2.1). These include the Protozoa (amebae, flagellates, ciliates, sporozoans, coccidia, and microsporidia), the Nematoda or roundworms, the Platyhelminthes or flatworms (cestodes, trematodes), the Pentastomids or tongue worms, the Acanthocephala or thorny-headed worms, and the Arthropoda (insects, spiders, mites, ticks, and so on). Although these categories appear to be clearly defined, there may be confusion in attempting to classify parasites, often due to the lack of known specimens. If organisms recovered from humans are very rare, it is difficult to determine their correct taxonomic positions. Type specimens must be deposited for study before a legitimate species name can be given. Also, even when certain parasites are numerous, they may represent strains or races of the same species with slightly different characteristics.
Reproductive mechanisms have been used as a basis for determining species definitions, but there are many exceptions within parasite groups. Another difficulty in species recognition is the ability and tendency of the organisms to alter their morphologic forms according to age, host, or nutrition, which often results in several names for the same organism. An additional problem involves alternation of parasitic and free-living phases in the life cycle; these organisms may be very different and difficult to recognize as belonging to the same species. However, newer molecular methods of grouping organisms have often confirmed taxonomic conclusions reached hundreds of years earlier by experienced taxonomists. As studies in parasitic genetics, immunology, and biochemistry continue, the species designation will be defined more clearly by highly sophisticated molecular techniques.
No attempt has been made to include every possible organism; only those that are considered to be clinically relevant in the context of human parasitology are addressed. Some human infections are represented by very few cases; however, they are well documented and are included here. Further information is provided in the tables at the end of this section.
Protozoa (Intestinal)
Amebae
Amebae are single-celled organisms characterized by having pseudopods (motility) and trophozoite and cyst stages in the life cycle. The cell’s organelles and cytoplasm are enclosed by a cell membrane, such that the cell obtains its food through phagocytosis. However, there are some exceptions in which a cyst form has not been identified. In environments which are potentially lethal to the cell, an ameba may become dormant by surrounding itself with a protective membrane to become a cyst. The cell remains in this form until it encounters more favorable conditions, at which time the organism excysts to release trophozoites. While in the cyst form, amebae do not replicate and may die if unable to excyst for a lengthy period.
Amebae are usually acquired by humans via fecal-oral transmission or mouth-to-mouth contact (Entamoeba gingivalis). In most species, after several nuclear divisions occur, comparable division of the cytoplasm follows excystation. Entamoeba histolytica is the most significant organism within this group.
Although Blastocystis hominis is an enteric protozoan parasite that is commonly found worldwide, the classification is undergoing review. B. hominis has extensive genetic diversity and infects humans and many other animals. Statistically, it may be the most common intestinal parasite recovered. Some of the subtypes are considered to be pathogens while others are probably nonpathogenic, a situation that leads to different opinions regarding pathogenicity. Unfortunately, these subtypes cannot be differentiated on the basis of microscopic morphology. Since there may be a relationship between numbers present and symptoms, this is one of the few parasites whose numbers should be specified in the report (rare, few, moderate, many, packed). It is recommended that quantitation be determined from the permanent stained smear.
Representative organisms include Entamoeba histolytica, Entamoeba dispar, Entamoeba coli, Entamoeba hartmanni, Entamoeba gingivalis, Endolimax nana, Iodamoeba bütschlii, and Blastocystis hominis.
Flagellates
Flagellates move by means of flagella and are acquired through fecal-oral transmission. With the exception of Dientamoeba fragilis (internal flagella) and organisms in the genus Pentatrichomonas, flagellates have both trophozoite and cyst stages in the life cycle. Reproduction is by longitudinal binary fission. Giardia lamblia is the most common pathogen in this group and is one of the most commonly found intestinal parasites. However, when permanent stained smears are routinely performed, D. fragilis is also found to be more common than suspected. Representative organisms include G. lamblia, D. fragilis, Pentatrichomonas hominis, Chilomastix mesnili, Enteromonas hominis, and Retortamonas intestinalis.
Ciliates
Ciliates are single-celled protozoa that move by means of cilia and are acquired through fecal-oral transmission. Balantidium coli is the only human pathogen in the group. Hosts include pigs, wild boars, rats, primates (including humans), horses, cattle, and guinea pigs. Infection is transmitted within or between these species by fecal-oral transmission of the infective cysts. Pigs are the most significant reservoir hosts, although they show few if any symptoms. Following ingestion, excystation occurs in the small intestine, and the trophozoites colonize the large intestine. The cilia beat in a coordinated rhythmic pattern, and the trophozoite moves in a spiral path. They have both trophozoite and cyst stages in the life cycle, and both stages contain a large macronucleus and a smaller micronucleus. These protozoa have a distinct cell mouth (cytostome), cytopharynx, and less conspicuous cytopyge (anal pore). These organisms are considerably larger than the majority of the intestinal protozoa and can be mistaken for debris or junk when seen in a permanent stained smear. The concentration wet preparation examination is recommended.
Coccidia
Coccidia are microscopic, spore-forming, single-celled, obligate intracellular parasites, which means that they must live and reproduce within an animal cell. These protozoa are acquired by ingestion of various meats or through fecal-oral transmission via contaminated food and/or water. In some cases, coccidia disseminate to other body sites, particularly in the severely compromised patient. These protozoa have both asexual and sexual cycles, the most common infective stage being the oocyst, containing sporocysts and/or sporozoites, all of which can be acquired through fecal-oral transmission. Representatives within this group include Cryptosporidium spp., Cyclospora cayetanensis, Isospora belli, and Sarcocystis spp.
Microsporidia
Currently, the most difficult intestinal protozoa to diagnose are the microsporidia (size range, 1 to 2.5 μm); the development of molecular biology-based methods should provide more specific and sensitive methods. These organisms have also been documented to disseminate from the intestinal tract to other body sites, including the kidneys and lungs. Routine parasitology stains are not useful; modified trichrome stains have been developed specifically for these organisms. Compared with the more common Wheatley’s trichrome for routine stool staining, the modified trichrome stain contains a 10-fold higher concentration of the main dye, chromotrope 2R. The infective form is called the spore; each spore contains a polar tubule that is used to penetrate new host cells, thus initiating or continuing the life cycle. Infections are acquired through ingestion, inhalation, or direct inoculation of spores from the environment. Currently at least two genera have been documented to cause human infection in the intestinal tract (Encephalitozoon intestinalis and Enterocytozoon bieneusi).
Protozoa (Other Body Sites)
Amebae
