Merozoites

Merozoites are a life-cycle stage of certain apicomplexan parasites, most notably Plasmodium species that cause malaria. They are produced by schizogony within infected hepatocytes or erythrocytes and are released when the schizont ruptures. Merozoites are the stage that invades new host cells, initiating a new round of asexual replication in the vertebrate host.

In malaria, merozoites released from liver schizonts invade red blood cells, entering as ring-stage parasites that mature into trophozoites and eventually schizonts within the erythrocyte. When these schizonts rupture, dozens of new merozoites are released to invade additional RBCs, sustaining the cycle and contributing to hemolysis and clinical symptoms. A portion of infection cycles can produce sexual forms, gametocytes, which are taken up by mosquitoes to continue transmission.

Structure and invasion: Merozoites possess an apical complex with secretory organelles (micronemes and rhoptries). They display surface antigens and invasion ligands that interact with erythrocyte receptors; in Plasmodium falciparum, essential interactions include PfRh5 with basigin and various EBA and Rh ligand families mediating alternative invasion routes. Entry involves formation of a moving junction and parasite motility powered by an actin–myosin system, followed by establishment of the parasitophorous vacuole and development to the ring stage.

Clinical relevance: Merozoites are central to malaria pathogenesis and are key targets for vaccines and drugs. Vaccine candidates such as merozoite surface proteins (for example MSP-1) and AMA-1 aim to block invasion. Antimalarial strategies often target invasion or intra-erythrocytic replication, and ongoing research seeks to overcome parasite diversity in invasion pathways.