All human Plasmodium spp. are transmitted by the bite of female Anopheles mosquitoes (Figure 3.1). At the time of feeding, sporozoites can leave the salivary ducts to enter the bloodstream, where they circulate for a short time before invading hepatocytes. Parasites develop in the liver over the next 7-10 days (pre-erythrocytic stage), with nuclear division to form schizonts. Hepatic infection is asymptomatic and may last from about 6 days to several weeks. When hepatocytes rupture, schizonts release into the bloodstream thousands of merozoites that in turn invade erythrocytes. In P. vivax and P. ovale malaria, some parasites may become dormant in
the liver (hypnozoites) and emerge at a later stage (up to 2 years or more after leaving an endemic area) to cause a relapse of disease. Delayed prepatent P. vivax infections also occur, with incubation periods of up to 6-9 months in some returned travellers.
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