Protozoa

J. Eckert

General information on parasites. A parasite (from the Greek word parasitos) is defined as an organism that lives in a more or less close association with another organism of a different species (the host), derives sustenance from it and is pathogenic to the host, although this potential is not always expressed. In the wider sense, the term parasite refers to all organisms with such characteristics. In medicine the term is used in a narrower sense and designates eukaryotic pathogens, which belong to the protozoa (unicellular organisms Chapter 9) and metazoa, including helminths (parasitic "worms," Chapter 10), arthropods (Chapter 11), and some other groups of lower medical significance (Annelida, Pentastomida, not covered in this book). Parasites cause numerous diseases (parasitoses) in humans, some being of extraordinary significance (e.g., malaria). Of practical concern in central Europe are both autochthonous and imported (tropical and travelers') parasitic infections.

A uniform disease nomenclature has been adopted in this book with the sole use of the suffix -osis (plural -oses)—for example trypanosomosis and not trypanosomiasis. This system, based on the Standardized Nomenclature of Parasitic Diseases (SNOPAD) (originally published in 1988 and recommended by the International Society of Parasitologists) avoids the inconsistent usage of disease names, such as leishmaniasis on the one hand and toxoplasmosis on the other.

A selection of the most important parasitoses is presented in the following chapters. In Table 1.10 (p. 28) zoonoses caused by parasites are listed.

Parasitic protozoa are eukaryotic, single-celled microorganisms about 1-150 im in size and enclosed by a trilaminated cell membrane. They possess one, rarely two nuclei (and multinuclear reproductive forms). Reproduction is asexual by binary or multiple fission of the cell, or sexual. The cellular construction of the protozoa is generally the same as in other eukaryotes but they also exhibit some special features. For example, during the course of evolution some protozoa (Giardia, Entamoeba) have lost the mitochondria secondarily, except several genomic traits that were laterally transferred to the nuclei. The apicoplast present in some species of Apicomplexa (see Toxoplasma) is a residual of a former plastid typical for their ancestors. Some protozoa contain specialized organelles, such as glycosomes (exclusively in trypanosomatids), hydrogenosomes (trichomonads and protozoa

Table 9.1 Provisional Classification of the Protozoa Mentioned in the Text

Phylum ■ Subphylum

Class

Order

Genus

Metamonada

Diplomonadea

Diplomonadida

Enteromonadida

Retortamonadida

Giardia Enteromonas Chilomastix, Retortamonas

Axostylata

Parabasalea

Pentatrichomonas,

■ Kinetoplasta

Trypanosomatidea

Trypanosomatida

Trypanosoma, Leishmania

Amoebozoa

Lobosea

Iodamoeba,

Endolimax,

Acanthamoeba,

Hartmanella,

Balamuthia

Heterolobosa

Schizopyrenidea

Schizopyrenida

■ Apicomlexa

Haematozoa Piroplasmea Litostomatea

Eimeriida

Haemosporida

Piroplasmida

Vestibuliferida

Toxoplasma,

Isospora,

Cyclospora,

Sarcocystis,

Cryptosporidium

Plasmodium

Babesia

Balantidium

Microspora1

Incerta2

Microsporea

Microsporida Pleistophorida

Brachiola,

Encephalitozoon,

Enterocytozoon,

Microsporidium,

Nosema, Vittaforma

Pleistophora,

Trachipleistophora

Blastocystis

1 Closely related to fungi. 2 Taxonomy uncertain.

1 Closely related to fungi. 2 Taxonomy uncertain.

of other groups), and mitosomes (Entamoeba) (see under specific protozoan groups). Motile stages of the parasitic protozoa mostly move by means of flagella, cilia, or pseudopodia. Some species produce resistant stages (cysts, oocysts) in which the parasites can survive outside of their hosts for longer periods.

According to current theories, the protozoa are a heterogeneous group consisting of different phyla within the regnum of Eukaryota. The term protozoa has no phylogenetic significance but is still used as a collective name for the various eukaryotic unicellular organisms. The classification of the protozoa is highly controversial. Therefore, all classification systems have to be regarded as provisional (Table 9.1).

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Responses

  • gundolpho
    How does the phylum apicomlexa move about?
    8 years ago

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