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 ||Download (.pdf) TABLE 52–1 Medically Important Stages in Life Cycle of Blood and Tissue Protozoa
|Organism ||Insect Vector ||Stage that Infects Humans ||Stage(s) in Humans Most Associated with Disease ||Important Stage(s) Outside of Humans |
|Plasmodium ||Female mosquito (Anopheles) ||Sporozoite in mosquito saliva ||Trophozoites and merozoites in red blood cells ||Mosquito ingests gametocytes → fuse to form zygote → ookinete → sporozoites |
|Toxoplasma ||None ||Tissue cyst (pseudocysts) in undercooked meat or oocyst in cat feces ||Rapidly multiplying trophozoites (tachyzoites) within various cell types; tachyzoites can pass placenta and infect fetus; slowly multiplying trophozoites (bradyzoites) in tissue cysts ||Cat ingests tissue cysts containing bradyzoites → gametes → ookinete → oocysts in feces |
|Pneumocystis ||None ||Uncertain; probably cyst ||Cysts ||None known |
|Trypanosoma cruzi ||Reduviid bug (Triatoma) ||Trypomastigote in bug feces ||Amastigotes in cardiac muscle and neurons ||Bug ingests trypomastigote in human blood → epimastigote → trypomastigote |
|Trypanosoma gambiense and Trypanosoma rhodesiense ||Tsetse fly (Glossina) ||Trypomastigote in fly saliva ||Trypomastigotes in blood and brain ||Fly ingests trypomastigote in human blood → epimastigote → trypomastigote |
|Leishmania donovani ||Sandfly (Phlebotomus and Lutzomyia) ||Promastigotes in fly saliva ||Amastigotes in macrophages in spleen, liver, and bone marrow ||Fly ingests macrophages containing amastigotes → promastigotes |
|Leishmania tropica and others ||Sandfly (Phlebotomus and Lutzomyia) ||Promastigotes in fly saliva ||Amastigotes in macrophages in skin ||Fly ingests macrophages containing amastigotes → promastigotes |
Malaria is caused primarily by four plasmodia: Plasmodium vivax, Plasmodium ovale, Plasmodium malariae, and Plasmodium falciparum. P. vivax and P. falciparum are more common causes of malaria than are P. ovale and P. malariae. P. 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.
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 ...