What are mosquitoes?
Mosquitoes are members of the Family Culicidae in the Order Diptera (the true flies). There are over 3,500 species of mosquito, most of which are of no medical significance because they do not feed on blood. Of those that do feed on blood it is only the females that take blood meals. The males survive by feeding on nectar. However, despite the relatively few vector species, over twenty diseases are transmitted to humans by mosquitoes.
What do they look like?
Mosquitoes are small, slender midge-like flies measuring 5 - 15mm long. Each one has a long, forward-projecting proboscis, narrow wings and long, thin legs. Male antennae are long and bushy, whereas the female antennae are sparsely haired.
How do they transmit disease?
In some species of mosquito it is necessary for the females to take a blood meal to develop the ovaries, and they need a separate meal for each batch of eggs they wish to produce. If a female mosquito takes a blood meal from an infected human, they may take up the pathogen (disease-causing organism) at the same time. This may undergo a developmental change within the female mosquito. The next time the female feeds, she injects an anticoagulant to stop the blood clotting in her proboscis, and the pathogen is passed on to the new, uninfected host in this anticoagulant. Therefore, mosquitoes do not cause diseases, they just transmit them, they are vectors.
What diseases can they transmit?
Most famously, mosquitoes are vectors of malaria, one of the biggest killers in the world. There are an estimated 250 million cases of malaria worldwide every year. However, they can also transmit filariases (infestation with roundworms), and a number of viral diseases. These include Dengue Fever, Yellow Fever, Rift Valley Fever, Congo Crimea Haemorrhagic Fever and several encephalitises.
What causes malaria?
Malaria is caused by parasitic protozoans of the genus Plasmodium, which destroy human blood and liver cells. The development of Plasmodium is dependent on two hosts, humans and mosquitoes - the reproduction takes place in mosquitoes and proliferation occurs in humans. Only one genus of mosquito, Anopheles, can transmit malaria, but 30 species within this genus are malaria vectors.
How can mosquitoes be controlled?
Mosquitoes are controlled in one of two ways: by killing the adults or by preventing larvae from reaching adulthood. Killing adult mosquitoes requires the use of insecticides either in aerosols for rapid knockdown, or in liquids for residual spraying of surfaces. However, almost as fast as new insecticides have been introduced, resistant strains of mosquitoes have emerged.
Thus, a more effective form of control is to control the larvae, either by actually killing them or by destroying the breeding grounds of the mosquito. Mosquitoes lay eggs in standing water, close to the water surface. The larvae, when they hatch remain under water either hanging down from the surface or laying parallel to it. Hence larvae can be controlled by removing all standing water in and around human habitation or draining the land using narrow, steep-sided ditches in the case of wet, marshy areas. A new technique that has proved successful in the case of pit latrines is to add expended polystyrene balls to the water which prevents larvae from reaching the surface to respire and adults from being able to lay eggs.
There are several biological methods of controlling larvae that have been developed recently. Adding the insect growth regulator methoprene to water will stop the larvae ever becoming adults. In a number of cases, predators of the larvae have successfully been used as control agents. These predators include fish such as Gambusia, and other mosquito larvae such as Toxorhynchites splendens.
On a personal level, the risks of becoming infected via a mosquito bite can be minimised by the use of repellants, nets, aerosols, correct clothing and immunisation.