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Condensed from CDC's laboratory reference (DPDx) comparative morphology tables. Sizes are length or diameter; the usual range is in parentheses.
Which preparation shows what
- Amebae: saline wet mounts show motility, inclusions (red cells, bacteria) and chromatoid bodies (clearer than in iodine); iodine shows cyst nuclei and glycogen; buffered methylene blue (or Quensel's stain) shows trophozoite nuclei; permanent stains show nearly everything. Formalin-fixed trophozoite nuclei are visible but rarely distinctive enough for species.
- Flagellates and ciliates: saline shows motility and shape; neutral red helps with trophozoites; iodine shows cyst nuclei and fibrils. The undulating membrane of Trichomonas and spiral groove of Chilomastix aren't always visible.
- Coccidia: oocysts show in most preparations; Cryptosporidium oocysts in acid-fast stains.
Amebae
Entamoeba histolytica/dispar trophozoite. CDC.
| Species | Trophozoite | Cyst |
|---|---|---|
| Entamoeba histolytica | 10–60 µm (15–20 µm commensal; over 20 µm invasive); progressive, finger-like pseudopods; fine, even peripheral chromatin, small central karyosome; may hold red blood cells | 10–20 µm (12–15); spherical; 4 nuclei when mature; chromatoid bars with blunt, rounded ends |
| E. hartmanni | 5–12 µm (8–10); like E. histolytica but small, karyosome often eccentric | 5–10 µm (6–8); 4 nuclei; blunt bars |
| E. coli | 15–50 µm (20–25); sluggish, blunt pseudopods; coarse, irregular peripheral chromatin; large eccentric karyosome; coarse, vacuolated cytoplasm with bacteria and yeasts | 10–35 µm (15–25); 8 nuclei (sometimes 16 or more); chromatoid bodies splinter-like with pointed ends |
| E. polecki | 10–25 µm (15–20); sluggish; many vacuoles | 9–18 µm (11–15); 1 nucleus (rarely 2); an "inclusion mass" often present |
| Endolimax nana | 6–12 µm (8–10); no peripheral chromatin; large, irregular "blot-like" karyosome | 5–10 µm (6–8); 4 nuclei |
| Iodamoeba buetschlii | 8–20 µm (12–15); large central karyosome ringed by granules | 5–20 µm (10–12); 1 nucleus; a compact glycogen mass staining dark brown with iodine |
| Dientamoeba fragilis | 5–15 µm (9–12); angular or serrated pseudopods; 2 nuclei (1 in about 20%), each with a cluster of 4–8 granules | no cyst |
Entamoeba coli cyst. CDC.
Flagellates, ciliate, coccidia and Blastocystis
Giardia duodenalis trophozoite. CDC.
| Species | Trophozoite | Cyst or oocyst |
|---|---|---|
| Pentatrichomonas hominis | 6–20 µm (11–12); pear-shaped; jerky; undulating membrane the length of the body | no cyst |
| Chilomastix mesnili | 6–24 µm (10–15); stiff, rotary motion; long cytostome; spiral groove | 6–10 µm (8–9); lemon-shaped with a clear knob |
| Giardia duodenalis | 10–20 µm (12–15); "falling leaf" motion; 2 nuclei; sucking disk over half to three-quarters of the underside | 8–19 µm (11–12); oval; usually 4 nuclei at one end; fibrils lengthwise |
| Enteromonas hominis | 4–10 µm (8–9); oval, one side flattened | 4–10 µm (6–8); 1–4 nuclei, usually 2 at opposite ends; resembles E. nana |
| Retortamonas intestinalis | 4–9 µm (6–7); cytostome about half the body | 4–9 µm (4–7); pear or lemon-shaped; resembles Chilomastix |
| Balantidium coli (ciliate) | 50–70 µm or more (40–50); rotary, boring; covered in cilia; large kidney-shaped macronucleus | 45–65 µm (50–55) |
| Cystoisospora belli | — | oocyst 25–30 µm (28–30), ellipsoidal; usually passed immature, with one granular mass |
| Sarcocystis hominis / S. suihominis | — | sporocysts 13–17 µm (14–16) / 12–13 µm usual; oval, 4 sporozoites each |
| Cryptosporidium | — | oocyst 3–6 µm (4–5); 4 "naked" sporozoites, no sporocysts |
| Blastocystis hominis | vacuolated form 5–30 µm (8–10); a large central "vacuole" inside a thin rim of cytoplasm holding 1–4 nuclei | — |
Giardia duodenalis cyst. CDC.
Cyclospora cayetanensis, wet mount. CDC.
Helminth eggs
Nematodes (roundworms)
- Enterobius (pinworm): 50–60 × 20–32 µm; flattened on one side; colorless; contains a C-shaped embryo; found more on anal swabs than in stool.
- Ascaris fertile: 45–70 × 35–45 µm; brown, thick shell with a bumpy (mammillated) coat, sometimes lost. Infertile: 85–95 × 35–45 µm; elongated or odd-shaped, full of granules.
- Trichuris (whipworm): 49–65 × 20–29 µm; barrel-shaped with a colorless plug at each end.
- Hookworm (Ancylostoma, Necator): 57–76 × 35–47 µm; thin shell, 4–8 cells. Species can't be told apart from eggs — report "hookworm."
- Trichostrongylus: 75–95 × 40–50 µm; like hookworm but larger and more pointed.
Cestodes (tapeworms)
- Taenia: 31–43 µm; thick, striated, walnut-brown shell around a six-hooked oncosphere. T. solium and T. saginata eggs look identical — report "Taenia spp."
- Hymenolepis nana: 40–60 × 30–50 µm; two membranes, with polar filaments between them.
- H. diminuta: 70–86 × 60–80 µm; yellow; no polar filaments.
- Dipylidium caninum: 31–50 µm eggs in packets of 5–15 or more.
- Diphyllobothrium latum: 58–76 × 40–51 µm; operculum (lid) at one end, a small knob at the other; unembryonated.
Trematodes (flukes)
- Schistosoma mansoni: 114–180 × 45–73 µm; lateral spine; in stool.
- S. haematobium: 112–170 × 40–70 µm; terminal spine; in urine.
- S. japonicum: 68–100 × 45–80 µm; small spine or knob; S. mekongi similar but smaller. S. intercalatum: 140–240 × 50–85 µm; long terminal spine.
- Small operculated eggs — Clonorchis (27–35 µm; lid on "shoulders"), Opisthorchis (no shoulders), Heterophyes and Metagonimus (broader lids).
- Paragonimus: 68–118 × 39–67 µm; flattened lid; usually in sputum; widest ahead of center, unlike D. latum.
- Fasciola hepatica and Fasciolopsis buski: large (120–159 µm long), thin-shelled, unembryonated, and hard to tell apart.
Relative sizes of helminth eggs, in micrometers. CDC.
Larvae: Strongyloides or hookworm?
| Strongyloides stercoralis | Hookworm | |
|---|---|---|
| Rhabditiform (first stage) | 200–300 × 16–20 µm; short buccal cavity; prominent genital primordium | 200–300 × 14–17 µm; long buccal cavity; genital primordium inconspicuous |
| Filariform (third stage) | 500–550 × 20–24 µm; esophagus about half the body; notched tail | 500–700 × 20–24 µm; esophagus about a quarter; pointed tail |
Tapeworm segments and heads
| Species | Gravid segment (proglottid) | Head (scolex) |
|---|---|---|
| Taenia solium | 12 mm long; 7–13 uterine branches per side | about 1 mm; 4 suckers; rostellum with a double row of 25–30 hooks |
| T. saginata | 16–20 mm long; 15–20 branches per side | 1–2 mm; 4 suckers; no hooks |
| Diphyllobothrium latum | broader than long; uterus in a rosette | almond-shaped; 2 grooves instead of suckers |
| Dipylidium caninum | pumpkin-seed shape, like rice grains in stool | rostellum with 30–150 small hooks |
| Hymenolepis nana / H. diminuta | tiny; rarely seen — the egg is the usual find | H. nana: 20–30 hooks; H. diminuta: no hooks |
Sources
Based on "Stool Specimens – Intestinal Parasites: Comparative Morphology Tables," DPDx, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The original gives every table in full, with sizes, and figures of each organism; its Sarcocystis suihominis size range reads "1-15 µm," which is taken as a misprint. Sizes of S. mekongi eggs follow Harinasuta and Kruatrachue (1962) and Taylor and Moose (1971); Sarcocystis sizes follow Rommel and Heydorn (1972) and Heydorn and others (1975).
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