Morphologically, the three species of Trichomonas of man are distinct, although T. vaginalis and T. tenax resemble each other more than either resembles T. hominis. T. vaginalis is much the larger, the other two being about the same length. T. vaginalis is more robust in shape and T. tenax is more slender.
T. vaginalis and T. tenax normally have a single compound blepharoplast which may occasionally appear as two approximately equal elements. In T. hominis there are characteristically two blepharoplasts of unequal size, the smaller being ventral in position. The oral and vaginal species have regularly four anterior flagella. T. hominis has a variable number, usually four or five, but more characteristically five. One of these is attached to the smaller ventral blepharoplast and the other four to the larger one.
The undulating membrane of T. tenax is relatively longer than that of T. vaginalis, although typically less than body length in both and without an extension of the marginal filament into a trailing flagellum. T. hominis has a full-length undulating membrane with a trailing flagellum. At the margin of the undulating membrane each of the three species has an accessory filament which is more slender than the posterior flagellum.
The costa of T. vaginalis and T. tenax is very slender and of uniform diameter while that of T. hominis is coarser, and thicker in its midregion. Both T. vaginalis and T. tenax have a well-developed parabasal apparatus, but this is apparently absent in T. hominis. The axostyle of T. hominis is relatively somewhat coarser than that of the other two species. In large individuals of T. vaginalis it tends to split into a number of fibrils.
The nucleus of T. vaginalis is typically much elongated while in the other two species it is more commonly rounded or oval. In T. vaginalis and T. tenax the chromatin is usually aggregated into granules which are uniformly distributed; these are larger and fewer in T. tenax. In T. hominis the chromatin is less often aggregated into granules and commonly forms a layer against the nuclear membrane.
The siderophilic cytoplasmic granules of T. vaginalis are numerous and scattered through the cytoplasm but tend to collect along the axostyle and costa. In T. tenax they are fewer, forming a row along the costa but not along the axostyle. In T. hominis there is a row of paracostal granules at the base of the undulating membrane and a zone of perinuclear granules or a chromatic cloud in the vicinity of the nucleus.
The cytostome is inconspicuous in T. vaginalis but more evident in the other two species, especially in T. hominis. In fresh material T. vaginalis usually shows no food inclusions except a leukocyte in an occasional individual while T. hominis usually shows a variety of food bodies.
T. vaginalis forms pseudopodia more often than the other two species, is more sluggish, and shows greater tendency to adhere to solid objects and to each other at the posterior ends.
Survey data show that T. vaginalis and T. tenax are much more prevalent than T. hominis even in the same group of persons, indicating that the latter species is not transferable to other parts of the body.
Available literature provides abundant evidence that, in the matters of culture conditions, inoculation into various animals and unsuccessful transmission of intestinal and oral trichomonads to the human vagina, these three species are physiologically as well as morphologically distinct.
Trichomonas vaginalis Donné is the type species of the genus and presents no taxonomic problem. T. tenax (O.F.M.) has priority for the oral species. The intestinal form appears to have a variable number of flagella, characteristically five. For the present, T. hominis (Davaine) is retained for it. Tritrichomonas fecalis Cleveland is shown to be distinct from T. hominis and very similar to, if not identical with, a species from the intestine of frogs and toads.