Protozoa is a subkingdom (formerly a phylum) comprised of organisms with eucaryotic cells that have many of the intracellular components characteristic of higher forms of life. Such other organisms as bacteria do not have this nucleus and are referred to as prokaryotes. Protozoa also have some form of active locomotion, which is a distinguishing feature in classifying them. Even though there are about 50,000 species of protozoa, relatively few are able to cause disease in humans. Protozoal diseases used to be limited to tropical, subtropical, and underdeveloped nations. Now, however, they are becoming a worldwide concern. Protozoa are generally free-living, but some exist as parasites or in a commensal relationship with another organism. The protozoa that are pathogenic parasites are of major interest because they cause such diseases in humans as malaria, trypanosomiasis, toxoplasmosis, and dysentery.
Protozoa generally exist in two basic forms: the active, growing form called the “trophozoite;” and the dormant, resistant form called the “cyst.” The trophozoite form proliferates tissues, causing damage that results in clinical disease. The cyst is able to survive in an external environment and is usually the form that is transmitted from host to host. Some protozoa go through an intermediate stage in blood-sucking insects.
Protozoology is the scientific study of protozoa. Classification of the organism is divided into seven phyla: Sarcomastigophora, Labyrinthomorpha, Apicoplexa, Microspora, Acetospora, Myxozoa, and Aliophora. In 1985, an extensive classification scheme was proposed for protozoa that included various phyla, subphyla, classes, etc.
The four groups of protozoa that are mainly responsible for human disease include the following: sarcodina, ciliophora, mastigophora, and sporozoa – all grouped according to their form of locomotion.
- Sarcodina, commonly known as amoebas, move by extending a section of their cytoplasm (called a pseudopodium or false foot) in one direction, causing the remainder to follow. They are usually found in marine and fresh water. Members include eight species (see Endoparasites). Three are parasitic to humans, with one causing more of a problem than the others. (Entamoeba histolytica causes the disease amebiasis.)
- Ciliophora, or Ciliates, move by using the many fine cilia that beat in rhythmic patterns to propel the organism. Members include and Paramecium, but only one species causes disease in humans; and that is of a dysentery nature. Balantidium coli is a large oval-shaped cell that is the largest intestinal protozoa found in humans. Increasingly, it is showing up in the human intestinal tract, where it can invade and destroy the intestinal lining. Its normal habitat is the intestinal tract of hogs, but it can also be found in marine and fresh water worldwide, causing the disease known as balantidiasis. The life cycle is similar to that of the amoeba E. histolytica and has been associated with chronic fatigue syndrome.
- Mastigophora is a subphylum of protozoa that has one or more whiplike flagella that propel the organism like swimmers. They are commonly known as Flagellates and are normally found in fresh water. Two relatively mild diseases, trichomoniasis and giardiasis, are produced from them, as well as the more serious diseases of trypanosomiasis and leishmaniasis.
- Sporozoa (singular sporozoon) is a class of parasitic protozoa that include Plasmodium and Toxoplasma. These two are commonly known as the parasites, found in vectors responsible for malaria and toxoplasmosis. They have both a sexual and asexual phase. They mainly target the epithelial cells of the intestinal tract, but can also be found in the liver and other organs.
Other, yet unclassified, sporozoa on the rise are Pneumocystis carinii and Cryptosporidium. They are of particular concern to the immunocompromised, particularly those with AIDS, cancer, or a recipient of transplants. P. carinii is responsible for a type of pneumonia, while Cryptosporidium produces a profuse watery diarrhea.