The life cycle of Malaria and of all species of human parasites is essentially the same. it comprises an exogenous sexual phase (sporogony) with multiplication in certain female anopheles mosquitoes and an endogenous asexual phase (schizogony) with multiplication in the vertebrate host. The later phase includes the development cycle in the red corpuscles in the blood erythrocytic schizogony and the phase taking place in the parenchyma cells in the liver (exo-erythrocytic schizogony).
Life Cycle in Man and Vertebrate Host.
Pre- erythrocytic phase: following the inoculation of sporozoites by the mosquito, there is a brief period of about one hour when the blood is infected. later the sporozoites disappear from the blood. many are destroyed by phagocytes but some enter the parenchyma cells of the liver (hepatocytes) directly or via the kupffer cells to undergo pre erythrocytic schizogony.
This takes about 7-10 days causing no symptoms. The first generation of merozoites is called cryptozoites while the subsequent ones are known as metacryptozoites. for P. vivax and P. ovale, development in the liver into a schizont is delayed in a proportion of invaded cells, and in these hepatocytes the parasites will lie dormant as hypnozoites.
Months or years later they will awaken and develop and cause the relapses of infection characteristics of these species. each fully developed hepatic Schizont of P. Vivax, P. Ovale, and P, Malariae liberates up to 15,000 Merozoites into the host’s bloodstream whereas the hepatic schizont of P. falciparum takes an average of 5-7 days and may be shorter compared to 7 days or longer in other plasmodium species.
This early amplification of the infection is greater in the potentially lethal falciparum malaria. Later, the exo-erythrocytic merozoites invade the bloodstream and penetrate the erythrocytes where they undergo schizogony. The trophozoite forms contain food vacuoles that displace the nucleus to one side like a signet ring. hence this stage is referred to as the signet ring stage. Later the vacuole disappears and haemozoin pigment granules are deposited.
This is segments the state. eventually, the red blood cells rupture to release the merozoites, haemozoin, and other metabolic wastes into the bloodstream. the rupture of parasitized cells is synchronized such that all ruptures at definite intervals are characteristics of each species.
This periodically accounts for the bout of fever at regular intervals caused by the metabolic wastes. The erythrocytic schizogony is repeated several times producing several generations of merozoites with the infection of new cells. some of the merozoites develop into gametocytes within the erythrocytes. No further development occurs until they are ingested by a mosquito while it feeds.