In most cases, malaria is transmitted by female Anopheles bites. There are more than 400 different mosquito species of Anopheles mosquitoes, of which about 30 are very important vectors of malaria. All important malaria vector species sting between dusk and dawn. The intensity of the transmission depends on factors related to the parasite, the vector, the human host and the environment.
The Anopheles lay their eggs in the water. These eggs hatch into larvae and then become adult mosquitoes. Female mosquitoes are looking for a blood meal to feed their eggs. Each species has its own preferences; Some for example prefer shallow freshwater such as puddles and the footprints left by Animal hoofs, which are abundant during the rainy season in tropical countries.
Transmission is more intense in places where mosquito species have a relatively long lifespan (allowing the parasite to complete its development cycle within the mosquito) and instead sting human beings rather than animals. The long life span and the strong human preference of African vector species explain that nearly 90% of malaria cases occur in Africa.
The transmission also depends on the climatic conditions that can influence the abundance and survival of mosquitoes, such as the precipitation regime, temperature and humidity. In many places, the transmission is seasonal with a peak during or just after the rainy season.
Epidemics of malaria can occur when climate and other conditions suddenly encourage transmission in areas where populations are little or not immune. They may also occur when low-immunized people move to areas of intense transmission, for example to find work or as refugees.
Human immunity is another important factor, especially in adults in areas of moderate to intense transmission. Immunity develops after years of exposure and, although it never confers total protection, it reduces the risk that malaria infection will cause severe disorders.
This is why most malaria deaths in Africa occur in young children, while in low-transmission areas and where the population is poorly immunized, all age groups are exposed.