Mosquitoes and Malaria

The bites of the common British species of mosquito (Culex pipiens) cause little more than irritation and swelling. In many tropical countries, the Middle East and Southern Asia, species of mosquito in the genus Anopheles transmit the disease malaria. It is estimated that about 280 million people suffer from malaria and over 2 million or more die each year.

The disease is caused by a single-celled parasite (Plasmodium spp.) which enters the blood stream as a result of a 'bite' from an infected mosquito. The parasites enter the red blood cells and reproduce there, feeding on the cytoplasm of the cell. The red cells eventually burst open and release the parasites into the blood stream, where they go on to infect other red cells. When hundreds of red cells burst simultaneously, they cause the patient to suffer from fever. The regular phases of reproduction and release produce regular bouts of fever.

If a mosquito sucks blood from an infected person, the parasites break out of the red cells and enter the mosquito's gut wall where they continue to reproduce. From here they migrate to the salivary glands. The next person to be bitten will receive saliva containing the parasite and will probably develop malaria. The parasites first enter the cells of the liver and reproduce there. When they break out, they go on to invade the red blood cells.

There are three lines of defence against the disease; destroying the malarial parasite in the blood, destroying the mosquitoes which transmit the disease and preventing people from being bitten.

There are drugs such as Chloroquine which destroy the parasites in the blood but not in the liver. The malarial parasite has, in many cases, developed resistance to these drugs and new drugs are constantly being sought.

Insecticides can be used to kill mosquitoes in dwellings but, here again, the mosquitoes have developed resistance to insecticides such as DDT. Other ways of reducing the mosquito population involve draining the swamps when they breed, increasing the flow of sluggish streams to carry away the eggs and larvae, spraying oil and insecticide to kill the larvae in the water, covering all stores of water and preventing water from accumulating in discarded tins, tyres etc.

To avoid being bitten by mosquitoes, screens can be placed over windows and doors and, since most bites occur at night, people can sleep under a tent of fine netting to keep mosquitoes out.

An intensive search for a vaccine has not been successful so far.

More information, and illustrations to accompany this article, can be found on

D G Mackean is the author of GCSE Biology, IGCSE Biology, and many other Biology text books. He has a site of Biology Teaching Resources at which includes a bank of experiments for teachers, sample PowerPoint presentations, and many biological drawings