Five species of blood protozoan parasites cause human malaria: the potentially lethal and often drug-resistant Plasmodium falciparum; the relapsing parasites P. vivax and P. ovale; P. malariae, which can persist at low densities for years; and P. knowlesi, a monkey parasite that causes occasional infections in humans in tropical forests in Southeast Asia. P. knowlesi resembles P. falciparum and P. malariae microscopically but is identified definitively by molecular methods (see Table e27-1, footnote a).

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Table e27-1 Morphologic Characteristics of Human Malaria Parasitesa 

The malaria parasites are readily seen under the microscope (×1000 magnification) in thick and thin blood smears stained with supravital dyes (e.g., Giemsa's, Field's, Wright's, Leishman's). The morphologic characteristics of the parasites are summarized in Table e27-1. In the thick film, lysis of red blood cells by water leaves the stained white cells and parasites, allowing detection of densities as low as 50 parasites/μL. This degree of sensitivity is up to 100 times greater than that of the thin film, in which the ...

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