Mosquitoborne Diseases

The mosquito is the most important vector of disease, because it is abundant, lives in close proximity to humans and needs to feed on blood (the female must have a blood meal for the development of its eggs). Incredibly it is a very delicate insect, being easily blown by the wind, is a weak and slow flier, and susceptible to climatic change. Its success lies in its opportunism and rapid developmental cycle, allowing large numbers to be produced in a short period of time. Once a suitable breeding place appears, be it a few puddles after a rainstorm or a man-made water storage tank, mosquitoes will quickly lay their eggs. These develop within a short period of time into a large number of adults. Each may become a vector, and although many will die, there will be a sufficient number left to seek out suitable blood meals and transmit infection.

Some parasites are specific to certain types of mosquitoes (e.g. malaria and the anophelines), while others, like the

© R. Webber 2005. Communicable Disease Epidemiology and Control, 2nd edition (Roger Webber)

arboviruses, are less selective and utilize many different species. Different kinds of mosquitoes may be required in a complex transmission cycle such as yellow fever.

Development of the parasite within the mosquito may be morphological without multiplication (as with filaria), asexual (arbovirus) or sexual reproduction (malaria). Each of these methods confer advantages and disadvantages, such as the sheer number of organisms produced by asexual reproduction, or the opportunity to produce strains of varying type with sexual reproduction, but if the mosquito does not live long enough for these developmental stages to take place, then all is lost.

There are two main groups of mosquitoes - the anophelines and the culicines (which includes Aedes), separated by characteristics found in all of the development stages (Fig. 15.1). The adult Anopheles mosquito raises its hind legs away from the surface, easily remembered by its stance being like one side of a letter 'A', while the lava lies horizontal to the surface. The eggs are laid singly and have little floats on each side. In contrast, culicine mosquitoes rest horizontal to the surface, their larvae hang down from a single siphon and their eggs have no floats and are often laid in rafts. It is better to try and differentiate an adult male, with bushy antennae, from a female before subsequently separating anophelines from culicines by the length of the palps. More precise species identification is required to identify which mosquitoes are principal vectors, but this needs entomological help.

Mosquitoes differ in their habits, some preferring to take blood meals on humans (anthropophilic) or on animals (zoophilia) or are non-specific depending on which is most readily available. They also have particular biting times, either only indoors, only outdoors or a mixture of the two. The biting period can be mainly during the night or predominantly in the daytime. All these different parameters need to be measured in determining the importance of each type of mosquito as a vector.

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