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The flagellated protozoa are widespread in nature, multiply by binary fission, and move about by means of a primary organelle of locomotion, the flagellum. This organelle arises from an intracellular focus known as a kinetosome (basal body), extends to the cell wall as a filamentous axoneme composed of microtubules arranged in the typical 9 pairs + 2 central microtubular pattern, and continues extracellularly as the free flagellum. A pair of dynein arms extends from each outer microtubule of a pair to an adjacent microtubular pair and is responsible for flagellar beating through ATP hydrolysis. The long, whip-like free flagella may be single or multiple. The number is distinctive for individual species. When more than one flagellum is present, each has its own associated basal body and axoneme. The entire flagellar unit and any associated organelles are referred to as a mastigont system.

In some flagellates, such as the trypanosomes, the flagellum becomes part of the cell surface and creates a structure called an undulating membrane. Movement occurs in helical waves and seems to be suited for organisms living within a viscous fluid environment such as that found in the bloodstream.

In other flagellates, the mastigont system includes a rod-like costa, which may serve as a supporting structure for the undulating membrane, or a tube-like axostyle, which arises from the base of flagella and probably functions in rotational motility and support. Trichomonads possess both these structures.

Although several flagellate genera parasitize humans, only four, Trichomonas, Giardia, Leishmania, and Trypanosoma, commonly induce disease. Trichomonas and Giardia are noninvasive organisms that inhabit the lumina of the genitourinary or gastrointestinal tract and spread without the benefit of an intermediate host. Disease is of low morbidity and cosmopolitan distribution. Leishmania and Trypanosoma, on the other hand, are invasive tissue and blood parasites that produce highly morbid, frequently lethal diseases. These hemoflagellates require an intermediate insect host for their transmission. Thus, their associated disease states are limited to the semitropical and tropical niches of these intermediate hosts.


Luminal flagellates can be found in the mouth, vagina, or intestine of almost all vertebrates, and it is common for an animal host to harbor more than one species. Humans may serve as host and reservoir to eight species (Table 53–1), but only two cause disease. Of these, Giardia duodenalis (=lamblia) inhabits the intestinal tract, and Trichomonas vaginalis inhabits the vagina and genital tract.

✺ Found in flora of vertebrates

TABLE 53–1Luminal Flagellates Infecting Humans

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