Malaria is transmitted from man to man by the female anopheles mosquito, one of the most capable vectors of human disease. Various species have been found to be the vectors in different parts of the world. A. gambiaecomplex is the chief vector in Africa and A. freeborni in N. America. Nearly 45 species of the mosquito have been found in India and A. culicifacies, A. fluviatilis, A. minimus, A. philippinensis, A. stephensi, A. sundaicus, and A. leucosphyrushave been implicated in the transmission of malaria. The areas of distribution are different for these mosquitoes: A. fluviatilis, A. minimus are found in the foot-hill regions, A. stephensi, A. sundaicus are found in the coastal regions, A. culicifacies and A. philippinensis are found in the plains. Species like A. stephensi are highly adaptable and are found to be very potent vectors of human malaria.
Mosquitoes choose the blood donor by odours and visual clues and can learn from experience! Human behaviour also plays a role and males are more frequently bitten.
The genome of A. gambiae has now been cracked and the effort is expected to help in future research into mosquito control strategies.
Apart from malaria, anopheles mosquitoes are also known to transmit W. bancrofti (filarial worm); the Timorese filaria, Brugiatimori; several arboviruses including eastern and western equine encephalitis, Venezualan equine encephalitis, onyong-nyong, tataguine etc.
The female mosquito has a specialised apparatus to penetrate the skin of its victim. At the end of the slender proboscis, there are two pairs of cutting stylets that slide against one another to slice through the skin. Once through the skin, the mosquito’s proboscis begins probing for a tiny blood vessel. If it does not strike one on the first try, the mosquito will pull back slightly and try again at another angle through the same hole in the skin. Inside the proboscis are two hollow tubes, one that injects saliva into the microscopic wound and one that withdraws blood. The mosquito’s saliva includes a combination of antihemostatic and anti- inflammatory enzymes that disrupt the clotting process and inhibit the pain reaction (so that the victim is unaware of the bite!)
The female mosquito lays 30-150 eggs every 2-3 days. Human blood is needed to nourish these eggs and Anopheles shows the most regular cycles of blood feeding and egg laying. As a corollary, by using personal protective measures against mosquito bites, like using mosquito nets, one can deny the blood meal and hence help in mosquito control.
Anopheles mosquitoes enter the house between 5 p.m. and 9.30 p.m. and again in early hours of morning. They start biting by late evening and the peak of biting activity is at midnight and early hours of morning. By keeping the windows and doors closed between 5 p.m. and 10 p.m. and again in early morning, one can prevent the entry of these mosquitoes into the house. Also protect yourself against the bites in the evenings and early mornings by wearing garments that cover the body as much as possible and at bedtime, by using mosquito nets without fail.
Mosquitoes can fly up to several kilometres! And they can reach far off places by taking shelter in motor vehicles, ships and aircraft.
The average life span of a mosquito is 2-3 weeks. It can be longer in ideal living conditions.
The adult mosquitoes hide themselves behind cupboards, clothes, curtains and other dark and cool corners during the day and come out to bite at night. It is important to minimize these hiding places. Therefore keep the cupboards and such other things closed; do not hang clothes at corners of the room, instead keep them inside the wardrobes or cupboards.
Stages of development of Anopheles – from eggs to adults
Anopheles mosquitoes breed in natural water collections. Therefore, breeding increases dramatically in the rainy season when water collects in bottles, tins, tender coconut shells, buckets, tyres etc., that are thrown out in the open and these provide ample breeding ground. Also wells, ponds, water tanks, paddy fields etc., act as breeding grounds. Construction sites provide ample breeding places for the mosquito – water on the concrete slabs (used for curing), water collected in tanks, water collected in and around the construction site owing to blockage of water drains – all these help breeding. It is very important to destroy these water collections or to keep them properly covered to prevent breeding.
Usually it takes about a week for the eggs to develop into adults.