P. hominis is probably the most commonly identified flagellate, other than G. lamblia and D. fragilis. P. hominis has been recovered from all parts of the world, in both warm and temperate climates, and is considered nonpathogenic and noninvasive. It is not known to have a cyst stage (see Figure 1). P. hominis trophozoites live in the cecum and feed on bacteria. The trophozoite measures 5 to 15 µm long and 7 to 10 µm wide. It has a pyriform shape and has both an axostyle and an undulating membrane, which aid identification of the organism. The undulating membrane extends the entire length of the body, in contrast to that seen in the pathogen T. vaginalis (on which the membrane extends halfway down the body).
Fig1. 1, Trophozoite of Pentatrichomonas hominis. 2, Trophozoite of Chilomastix mesnili. 3, Cyst of C. mesnili. 4, Trophozoite of Giardia lamblia (front view). 5, Trophozoite of G. lamblia (side view). 6, Cyst of G. lamblia. 7, Trophozoite of Enteromonas hominis. 8 to 10, Cysts of E. hominis. 11, Trophozoite of Retortamonas intestinalis. 12 to 13, Cysts of R. intestinalis. (From Garcia LS, Bruckner DA: Diagnostic medical parasitology, Washington, DC, 1993, ASM Press; illustration 5 by Nobuko Kitamura; illustrations 7 to 13 modified from Markell EK, Voge M: Medical parasitology, ed 5, Philadelphia, 1981, WB Saunders.)
Epidemiology
Because P. hominis is not known to have a cyst stage, transmission probably occurs in the trophic form. If ingested in a substance such as milk, these organisms apparently can survive passage through the stomach and small intestine in patients with achlorhydria. P. hominis cannot be transplanted into the vagina, the natural habitat of T. vaginalis. The incidence of this organism is relatively low, but it tends to be recovered more often than E. hominis or R. intestinalis, two small nonpathogenic flagellates that are rarely seen and extremely difficult to identify (see Figure 1).
Pathogeneis and Spectrum of Disease
P. hominis is considered nonpathogenic and does not cause disease.
Laboratory Diagnosis
P. hominis trophozoites can sometimes be seen on a permanent stained smear, but definitive identification can be difficult. However, it is important to report the presence of the organism if seen.
Therapy
Specific treatment is not recommended for this nonpathogen.
Prevention
Prevention depends on adequate disposal of human excreta and improved personal hygiene, preventive measures that apply to most of the intestinal protozoa.
