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).
Table e27-1 Morphologic Characteristics of Human Malaria Parasitesa |Favorite Table|Download (.pdf)
Table e27-1 Morphologic Characteristics of Human Malaria Parasitesa
|P. falciparum||P. vivax||P. ovale||P. malariae|
|Asexual parasites||Usually only fine blue ring forms (some resembling stereo headsets) are seen. Parasitemia level may exceed 2%.||Irregular, large, fairly thick rings become highly pleomorphic as the parasite grows. Parasitemia level is low.||Regular, dense ring enlarges to compact, blue, mature tropho-zoite (rectangular or band-form). Parasitemia level is low.||Dense, thick rings mature to dense, round trophozoites. Parasitemia level is low.|
|Schizonts||Rare in peripheral blood; 8–32 merozoites, dark brown-black pigment||Common; 12–18 merozoites, orange-brown pigment||8–14 merozoites, brown or black pigment||8–10 merozoites, dark brown or black pigment|
|Gametocytes||Banana-shaped; male: light blue; female: darker blue; a few scattered blue-black pigment granules in cytoplasm||Round or oval; male: round, pale blue; female: oval, dark blue; triangular nucleus, a few orange pigment granules||Large, round, dense, and blue (like P. malariae), but prominent James's dots; brown pigment||Large, oval; male: pale blue; female: dense blue; large black pigment granules|
|RBC changes||RBCs are normal in size. As the parasite matures, the RBC cytoplasm becomes pale, the cells become crenated, and a few small red dots may appear over the cytoplasm (Maurer's clefts).||RBCs are enlarged. Pale red Schüffner's dots increase in number as the parasite matures.||RBCs become oval with tufted ends. Red James's dots are prominent.||RBCs are normal in size and shape. No red dots are seen.|
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 ...