Six species of blood protozoan parasites cause human malaria (Chap. 219): the potentially lethal and often drug-resistant Plasmodium falciparum; the relapsing parasites Plasmodium vivax and Plasmodium ovale (with what appear to be two morphologically identical sympatric species of P. ovale); Plasmodium malariae, which can persist at low densities for years; and, in infections in individuals living in or close to tropical forests in Southeast Asia, Plasmodium knowlesi, a monkey parasite that microscopically resembles P. falciparum (young forms) and P. malariae (older forms) but is identified definitively by molecular methods.
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 A6-1. 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 cells are fixed and the malaria parasites are seen inside the red cells. The thin film is better for speciation and provides useful prognostic information in severe falciparum malaria. Several findings are associated with increased mortality risk: high parasite counts, more mature parasites (>20% containing visible malaria pigment), and phagocytosed malaria pigment in >5% of neutrophils.
Babesia microti (Chap. 220) appears as a small ring form resembling P. falciparum. Unlike Plasmodium, Babesia does not cause the production of pigment in parasites, nor are schizonts or gametocytes formed.