Malaria is not caused by a mosquito but by a parasite in the mosquito. The malaria parasite needs the mosquito to reproduce and spread. The gametocyte is the developmental stage of the parasite that can be transmitted from people to the mosquito. In the mosquito's stomach the gametes are released and fertilisation takes place. The parasite develops further until the final stage (the sporozoite) in the salivary gland. The sporozoite can be transmitted with the saliva to a person if he/she is bitten by this mosquito. There the parasite reproduces rapidly and the person becomes ill.
People who live in areas where malaria is prevalent, can develop a natural immunity that stops the development of the parasite in the mosquito. This prevents the parasite from spreading further. The presence of this immunity, the so-called transmission-reducing activity, is determined using a laboratory test. Van der Kolk discovered that people who are often infected with malaria could quickly acquire this immunity. He also found that people with higher numbers of gametocytes are more frequently immune.
Each year more than 200 million people develop malaria. More than one million people die from malaria each year. In Cameroon, the researchers recorded how often people were bitten by a mosquito that carried malaria parasites. They also examined the number of transmittable parasites in the blood of infected persons. In a neighbourhood of the capital Yaoundé, 34 infectious bites per person per yea r were found to occur. The number of gametocytes per person was season and age dependent. Children were found to be by far the most important source of malaria transmission in the area. In the village Koundou, the number of infectious bites was about five times as high as in the capital. There previous research had revealed 177 infectious bites.
The existing laboratory test for malaria immunity did not yet work optimally. Therefore the researchers first modified this method before starting the research on immunity. With the improved test, the researchers studied how people not previously exposed to malaria become infected or immune. Migrants in the province of Papua in Indonesia, who had not previously been exposed to malaria were investigated for this purpose. Malaria is highly prevalent in Papua. After just one to four malaria infections the immunity against the infectious gametocytes increased. Immunity therefore develops quickly after exposure to an infection. The researchers expect that the modified methods will make it possible to carry out more detailed studies into the development and maintenance of immunity within the population.