Parasitology

The female mosquito becomes infected after taking a blood meal containing gametocytes, the sexual form of the malarial parasite. The developmental cycle in the mosquito usually takes 7-20 days (depending on temperature), culminating in infective sporozoites migrating to the insect's salivary glands. The sporozoites are inoculated into a new human host, and those which are not destroyed by the immune response are rapidly taken up by the liver. Here they multiply inside hepatocytes as merozoites: this is pre-erythrocytic (or hepatic) sporogeny. After a few days the infected hepatocytes rupture, releasing merozoites into the blood from where they are rapidly taken up by erythrocytes. In the case of P. vivax and P. ovale, a few parasites remain dormant in the liver as hypnozoites. These may reactivate at any time subsequently, causing relapsing infection.

Inside the red cells the parasites again multiply, changing from merozoite, to trophozoite, to schizont, and finally appearing as 8-24 new merozoites. The erythrocyte ruptures, releasing the merozoites to infect further cells. Each cycle of this process, which is called erythrocytic schizogony, takes about 48 hours in P. falciparum, P. vivax and P. ovale, and about 72 hours in P. malariae. P. vivax and P. ovale mainly attack reticulocytes and young erythrocytes, while P. malariae tends to attack older cells; P. falciparum will parasitize any stage of erythrocyte.

A few merozoites develop not into trophozoites but into gametocytes. These are not released from the red cells until taken up by a feeding mosquito to complete the life cycle.