Figure 4.

Malaria-causing Plasmodium needs both a female mosquito vector and a vertebrate host to complete its lifecycle. Different stages of Plasmodium parasite (orange) are infective to mosquito vectors (green) and human hosts (blue). A female mosquito needs to blood-feed at least twice to transmit malaria and she can only be infected following ingestion of the sexual gametocyte stage of Plasmodium when she bites an infected host (solid green lines and arrows). Once inside a mosquito, male and female Plasmodium gametocytes form motile zygotes (ookinetes), which invade the mosquito midgut wall. Ookinetes develop into oocysts, which eventually rupture releasing sporozoites, which invade the mosquito’s salivary glands. Sporozoites are the only stage of Plasmodium that can infect human hosts after a female mosquito injects them along with her saliva during a blood meal (dashed blue lines and arrows). Sporozoites invade the liver and mature into schizont, which release merozoites. Merozoites in turn invade red blood cells, where Plasmodium goes through repeated cycles of asexual reproduction. Merozoites produce immature ring stages, which develop into mature trophozoites and produce schizonts again (which will go through the cycle and infect new red blood cells). Small proportion of the immature ring stage trophozoites develop into male and female gametocytes (Plasmodium sexual stages). The human host now harbors the only Plasmodium stage infective to female mosquitoes taking a blood meal. The critical step for infection of both mosquito vectors and vertebrate hosts is highly dependent on mosquito olfaction. P, Plasmodium parasite; V, mosquito vector; H, human host.