Informational Tables

4.2. Tissue protozoa: characteristics

Species

Shape and size

Other features^a

Toxoplasma gondii

Trophozoites (tachyzoites)

Crescent shaped; 4–6 μm long by 2–3 μm wide

Found in peritoneal fluid of experimentally infected mice (research, not used in clinical laboratory); intracellular forms small and may be more difficult to identify than the cyst forms in human clinical specimens. May be isolated in tissue culture, particularly from CSF. Diagnosis is most frequently based on clinical history and serological evidence (acute- and convalescent-phase sera). NAAT highly recommended.

Cysts (bradyzoites)

Generally spherical; 200 μm to 1 mm in diam

Occur in many body tissues (approx 15% of the U.S. population have these organisms in tissues, indicating past infection). Many infections are asymptomatic. Infections in compromised hosts are very serious and involve the CNS. For these patients, particularly those with AIDS, diagnostic serological titers may be very difficult to demonstrate.
Note:Cysts identified in histological preparations may or may not be causative agents of symptoms (additional testing required to demonstrate relevant serologic changes and/or the presence of tachyzoites in clinical specimens).

Cryptosporidium spp. Apicomplexa

Oocyst usually round, 4–6 μm, each mature oocyst containing sporozoites (infective on passage)

Oocyst usually diagnostic stage in stool, sputum, and possibly other body specimens. Various other stages in life cycle can be seen in biopsy specimens taken from GI tract (brush border of epithelial cell-intestinal tract) and other tissues (lung, gallbladder). Several modified acid-fast stains have been used successfully. Direct detection methods using immunoassay reagents are also available. NAAT also recommended.
Note: Infection in the immunocompetent host is self-limiting; however, in immunodeficient patients (AIDS), infection is chronic because of an autoinfective capability in the life cycle. The number of oocysts usually correlates with symptoms (watery diarrhea = many oocysts in specimen). The more normal the stool, the more difficult it is to find oocysts. Risk groups include animal handlers, travelers, immunocompromised individuals, children in day care centers, and those who come in contact with these individuals. Since oocysts are immediately infective, nosocomial transmission has been documented.

Cyclospora cayetanensis
Coccidia

Oocyst usually round, 8–10 μm; each oocyst is immature on passage; no internal morphology visible; oocysts appear as “wrinkled” cellophane

Oocyst is the diagnostic stage in stool. Various other stages in the life cycle can be seen in biopsy specimens taken from the GI tract (within epithelial cells and intestinal tract). The morphology is similar to that of Cystoisospora (Isospora) belli. A number of modified acid-fast stains have been used successfully to demonstrate the oocysts (quite acid-fast variable). Detection methods involving immunoassay reagents are under development.

Cystoisospora belli Coccidia

Ellipsoidal oocyst; usual range, 20–30 μm long by 10–19 μm wide; sporocysts rarely seen broken out of oocysts but measure 9–11 μm

Mature oocyst contains two sporocysts with four sporozoites each; usual diagnostic stage in feces is immature oocyst containing spherical mass of protoplasm (diarrhetic stool). Developing stages can be recovered from intestinal biopsy specimens. Oocysts are also acid fast and can be detected during modified acid-fast staining of stool for Cryptosporidium spp. Oocysts are often detected in concentration sediment (wet preparation).

Microsporidia (now classified with the fungi)

Brachiola spp.

Nosema spp.

Encephalitozoon spp.

Pleistophora spp.

Trachipleistophora spp.

Anncaliia spp.

Enterocytozoon bieneusi.

Microsporidium spp.

Vittaforma corneae

Spores are extremely small and have been recovered from all body sites, including the eye.

These organisms have been found as insect or other animal parasites; the route of infection may be ingestion, inhalation, or direct inoculation (eye). NAAT is recommended; histology results vary (spores are acid fast); PAS (positive granule at one end of spore/difficult to see), silver (spores positive), tissue Gram, and Giemsa stains are recommended for spores. Animal inoculation is not recommended; laboratory animals may carry occult infection; electron microscopy or NAAT may be necessary for confirmation and identification to the genus and species levels. Although difficult to diagnose, infections have been found in a large number of AIDS patients (Enterocytozoon bieneusi, Encephalitozoon intestinalis in the intestinal tract, Pleistophora spp. in muscle, and various other microsporidia in other tissues, including the CNS). To date, it is still somewhat difficult to diagnose this infection by examining stool specimens prepared with optical brightening agents (calcofluor) or routine stains (modified trichrome, acid-fast trichrome stains). Diagnostic immunoassay reagents are available commercially (check for FDA clearance)

Sarcocystis hominis, S. heydorni (beef) or S. suihominis (pork)

Thin-walled oocyst contains two mature sporocysts, each containing four sporozoites; frequently thin oocyst wall ruptures; ovoidal sporocysts are 9–16 μm long by 7.5–12 μm wide

Thin-walled oocyst or ovoidal sporocysts occur in stool. Compromised host may show fever, severe diarrhea, abdominal pain, and weight loss, although the number of patients has been small (may be unrecognized), in some areas of the world the estimated incidence is as high at 23% (Asia). Infections occur from ingestion of uncooked pork or beef. Life cycle occurs within intestinal cells, with eventual production of sporocysts in stool

Sarcocystis spp. (non-human Sarcocystis species)

Shapes and sizes of skeletal and cardiac muscle sarcocysts vary considerably.

When humans ingest non-human Sarcocystis sporocysts, they become dead-end (intermediate) hosts; extraintestinal symptoms range from asymptomatic muscle cysts to severe, acute eosinophilic myositis. The term Sarcocystis lindemanni is no longer considered correct.