Q.What is the exo-erythrocytic cycle of Plasmodium vivax?

Plasmodium vivax is a digenetic sporozoan parasite that causes vivax (benign tertian) malaria in humans, requiring two hosts to complete its life cycle: humans, in which only asexual multiplication (schizogony) occurs, and the female Anopheles mosquito, in which the sexual phase (gametogony and sporogony) occurs.

In man — exo-erythrocytic (hepatic) cycle. An infective mosquito bite injects sporozoites into the blood; these invade liver cells (hepatocytes) and multiply asexually by schizogony into cryptomerozoites. The time from the infective bite to the first appearance of parasites in blood is the prepatent period; some sporozoites remain dormant in the liver as hypnozoites, which can reactivate later and cause relapses long after apparent recovery.

In man — erythrocytic cycle. Cryptomerozoites invade RBCs, passing through ring and trophozoite stages that digest haemoglobin and deposit the pigment haemozoin; infected RBCs show Schuffner's dots. Schizogony in the RBC produces a schizont with 12–24 merozoites; the RBC ruptures (triggering the fever/chill paroxysm), and merozoites invade fresh RBCs. Because this cycle repeats roughly every 48 hours, P. vivax malaria produces benign tertian fever (contrasted with the malignant tertian fever of P. falciparum). After several cycles, some merozoites differentiate instead into sexual gametocytes (micro- and macrogametocytes), which cause no further symptoms but are essential for transmission to a mosquito. …