The life cycle of Plasmodium falciparum malaria parasite

The life cycle of Plasmodium falciparum malaria parasite.

Malaria is caused by infection with an obligate, intracellular protozoan parasite of the genus Plasmodium. Of the four species that infect humans (Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale and Plasmodium malariae), it is P. falciparum that is responsible for virtually all deaths. The life cycle of Plasmodium spp.is complex and somewhat specific to the parasite species.

(a) P. falciparum infection in humans begins when an infected Anopheles sp. mosquito takes a blood meal and injects infective sporozoites into the peripheral circulation.

(b) Within minutes, these sporozoites invade hepatocytes in the liver and, over approximately one week, undergo asexual multiplication, producing tens of thousands of merozoite forms of the parasite.

(c) When the infected hepatocyte ruptures, merozoites are released into the peripheral circulation.

(d) The merozoites invade red blood cells (rbcs) and complete another round of multiplication within 48–72 h, with the production of 16–20 additional merozoites per rbc, which devour the rbc haemoglobin in the process.

(e) The released merozoites invade additional rbcs and carry on the cycle. It is the synchronous release of merozoites that is thought to be responsible for the periodic fevers associated with malaria.

(f) Some invading merozoites do not divide, but differentiate into male (microgametocyte) and female (macrogametocyte) sexual forms.

(g) These sexual forms are taken from the bloodstream by a feeding Anopheles sp. mosquito and fertilise in the mosquito midgut to form zygotes.

(h) These zygotes further differentiate into motile forms, called ookinetes, migrate through the mosquito gut wall and divide within oocysts on the external gut wall to form thousands of sporozoites.

(i) The infective sporozoites are released into the mosquito haemocoele and move to the salivary gland, where they await injection into another human host, thus completing the life cycle