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Anopheles freeborni mosquito pumping blood Larger Picture. Sequential images of the mosquito taking its blood meal. There are approximately 3, species of mosquitoes grouped into 41 genera.
Human malaria is transmitted only by females of the genus Anopheles. Of the approximately Anopheles species, only transmit malaria i. The rest either bite humans infrequently or cannot sustain development of malaria parasites. Anophelines are found worldwide except Antarctica. Malaria is transmitted by different Anopheles species in different geographic regions. Within geographic regions, different environments support a different species. Anophelines that can transmit malaria are found not only in malaria-endemic areas, but also in areas where malaria has been eliminated.
These areas are thus at risk of re-introduction of the disease. Like all mosquitoes, anopheles mosquitoes go through four stages in their life cycle: egg, larva, pupa, and adult. The first three stages are aquatic and last days, depending on the species and the ambient temperature. The biting female Anopheles mosquito may carry malaria. Male mosquitoes do not bite so cannot transmit malaria or other diseases. The adult females are generally short-lived, with only a small proportion living long enough more than 10 days in tropical regions to transmit malaria. Adult females lay eggs per oviposition.
Eggs are laid singly directly on water and are unique in having floats on either side. Eggs are not resistant to drying and hatch within days, although hatching may take up to weeks in colder climates. Mosquito larvae have a well-developed head with mouth brushes used for feeding, a large thorax, and a segmented abdomen. 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. Top: Anopheles Egg; note the lateral floats.
Bottom: Anopheles eggs are laid singly. Larvae breathe through spiracles located on the 8th abdominal segment and therefore must come to the surface frequently. The larvae spend most of their time feeding on algae, bacteria, and other microorganisms in the surface microlayer. They do so by rotating their head degrees and feeding from below the microlayer. Larvae dive below the surface only when disturbed. Larvae swim either by jerky movements of the entire body or through propulsion with the mouth brushes. Larvae develop through 4 stages, or instars, after which they metamorphose into pupae.
At the end of each instar, the larvae molt, shedding their exoskeleton, or skin, to allow for further growth. Anopheles Larva. Note the position, parallel to the water surface.
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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.
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The pupa is comma-shaped when viewed from the side. This is a transitional stage between larva and adult. The pupae does not feed, but undergoes radical metamorphosis. The head and thorax are merged into a cephalothorax 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 onto the surface of the water.
The duration from egg to adult varies considerably among species and is strongly influenced by ambient temperature.
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Mosquitoes can develop from egg to adult in as little as 7 days but usually take days in tropical conditions. Anopheles Adults. Note bottom row the typical resting position. 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. The antennae are important for detecting host odors as well as odors of aquatic larval habitats where females lay eggs.
The head also has an elongate, forward-projecting proboscis used for feeding, and two sensory palps. The thorax is specialized for locomotion. Three pairs of legs and a single 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.
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 some species, the males form large swarms, usually around dusk, and the females fly into the swarms to mate. The mating habitats of many species remain unknown. Males live for about a week, feeding on nectar and other sources of sugar. 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 days in tropical conditions.
Once the eggs are fully developed, the female lays them then seeks blood to sustain another batch of eggs. The cycle repeats itself until the female dies. Females can survive up to a month or longer in captivity but most do not live longer than weeks in nature. Their chances of survival depend on temperature and humidity, but also upon their ability to successfully obtain a blood meal while avoiding host defenses. Understanding the biology and behavior of Anopheles mosquitoes can aid in designing appropriate control strategies.
Long-lived species that prefer human blood and support parasite development are the most dangerous. Factors that should be taken into consideration when designing a control program include the susceptibility of malaria mosquitoes to insecticides and the preferred feeding and resting location of adult mosquitoes.
One important behavioral factor is the degree to which an Anopheles species prefers to feed on humans anthropophily or animals such as cattle zoophily. Anthrophilic Anopheles are more likely to transmit the malaria parasites from one person to another. Most Anopheles mosquitoes are not exclusively anthropophilic or zoophilic; many are opportunistic and feed upon whatever host is available.
However, the primary malaria vectors in Africa, An. 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 takes 9 days or longer, 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 many studies have indirectly measured longevity by examination of their reproductive status or via marking, releasing, and recapturing adult mosquitoes. The majority of mosquitoes do not live long enough to transmit malaria, but some may live as long as three weeks in nature.
Though evidence suggests that mortality rate increases with age, most workers estimate longevity in terms of the probability that a mosquito will live one day. Usually these estimates range from a low of 0. Any control measure that reduces the average lifespan of the mosquito population will reduce transmission potential. Insecticides thus need not kill the mosquitoes outright, but may be effective by limiting their lifespan. Most Anopheles mosquitoes are crepuscular active at dusk or dawn or nocturnal active at night.
Some Anopheles mosquitoes feed indoors endophagic while others feed outdoors exophagic. After blood feeding, some Anopheles mosquitoes prefer to rest indoors endophilic while others prefer to rest outdoors exophilic. Biting by nocturnal, endophagic Anopheles mosquitoes can be markedly reduced through the use of insecticide-treated bed nets ITNs or through improved housing construction to prevent mosquito entry e.
Endophilic mosquitoes are readily controlled by indoor spraying of residual insecticides. Insecticide-based control measures e. However, after prolonged exposure to an insecticide over several generations, mosquitoes, like other insects, 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 within a few years after the insecticides were introduced. There are over mosquito species 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, particularly via rotation of different classes of insecticides used for control. Monitoring of resistance is essential to alert control programs to switch to more effective insecticides. Some Anopheles species are poor vectors of malaria, 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 for strains of An. Scientists are studying the genetic 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 parasites are micro-organisms that belong to the genus Plasmodium. There are more than species of Plasmodium , which can infect many animal species such as reptiles, birds, and various mammals. Four species of Plasmodium have long been recognized to infect humans in nature. In addition there is one species that naturally infects macaques which has recently been recognized to be a cause of zoonotic malaria in humans.
There are some additional species which can, exceptionally or under experimental conditions, infect humans. See all free Kindle reading apps. Don't have a Kindle? Product details Paperback: 84 pages Publisher: Springer 4 Oct. No customer reviews. Share your thoughts with other customers.
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