. vivax. Conversely, some people argue that P. vivax emerged in Africa, due to the high prevalence of certain genetic traits in African populations (such as the Duffy negative antigen), which protect against P. vivax.

In contrast, P. falciparum is agreed to have emerged in sub-Saharan Africa, and likely in the Congo basin, though the exact source of its origin has been under recent scientific dispute. Until 2010, it was thought that P. falciparum had crossed over to humans from chimpanzees, as chimps are known to be infected with P. reichenowi, a species very closely related to P. falciparum. However, a paper was published in 2010 which had sampled Plasmodium parasites of gorillas and revealed new species of Plasmodium which are even more closely related to P. falciparum, suggesting that the cross-over occurred from gorillas to humans.

As you can see, humans are not the only primates to get malaria; many species of monkey and ape are also susceptible to Plasmodium species, and even lemurs have their own suite of Plasmodium parasites. Among the mammals, rodents also can get malaria, and bats are infected with Hepatocystis, a malaria-like parasite which also infects hippos, primates and rodents. However, no other species of mammal appears to be susceptible to Plasmodium/Hepatocystis, and the reasons for this are not entirely clear.

Plasmodium probably crossed over to mammals from birds or lizards, both of which are infected with a vast number of species of Plasmodium. It is unclear in which of these groups Plasmodium first emerged, though it likely evolved originally from another type of blood-borne parasite called Leucocytozoon, which infects birds and uses blackflies (genus Simulium) as vectors.

A sister group to Plasmodium, called Haemoproteus, also evolved from Leucocytozoon but utilises a variety of different vectors, including mosquitoes, biting midges (Culicoides), louse flies (Hippoboscidae) and tabanids (Tabanidae). Plasmodium, by contrast, exclusively uses mosquitoes as its vectors (apart from one species of lizard Plasmodium, P. mexicanum, which uses sandflies), but while mammalian Plasmodium is only transmitted by Anopheles mosquitoes, bird and lizard Plasmodium can be transmitted by Culex, Aedes, Culiseta, Anopheles, Mansonia and Psorophora. As such, understanding the patterns of vector and host switches within Plasmodium and related taxa can actually provide interesting insights into the genus’ evolutionary history.

What are mosquito larvae?

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QUESTION

What are mosquito larvae?

ANSWER

Larvae are one of the life stages of mosquitoes; they are baby mosquitoes, if you will. Adult mosquitoes lay eggs as a “raft” on the surface of a body of fresh water—they prefer still and stagnant pools. These eggs then hatch into the mosquito larvae, which live in the freshwater pool until they form a pupa, just under the surface. These pupae then hatch into adults again, completing the life cycle.

Mosquito larvae are omnivorous, eating algae and small organisms also living in the water. Despite living immersed in water, they require oxygen to breathe, which they inhale using two different methods: Aedes and Culex mosquitoes (the vectors of a number of diseases, including West Nile disease, dengue fever, yellow fever, encephalitis and filarisasis) have a specialised breathing organ, a bit like a snorkel, called a siphon, which they use to suck in air, whereas Anopheles mosquitoes (the main vectors of malaria) lack this organ and so have to lie next to the surface to take in air. The larvae moult four times while they live in water; after the fourth time, they are ready to pupate and become adults. The entire larval stage of a mosquito’s life usually take between one and two weeks, depending on the ambient temperature.

Screening Mosquito House Entry Points as a Potential Method for Integrated Control of Malaria

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Partial mosquito-proofing of houses with screens and ceilings has the potential to reduce indoor densities of malaria mosquitoes. We wish to measure whether it will also reduce indoor densities of vectors of neglected tropical diseases.

Methodology: The main house entry points preferred by anopheline and culicine vectors were determined through controlled experiments using specially designed experimental huts and village houses in Lupiro village, southern Tanzania. The benefit of screening different entry points (eaves, windows and doors) using PVC-coated fibre glass netting material in terms of reduced indoor densities of mosquitoes was evaluated compared to the control.

Findings: 23,027 mosquitoes were caught with CDC light traps; 77.9% (17,929) were Anopheles gambiae sensu lato, of which 66.2% were An. arabiensis and 33.8% An. gambiae sensu stricto. The remainder comprised 0.2% (50) An. funestus, 10.2% (2359) Culex spp. and 11.6% (2664) Mansonia spp. Screening eaves reduced densities of Anopheles gambiae s. l. (Relative ratio (RR) = 0.91; 95% CI = 0.84, 0.98; P = 0.01); Mansonia africana (RR = 0.43; 95% CI = 0.26, 0.76; P<0.001) and Mansonia uniformis (RR = 0.37; 95% CI = 0.25, 0.56; P<0.001) but not Culex quinquefasciatus, Cx. univittatus or Cx. theileri. Numbers of these species were reduced by screening windows and doors but this was not significant.

Significance: This study confirms that across Africa, screening eaves protects households against important mosquito vectors of filariasis, Rift Valley Fever and O’Nyong nyong as well as malaria. While full house screening is required to exclude Culex species mosquitoes, screening of eaves alone or fitting ceilings has considerable potential for integrated control of other vectors of filariasis, arbovirus and malaria.

Author Summary: Mosquito vectors that transmit filariasis and several arboviruses such as Rift Valley Fever, Chikungunya and O’Nyong nyong as well as malaria co-occur across tropical Africa. These diseases are co-endemic in most rural African countries where they are transmitted by the same mosquito vectors. The only control measure currently in widespread use is mass drug administration for filariasis. In this study, we used controlled experiments to evaluate the benefit of screening the main mosquito entry points into houses, namely, eaves, windows and doors.

This study aims to illustrate the potential of screening specific house openings with the intention of preventing endophagic mosquitoes from entering houses and thus reducing contact between humans and vectors of neglected tropical diseases. This study confirms that while full house screening is effective for reducing indoor densities of Culex spp. mosquitoes, screening of eaves alone has a great potential for integrated control of neglected tropical diseases and malaria.

Citation: Ogoma SB, Lweitoijera DW, Ngonyani H, Furer B, Russell TL, et al. (2010) Screening Mosquito House Entry Points as a Potential Method for Integrated Control of Endophagic Filariasis, Arbovirus and Malaria Vectors. PLoS Negl Trop Dis 4(8): e773. doi:10.1371/journal.pntd.0000773

Editor: Neal D. E. Alexander, London School of Hygiene and Tropical Medicine, United Kingdom

Funding: SBO was supported by a scholarship kindly provided by Valent Bioscience Corporation. This study was also supported by the Centers for Disease Control and Prevention and the United States Agency for International Development through the U.S. President’s Malaria Initiative (Award Number 621-A-00-08-0007-00), the Addessium Foundation (Reenwijk, The Netherlands) and a Research Career Development Fellowship (076806) provided to GFK by the Wellcome Trust. The funders of this study had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Copyright: © 2010 Ogoma et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

More information: Full text: Screening Mosquito House Entry Points as a Potential Method for Integrated Control of Endophagic Filariasis, Arbovirus and Malaria Vectors (PDF)