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topic2b-lifecycle.qmd
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topic2b-lifecycle.qmd
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# Anopheles Mosquito Vector
Malaria is a mosquito-borne parasitic disease that infects humans through the bite of an Anopheline mosquito vector. Of the approximately 70 Anopheles species that are able to transmit malaria to humans, an estimated 30--40 are dominant vector species, and are therefore of relevance to public health. Anopheles species are found in varying geographic regions, and within regions distinct environments support different species, which affects malaria epidemiology and transmission (@fig-2). The most important species vary by region and include *Anopheles gambiae* and *Anopheles funestus* in Africa, *Anopheles stephensi* in South Asia, and *Anopheles darlingi* in South America (@fig-2).
## Key Species
- **Anopheles gambiae Complex**: This group includes several species that are among the most efficient malaria vectors, particularly in Sub-Saharan Africa. *A. gambiae* is known for its strong preference for biting humans (anthropophilic behavior), which makes it an especially effective vector.
- **Anopheles funestus**: Another major vector in Africa, *A. funestus* is highly efficient in transmitting *P. falciparum*, the most deadly malaria parasite. It is often found in more permanent water bodies and is also anthropophilic.
- **Anopheles stephensi**: Found primarily in urban and peri-urban areas of South Asia and parts of the Middle East, *A. stephensi* is a significant vector of urban malaria. Its ability to breed in man-made water containers makes it particularly difficult to control.
- **Anopheles dirus**: This species is a primary vector in Southeast Asia and is known for its adaptation to forested areas, making it a significant concern in rural and forested regions.
- **Anopheles darlingi**: The main malaria vector in the Amazon Basin, *A. darlingi* is highly adaptable, breeding in a variety of natural and man-made water bodies.
![A global map of dominant malaria vector species. Reproduced from Sinka et al 2014.[^topic2b-lifecycle-1]](figures/anopheles-spp-dist.png){#fig-2}
[^topic2b-lifecycle-1]: Sinka, M.E., Bangs, M.J., Manguin, S. *et al.* A global map of dominant malaria vectors. *Parasites Vectors* **5**, 69 (2012). <https://doi.org/10.1186/1756-3305-5-69>
[^topic2b-lifecycle-2]: Sinka, M.E., Bangs, M.J., Manguin, S. *et al.* A global map of dominant malaria vectors. *Parasites Vectors* **5**, 69 (2012). <https://doi.org/10.1186/1756-3305-5-69>
## Anopheles Lifecycle
Malaria parasites are transmitted by the female mosquito of the Anopheles genus. Anopheles species have four distinct life stages: egg, larva and pupa make up the juvenile aquatic stages before the final adult stage. Juvenile stages last for around 5--14 days depending on the species and the ambient temperature. Once at the adult stage mosquitos tend to mate within a few days of emergence and feed on sugar sources for energy. Female mosquitos will also require a blood meal for the development of her eggs. It is this stage that links the female mosquito and the human hosts in the malaria transmission cycle.
Following a blood meal, the female must rest while the eggs are developed, again this process depends on the ambient temperature taking around two to three days in tropical conditions. Females will then lay their eggs in standing water and continue to seek further blood meals to sustain further egg production. This cycle continues until the female dies, around one to two weeks later.
Chances of survival are dependent on temperature and humidity and the ability of the female mosquito to find a blood meal. In order to transmit parasites mosquitos must survive for longer than the extrinsic incubation period of plasmodium which is around 9--18 days depending on species and temperature (higher temperatures accelerate parasite growth). Many Anopheles species are opportunistic in their feeding behaviour and will take a blood meal from whatever host is available either human or animal. The degree to which a species favours humans, known as anthrophily, determines their efficiency as a vector of malaria. *Anopheles gambiae* and *Anopheles funestus* are two highly anthropophilic species that makes them the primary vector in much of sub-Saharan Africa.
## Feeding Behaviors
A summary of some key terms and behviours in mosuqito feeding behaviours:
- **Anthropophily**: Many *Anopheles* species exhibit a strong preference for human blood, a behavior known as anthropophily. This preference significantly enhances their role as malaria vectors. For example, *A. gambiae* is highly anthropophilic, which, coupled with its breeding in proximity to human habitation, makes it one of the most effective malaria vectors globally.
- **Crepuscular and Nocturnal Activity**: Most *Anopheles* mosquitoes are active during twilight hours (dusk and dawn) and at night. This behavior is critical for malaria transmission, as it coincides with human sleeping patterns, making bed nets an effective intervention.
- **Endophagy and Exophagy**:
- **Endophagic** mosquitoes prefer to feed indoors, making indoor residual spraying (IRS) and insecticide-treated nets (ITNs) effective control measures.
- **Exophagic** species feed outdoors, requiring different strategies such as outdoor spraying and environmental management.
## Breeding Environments
- **Natural Habitats**: *Anopheles* mosquitoes typically breed in clean, unpolluted water. Natural breeding sites include:
- **Swamps and Marshes**: Often found in tropical and subtropical regions.
- **Forest Pools and Streams**: Common in rural and forested areas.
- **Rice Fields and Irrigation Channels**: These provide extensive breeding habitats in agricultural regions.
- **Man-Made Habitats**: Urbanization and human activities have created additional breeding sites:
- **Water Containers**: In urban and peri-urban areas, *A. stephensi* and other species can breed in water storage containers, discarded tires, and other small water bodies.
- **Construction Sites**: Pools of stagnant water at construction sites can serve as breeding grounds, especially in urban areas.