TABLE A2.
Key characteristics of protozoa of the intestinal tract and urogenital systema
| Organism(s) (characteristic[s]) | Trophozoite or tissue stage | Cyst or other stage in specimen | Comments |
|---|---|---|---|
| Amoebae (shrinkage occurs in cyst forms in stained preparations, creating a halo which should be included in the measurement) | |||
| Entamoeba histolytica (pathogenic) | Cytoplasm is clean; the presence of RBCs is diagnostic, but the cytoplasm may also contain some ingested bacteria; peripheral nuclear chromatin is usually evenly distributed, with a central, compact karyosome | A mature cyst contains 4 nuclei; chromatoidal bars have smooth, rounded ends; the organism cannot be differentiated from E. dispar | Considered pathogenic; should be reported to public health authorities; trophozoites can be confused with macrophages and cysts can be confused with PMNs in stools |
| Entamoeba dispar (nonpathogenic) | Morphology identical to that of E. histolytica (confirmed by the presence of RBCs in the cytoplasm); if no RBCs are present, molecular testing or fecal immunoassays are necessary to confirm species designation | A mature cyst has a morphology identical to that of E. histolytica | Nonpathogenic; morphology resembles that of E. histolytica; these organisms should be reported as Entamoeba histolytica/E. dispar and reported to public health authorities; immunoassay reagents are now available to identify the Entamoeba histolytica/E. dispar group and to differentiate pathogenic E. histolytica and nonpathogenic E. dispar; some laboratories may decide to use these reagents on a routine basis, depending on the positivity rate and cost |
| Entamoeba histolytica/E. dispar (“group” or “complex” should be added to indicate that the two organisms are indistinguishable and require additional testing; some like to add the word “group” to indicate that the two organisms cannot be differentiated on the basis of morphology unless RBCs are seen within the cytoplasm or E. histolytica is confirmed using species-specific immunoassays) | Use the correct way to report, unless a species-specific immunoassay is used to identify E. histolytica or trophozoites are seen with ingested RBCs (E. histolytica) | ||
| Entamoeba hartmanni (nonpathogenic) | Looks identical to E. histolytica/E. dispar but is smaller (<12 μm); RBCs are not ingested | A mature cyst contains 4 nuclei but often has only 2; chromatoidal bars are often present and look like those of E. histolytica/E. dispar (size, <10 μm); very fine-looking organism | Shrinkage occurs on the permanent stain due to dehydration steps (especially in the cyst form); E. histolytica/E. dispar may actually be below the 12- and 10-μm cutoff limits and can be as much as 1.5 μm below the limits quoted for wet prepn measurements |
| Entamoeba coli (nonpathogenic) | Cytoplasm is dirty and may contain ingested bacteria or debris; peripheral nuclear chromatin is unevenly distributed, with a large, eccentric karyosome | A mature cyst contains 8 nuclei; more may be seen; chromatoidal bars (if present) tend to have sharp, pointed ends | If a smear is too thick or thin and if the stain is too dark or light, E. histolytica/E. dispar and E. coli can often be confused, since there is much overlap in morphology |
| Endolimax nana (nonpathogenic) | Cytoplasm is clean, not diagnostic, with a great deal of nuclear variation; there may even be some peripheral nuclear chromatin; perikaryosomal space is usually clean looking; normally only karyosomes are visible | The cyst is round to oval, with the 4 nuclear karyosomes being visible as miniature versions of the trophozoite karyosome | There is more nuclear variation in this amoeba than in any others; the organisms can be confused with Dientamoeba fragilis and/or E. hartmanni by inexperienced microscopists |
| Iodamoeba bütschlii (nonpathogenic) | Cytoplasm contains much debris; organisms are usually larger than E. nana but may look similar; large karyosome | The cyst contains a single nucleus (may be a basket nucleus) with bits of nuclear chromatin arranged on the nuclear membrane (the karyosome is the basket, the bits of chromatin are the handle); large glycogen vacuole; the perikaryosomal space is slightly darker due to the presence of chromatin fibrils | The glycogen vacuole stains brown with the addition of iodine in the wet prepn; a basket nucleus is more common in cysts but can be seen in trophozoites; the vacuole may be so large that the cyst collapses on itself |
| Blastocystis spp. (pathogenic; the organisms are undergoing review for possible reclassification; multiple strains or subtypes look the same [approx half are pathogenic, half are nonpathogenic]; numerous subtypes from different species are not all pathogenic for humans) | Trophozoites may/may not be seen; often in patients with diarrhea; difficult to identify | Central-body forms are the most common; there is tremendous size variation; the central area may or may not stain; the outer perimeter contains multiple nuclei (often seen as variously sized dots) | This is the most common gastrointestinal tract organism worldwide; it is much more common than Giardia or Dientamoeba (whose numbers tend to be equal, although Dientamoeba organisms are more common than Giardia organisms in many areas); symptomatic patients tend to be treated when >5 cysts/high-power field are reported |
| Flagellates | |||
| Giardia lamblia (pathogenic) | Trophozoites are teardrop shaped from the front and like a curved spoon from the side; they contain 2 nuclei, linear axonemes, and curved median bodies | Cysts are round to oval, containing 4 nuclei, axonemes, and median bodies | Organisms live in the duodenum, and multiple stool specimens may be negative; additional sampling techniques (aspiration, Entero-Test) may be needed; fecal immunoassays are helpful; assemblages A and B are pathogenic to humans; other assemblages have a narrow host specificity |
| Chilomastix mesnili (nonpathogenic) | Trophozoites are teardrop shaped; the cytostome is usually visible for identification; the nucleus is usually situated at the anterior end | The cyst is lemon shaped with 1 nucleus and a curved fibril, called the shepherd's crook (cytostome remnant) | The cyst can be identified much more easily than the trophozoite form; the trophozoite looks like some of the other small flagellates |
| Dientamoeba fragilis (pathogenic) | Cytoplasm contains debris; may contain 1 or 2 nuclei (chromatin is often fragmented into 4 packets) | The cyst form has now been identified; it appears to have a double wall; the percentage is quite low (∼1–2%); thus, it can be very difficult to find and identify | Tremendous size and shape range on a single smear; trophozoites with 1 nucleus can resemble E. nana; staining quality is important to produce packets, not a single “blob” |
| Trichomonas vaginalis (pathogenic) | Supporting rod (axostyle) is present; the undulating membrane comes halfway down the organism; small dots may be seen in the cytoplasm along the axostyle | No known cyst form | Recovered from the genitourinary system; often diagnosed at bedside with wet prepn (motility) |
| Pentatrichomonas hominis (nonpathogenic) | Supporting rod (axostyle) is present; the undulating membrane comes all the way down the organism; small dots may be seen in the cytoplasm along the axostyle; karyosome appears granular | No known cyst form | Recovered in stool; trophozoites may resemble other small flagellate trophozoites |
| Ciliates | |||
| Balantidium coli (pathogenic) | Very large trophozoites (50–100 μm long) covered with cilia; a large bean-shaped macronucleus is present; the very small micronucleus is difficult to see | Morphology is not significant, with the exception of a large, bean-shaped macronucleus; a small micronucleus is difficult to see | Rarely seen in the United States; causes severe diarrhea with large fluid loss; organisms are seen in proficiency test specimens or possibly people who work around pigs |
| Apicomplexa, coccidia | |||
| Cryptosporidium spp. (pathogenic) | Seen in the intestinal mucosa (edge of brush border), gallbladder, and lungs; present in biopsy specimens | Oocysts are seen in stool and/or sputum; organisms are acid fast and measure 4–6 μm; they are hard to find if only a few are present | Chronic infection occurs in a compromised host (internal autoinfective cycle), and self-cure occurs in an immunocompetent host; numbers of oocysts correlate with stool consistency; organisms can cause severe, watery diarrhea; oocysts are immediately infective when passed |
| Cyclospora cayetanensis (pathogenic) | Experience with this organism is not extensive; it may be difficult to identify in tissue; since patients are immunocompetent, biopsy specimens will rarely be required or requested | Oocysts are seen in stool (approx 8–10 μm in size); they are unsporulated and thus difficult to recognize as coccidia; they mimic Cryptosporidium on modified acid-fast-stained smears, they are larger, and they may appear almost colorless or darkly stained in acid-fast smears | Most infections are associated with immunocompetent individuals but may also be seen in immunosuppressed patients; may be associated with traveler's diarrhea; oocysts are not immediately infective when passed; within the United States, infections have been associated with contaminated food, including raspberries, basil, snow peas, and mesclun (baby lettuce leaves), which are considered “transmission vehicles; PCR can detect 40 or fewer oocysts per 100 g of raspberries or basil but has a detection limit of around 1,000 per 100 g in mesclun lettuce |
| Cystoisospora belli (pathogenic) | Seen in intestinal mucosal cells; seen in biopsy specimens; not as common as Cryptosporidium | Oocysts are seen in stool; organisms are acid fast; the best technique is concn, not a permanent-stain smear | Thought to be the only Cystoisospora sp. that infects humans; oocysts are not immediately infective when passed |
| Microsporidia Nosema spp. Encephalitozoon spp. Pleistophora spp. Trachipleistophora spp. Anncaliia sp. Enterocytozoon spp. Microsporidium spp. Vittaforma corneae Tubulinosema sp. | Developing stages are sometimes difficult to identify; spores can be identified by size, shape, and the presence of polar tubules | Depending on the genus involved, spores can be identified in stool or urine using the modified trichrome stain, calcofluor white, or immunoassay reagents (available outside the United States) | Spores are generally quite small (1–2.0 μm for Enterocytozoon spp.) and can easily be confused with other organisms or artifacts (particularly in stool); infections tend to be present in immunosuppressed patients; however, they are not limited to this patient group |
a
Modified from reference 10. RBC, red blood cells; PMN, polymorphonuclear neutrophils.