Page 56 - parasitology for medical and clinical laboratoryprofessionals

36 CHAPTER 3

Pinocytosis Pseudopodia Urethritis
Protista Tenesmus Vacuoles
Pseudomembranous colitis Trophozoite Zoonoses

INTRODUCTION physician must be alert to the underlying causes of new
infections based on diseases already present but also to a
Numerous protozoa live in the gastrointestinal tract of lifestyle placing the patient at increased risk for contract-
humans and a source of those that cause human infec- ing infectious diseases.
tions may come from animals which harbor similar or
the same intestinal parasites as humans but that may not CLASSIFICATION OF PROTOZOA
be harmful to the animal. It is possible that mutations
of animal protozoa and even other classes of parasites Some protozoa are similar under microscopic exami-
may have made it possible for these organisms to inhabit nation and are easily confused as pathogens when they
and cause disease in humans. Some protozoa are non- may actually be nonpathogenic. Morphological and ana-
pathogenic, whereas other species may cause only mild tomical features are used to differentiate both protozoa
discomfort in an episodic manner. A number of medi- and other classes of parasites such as worms that may
cally important single-celled organisms called protozoa be differentiated both by the appearance of larvae and
may be found that are classified as parasites in humans. adults and by the eggs they produce. Comparison of wet
These protozoal organisms are found in the phylum of mounts and stained specimens of protozoa that may con-
the kingdom called Protista and the group includes uni- tain either or both cyst stages and trophozoite stages are
cellular and animal-like organisms as opposed to other helpful.
one-celled free-living organisms such as bacteria that The taxonomic phylogeny of parasites is complex,
may also be either pathogenic or in some cases helpful. and those causing human diseases are no exception. The
At any rate, both parasites and many species of bacteria kingdom Protista includes the phylum Protozoa, which
may produce serious illnesses. includes a number of species of parasites with which
Intestinal parasites classified as protozoa are most humans become infected. Ordinarily these organisms
often transmitted by the fecal-oral route and are due to are loosely divided into three groups and the first set of
poor hygiene and living conditions in many instances. nomenclature used to differentiate protozoa relates to
Most exist as a cyst or resistant stage, whereas a few are the type of locomotion, if any, that is present. Ciliates
found only in the trophozoite or active stage. The cyst use cilia or short hairlike projections to effect movement
stage with a protective wall offers some protection from and belong to the phylum Ciliophora. Basically only one
adverse conditions and is a resting or dormant stage entity of the phylum Ciliophora is described as one that
upon which the cysts become trophozoites upon intro- is capable of causing human disease (Balantidium coli).
duction to the appropriate host. Trophozoites are active Flagellates possess longer modified cilia of which there
in their metabolism and are usually motile, making them are usually only a few species characterized by this type
easier to identify that cyst stages. These intestinal proto- of ciliate. Amoebae include some of the most patho-
zoa often are contracted when a victim is immunocom- genic types of protozoa and most movement is achieved
promised by other disease states such as AIDS and may by amoeboid movement where some organisms extend
cause life-threatening diarrhea by causing dehydration pseudopodia, or “false feet,” by which they pull them-
and aggravation of existing conditions. In HIV-positive selves along a surface with only minimal movement.
patients and those who have progressed to the chronic These classes of protozoa are presented with representa-
disease of AIDS, the patient may have an incompetent tives of each of the basic types of protozoa that will be
immune condition in which the individual has lost the presented in the following sections. Table 3-1 depicts
ability of the body to ward off and prevent infection the divisions of both pathogenic and nonpathogenic
because of a form of immunological dysfunction. The protozoa.