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The medically important organisms in this category of protozoa consist of the sporozoans Plasmodium and Toxoplasma and the flagellates Trypanosoma and Leishmania. Pneumocystis is discussed in this book as a protozoan because it is considered as such from a medical point of view. However, molecular data indicate that it is related to yeasts such as Saccharomyces cerevisiae. Table 51–2 summarizes several important features of these blood and tissue protozoa.

The medically important stages in the life cycle of the blood and tissue protozoa are described in Table 52–1.

TABLE 52–1Medically Important Stages in Life Cycle of Blood and Tissue Protozoa



Malaria is caused primarily by four plasmodia: Plasmodium vivax, Plasmodium ovale, Plasmodium malariae, and Plasmodium falciparum. Plasmodium vivax and P. falciparum are more common causes of malaria than are P. ovale and P. malariae. Plasmodium vivax is most widely distributed and P. falciparum causes the most serious disease. A fifth species, Plasmodium knowlesi, is found in Southeast Asia.

Worldwide, malaria is one of the most common infectious diseases and one of the leading causes of death.

Important Properties

The life cycle of Plasmodium species is shown in Figure 52–1. The vector and definitive host for plasmodia is the female Anopheles mosquito (only the female takes a blood meal). There are two phases in the life cycle: the sexual cycle, which ...

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