merozoite

In malaria caused by Plasmodium species (P. falciparum, P. vivax, P. ovale, P. malariae), the life cycle begins when an infected Anopheles mosquito injects sporozoites into the bloodstream. Sporozoites travel to the liver and invade hepatocytes, where they undergo exoerythrocytic (pre-erythrocytic) schizogony to form merozoites. These hepatic merozoites are released into the blood and invade red blood cells (RBCs), initiating the erythrocytic cycle. Within RBCs, the parasite develops through ring, trophozoite, and schizont stages; the schizont ruptures to release new merozoites, which then infect additional RBCs. This synchronous rupture of infected RBCs causes the characteristic cyclical fevers, chills, and hemolytic anemia seen in malaria—occurring every 48 hours (tertian) for P. vivax/ovale or every 72 hours (quartan) for P. malariae; P. falciparum has less synchronous, more severe cycles causing life-threatening complications (cerebral malaria, severe anemia, renal failure). P. vivax and P. ovale can also form dormant liver hypnozoites, which release merozoites later and cause relapses. Diagnosis is made by light microscopy of Giemsa-stained thin/thick blood smears, identifying characteristic ring forms, schizonts, and gametocytes within RBCs. Clinically, the merozoite stage is the target of several antimalarial drugs (e.g., chloroquine, which is schizonticidal and works by inhibiting heme polymerization in the food vacuole). Babesia microti also produces intraerythrocytic merozoites, causing a malaria-like illness, often seen in asplenic patients or those in endemic Northeast US regions; blood smears show characteristic Maltese cross tetrads (unlike Plasmodium rings).