Malaria life cycle

Life cycle of human malaria parasites (Plasmodium spp.) without reference to species-specific variations.1 Elongate sporozoites are injected during bite of the female mosquito (Anopheles spp.). The sporozoites are distributed by bloodstream and enter liver cells within 2 minutes after infection. 2, 3 Formation of schizonts and merozoites in liver parenchymal cells (exoerythrocytic phase). In some species this cycle may be preserved intracellularly via ``hypnozoites'' (dormozoites) for a long time (years) and cause relapses. 4±8 Erythrocytic cycle; liver merozoites enter (after typical prepatent periods,) erythrocytes, grow to ``signet-ring stages'' (5) and finally form, as schizonts (6), several merozoites (7, 8). During the digestion of hemoglobin the parasites produce pigment granules (6, 7; PG) of hemozoin. The development of such schizonts becomes synchronous and is repeated (4±8) in a 1±3 day cycle (depending on the species). 9 After an indeterminate number of such asexual generations, some merozoites enter erythrocytes and become macro- (9.1) or microgamonts (9.2). The size and shape are species specific (banana-shaped in P. falciparum). 10±11 When mosquitoes bite, they ingest erythrocytes containing such gamonts, which are released inside the gut from their enclosing erythrocytes. 12, 13 The microgamonts develop four to eight migrogametes in 10±15 min. 14 Fertilization of macrogamete. 15±19 The resultant zygote quickly elongates and becomes a motile ookinete (17) which penetrates (the not) drawn peritrophic membrane in the mosquito's gut, migrates through the cytoplasm of a gut cell and begins its transformation into an oocyst (situated between basal membrane and epithelial cells, 19). 20±22 Formation of multinucleate sporoblasts (20) which give rise to thousands of sporozoites (after 10±14 days). The latter become liberated into the hemocoel (body cavity) and migrate to salivary glands. These slender sporozoites (10±15 x 1 mm), which form a protecting surface coat, are finally injected into a new host at the next feeding act. BM, basal membrane of intestine; E, erythrocyte; IN, intestinal cell; LP, liver parenchymal cell; N, nucleus;PG,pigment; PV, parasitophorous vacuole; SG, salivary gland.