One scientific way to group protozoa is by their method of locomotion. Although this classification does not necessarily convey genetic relatedness, it does group them by phenotypic similarity and thus makes sense diagnostically.
The protozoan groups found in the human intestine include:
1. Amoebae (also known as Sarcodina)
These organisms use pseudopodia to both move and feed. Pseudopodia are projections of the organisms' cytoplasm that occur through a reorganization of their cytoskeletal filaments and myosin (the image to the right shows several extended pseudopods of an amoeba).
2. Flagellates (also known as Mastigophora)
As the word denotes, these organisms move using flagella. Flagella are long, rod-like structures composed of a filament called an axoneme (comprised of nine pairs of microtubules) surrounded by a cell membrane. The flagellum (singular) is anchored in a basal body. Some organisms can have several flagella.
3. Ciliates
Cilia are similar to flagella but are much shorter and more numerous. They beat in a coordinated rhythmic manner, thus moving the organism in a spiral path.
4. Coccidia
These protozoa belong to the larger group known as Sporozoa, are non-motile, and live intracellularly, unlike the above three groups. Also, the above three groups reproduce asexually, whereas the coccidia can reproduce both asexually and sexually.
5. Microsporidia
These extremely small parasites are no longer considered protozoa but are now classified as fungi. They are non-motile and live intracellularly. They will be mentioned here because some parasitology sources still include them.
After an overview of diagnostic techniques, we will discuss some of the major parasites in each group, including their life cycles, epidemiology, and pathology, with a focus on diagnosis.
