Protozoa

Protozoa are the most basic form of animal life carrying nuclei; most have a Golgi, an endoplasmatic reticulum, and at least a rudimentary mitochondrion. Only 73 of the approximately 40000 protozoa found in nature are of medical importance and parasitize man (Ashford and Crewe 1998). For further details of protozoa indigenous to the North, see Chapters 3-7.

The protozoa (protos = first; zoon = animal; greek) form their own sub-kingdom within the kingdom of single-cell eukaryotes, the Protista (Figure 2.2). They comprise the phyla Apicomplexa, Ciliata, Microspora, and Sarcomastigophora. Apicomplexan parasites include,

Figure 2.2 Classification of protozoa according to Levine et al. (1980).

for example, the malaria parasites (Plasmodium falciparum, P. malariae, P. ovale, P. vivax) and Toxoplasma gondii. The Apicomplexa derive their name from the presence of an apical complex composed of rhoptries, micronemes, and other structures used by the parasite during the invasion of cells. Most of them also have a cytostome (micropore) for the acquistion of nutrients and carry a rudimentary organelle called the apicoplast, a modified chloroplast, which in plants is involved in the generation of chlorophyll. It lacks genes of importance for photosynthesis but still carries those encoding proteolytic enzymes as well as molecules involved in fatty-acid synthesis. The phylum Ciliata does not include any parasites pathogenic to man but the rather uncommon ciliated Balantidium coli which causes diarrhoea (first identified in 1851 in Stockholm by Malmsten). The phylum Microspora includes intracellular spore-forming parasites which infect nearly all major animal groups. Microsporan parasites only (or mainly) cause gastro-intestinal disease in immunosuppressed individuals, such as those with AIDS. In contrast, a large number of pathogeneic parasites that affect the immunocompetent host are included in the phylum Sarcomastigophora. Protozoa of the order Kinetoplastida, for example, trypanosomes and leishmania parasites, typically carry a flagellum and a DNA-containing kinetoplast located near the flagellar basal body (kinetosome). Some of the kinetoplastids (trypanosomes) have additional organelles called glycosomes for storing carbohydrates. Giardia lamblia, which causes diarrhoea, belongs to the order diplomonida and are characterized by multiple flagella, the absence of a complete mitochondrion, and their capacity to form cysts. Protozoa of the sub-phylum Sarcodina are uninucleated and possess mitochondria; many have the capacity to form cysts, for example, Entamoeba histolytica. The order Thrichomonadida, which comprises trichomonas and dientamaoebas, frequently carry flagella and lack a mitochondrion but have hydrogenosomes which generate free H-ions

(trichomonatids).

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