Hypnozoite

Hypnozoite refers to a dormant, latent form of certain malaria parasites that resides in the liver after a sporozoite infects a person. Unlike the actively replicating liver-stage schizonts that immediately produce merozoites, hypnozoites can persist in hepatocytes for extended periods and may reactivate weeks to months later to initiate a new blood-stage infection. This capacity for dormancy underpins relapse malaria, in which recurrent fevers and parasitemia occur without a new mosquito bite.

Species and distribution: Hypnozoite formation is characteristic of Plasmodium vivax and Plasmodium ovale, the species most associated with relapsing malaria. Other human malaria parasites, notably Plasmodium falciparum, do not form hypnozoites and relapses are not typically due to dormant liver stages. Hypnozoites can be heterogeneous in their duration, and the timing of relapse can vary between individuals and geographic strains.

Clinical implications: Relapses can occur after apparent clearance of the initial infection, contributing to ongoing transmission and disease burden, especially in areas where vivax and ovale malaria are common. Distinguishing relapse from recrudescence or reinfection can be challenging and has implications for treatment and surveillance.

Treatment and prevention: Eradication of hypnozoites requires medications with activity against dormant liver stages, such as primaquine or tafenoquine. Prior to radical cure, patients should be screened for G6PD deficiency due to the risk of hemolysis, and primaquine is typically avoided in pregnancy. In addition to liver-stage treatment, standard antimalarial therapy addresses the blood-stage infection.