The trichomonads belong to the family Trichomonadidae within the order Trichomonadorida. The body is usually piriform, with a rounded anterior end and a pointed posterior end. There is a single nucleus in the anterior part of the body. Anterior to the nucleus is a blepharoplast composed of several basal granules. Two to five anterior flagella and a posterior flagellum arise from the blepharoplast. The posterior flagellum passes along the border of an undulating membrane which extends along the side of the body; a secondary or accessory filament may be associated with it. The posterior flagellum may or may not extend beyond the undulating membrane as a free flagellum. A filamentous costa arises from the blepharoplast and runs along the base of the undulating membrane. A parabasal body arises from the blepharoplast; there may or may not be a parabasal filament at its posterior end. A clear, rod-like axostyle also arises from the blepharoplast and passes thru the center of the body to emerge from the posterior end. The anterior end of the axostyle is enlarged to form a capitulum. There may or may not be a chromatic ring at the point of emergence of the axostyle. There may or may not be a cytostome near the anterior end. Just anterior to the blepharoplast and lying along the anterior margin of the body is a pelta which stains with silver. In addition to these structures, there may be various granules within or along the axostyle, along the costa, or in other locations.
An electron micrograph study of Tritrichomonas muris by Anderson (1955) revealed the fine structures of these organelles which may be taken to represent the group. He found that the blepharoplast appears to be limited by a membrane and to contain basal granules for each organelle. The anterior and posterior flagella are composed of 2 central and 9 peripheral fibrils. The accessory filament is composed of two differentiated meshwork areas. The undulating membrane is composed of a series of lamellae 300 to 400 A thick; it is attached to the outer surface of the body by fine fibers 167 to 300 A thick. The costa consists of a series of discs about 370 A thick and 490 A apart, embedded in a matrix; it is attached to the inner surface of the body wall by extensions of the discs. The axostyle is limited by a double, corrugated membrane. The chromatic ring is composed of a series of rods about 640 A thick. The parabasal bodyoconsists of a series of filaments about 190 A thick. The chromatic granules along the costa, inside the axostyle and scattered in the cytoplasm are irregular in shape and vacuolated. The mitochondria are spherical and contain a varying number of projections internally.
Trichomonads are divided into several genera on the basis of the number of their anterior flagella. Ditrichomonas has 2, Tritrichomonas has 3, Trichomonas has 4, and Pentatrichomonas has 5. These genera are closely related; Mehra, Levine and Reber (I960), for example, found in a column chromatographic study of the hydrolysates of Tritrichomonas foetus, T. suis, Trichomonas gallinae, T. gallinarum and T. buttreyi, that they are all composed of the same amino acids but that there are some differences in the amounts of each amino acid present in the different species.
Gabel (1954) established the genus Paratrichomonas for P. marmotae from the woodchuck and possibly T. batrachorum from the frog. Paratrichomonas differs from Tritrichomonas principally in having a ring-shaped parabasal body. T. buttreyi of the pig resembles it, but has 4 anterior flagella. There does not seem to be sufficient justification for accepting this genus, at least at present. Morgan (1943, 1946) and Trussell (1947) have given host-parasite lists of the trichomonad species.
There are several species of trichomonads in domestic animals and man, but the nomenclatorial status and host-parasite relations of many of them are not yet clear. They have been found in the cecum and colon of practically every species of mammal or bird that has been examined for them, and they also occur in reptiles, amphibia, fish and many invertebrates. Those in the termite gut are particularly well known. Many of the cecal trichomonads look alike, and cross-transmission studies have shown that many of them can be easily transmitted from one host species to another. Some mammalian trichomonads have even been transmitted successfully to day-old chicks, altho they will not become established in older birds. Further and extensive studies are needed to establish the correct names and host spectra of all but a few trichomonads.
Most trichomonads are non-pathogenic commensals, but a few are important pathogens. None of the cecal trichomonads has ever been proven to be pathogenic, altho some people have thought that they were because they were found in animals which had enteritis or diarrhea. However, the mere presence of an organism in a diseased animal does not mean that the organism caused the disease. The latter may have set up conditions favorable to the organism's growth and multiplicationo This is especially true of the cecal trichomonads, which flourish in a fluid or semi-fluid habitat.
The life cycle of trichomonads is simple. They reproduce by longitudinal binary fission. No sexual stages are known. There are no cysts, altho degenerating or phagocytized individuals (or entirely different organisms such as Blastocystis) have been mistaken for them.