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Stool parasite methods, amebae and coccidia

16 min

Skills you'll build3 skills
  • Choose stool parasite methods that match the collection and preservative
  • Distinguish pathogenic intestinal amebae from morphologic look-alikes
  • Select a targeted stain or NAAT when intestinal coccidia are suspected

Try first

Try first

A gastrointestinal multiplex NAAT panel is ordered. The stool arrives in a 10% formalin vial. What do you do?

Keep going to see the reasoning.

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Stool collection and preservatives

A stool parasite test is valid only when the collection is suitable for the method. Liquid stool is examined or preserved within 30 minutes of passage and soft stool within 1 hour, because trophozoites degenerate quickly. Formed stool is acceptable for up to 24 hours.2

CollectionUsed forLimitations
Fresh stoolSaline wet mount for motility; antigen tests and NAAT that list itExamined or preserved within the time limits
10% formalinConcentration; modified acid-fast and other special stainsPoor trophozoite detail; can inhibit NAAT
Polyvinyl alcohol (PVA)Permanent trichrome or iron hematoxylin stainUnsuitable for many concentration, antigen, acid-fast and molecular methods

Each antigen test and NAAT accepts only the specimen types and preservatives in its instructions. A specimen in the wrong vial can give a false-negative result.2,3

Amebae

Nuclear detail is read on the permanent stain. Size is measured with a calibrated ocular micrometer whenever it separates look-alikes.1 A mature cyst of 10 to 20 µm with 4 nuclei, fine even peripheral chromatin and blunt chromatoid bodies belongs to the Entamoeba histolytica/E. dispar complex. E. dispar, E. moshkovskii and E. bangladeshi look the same as E. histolytica under the light microscope. The report names the complex, and a validated E. histolytica-specific antigen assay or NAAT identifies the species. Ingested red cells in a trophozoite favor E. histolytica. They do not identify the species.4 A 4-nucleus cyst of 5 to 10 µm fits E. hartmanni or Endolimax nana, and a mature cyst with 8 nuclei is E. coli.5

Coccidia

Routine trichrome does not reliably show Cryptosporidium, Cyclospora or Cystoisospora. A modified acid-fast stain of concentrated sediment shows their oocysts.1,6

  • Cryptosporidium: round, 4 to 5 µm. DFA, antigen tests and NAAT detect it more sensitively than the acid-fast stain.
  • Cyclospora: round, 7.5 to 10 µm, variably acid-fast and autofluorescent.
  • Cystoisospora: ellipsoidal, about 25 to 30 µm long, variably acid-fast.

A NAAT detects only its listed targets, and many gastrointestinal panels omit Cystoisospora.3,5 When the history suggests coccidia, the targeted test is requested by name.

Follow the bench demonstration

Sources for this step
References
  1. Garcia LS, Arrowood M, Kokoskin E, et al. Practical guidance for clinical microbiology laboratories: laboratory diagnosis of parasites from the gastrointestinal tract. Clin Microbiol Rev. 2018;31(1):e00025-17. doi:10.1128/CMR.00025-17
  2. Centers for Disease Control and Prevention. DPDx: Stool specimens, specimen collection. Accessed September 27, 2026. https://www.cdc.gov/dpdx/diagnosticprocedures/stool/specimencoll.html
  3. Centers for Disease Control and Prevention. DPDx: Stool specimens, molecular diagnosis. Accessed September 27, 2026. https://www.cdc.gov/dpdx/diagnosticprocedures/stool/moleculardx.html
  4. Centers for Disease Control and Prevention. DPDx: Amebiasis. Accessed September 27, 2026. https://www.cdc.gov/dpdx/amebiasis/index.html
  5. Centers for Disease Control and Prevention. DPDx: Intestinal parasites, comparative morphology tables. Accessed September 27, 2026. https://www.cdc.gov/dpdx/diagnosticprocedures/stool/morphcomp.html
  6. Centers for Disease Control and Prevention. DPDx: Stool specimens, staining procedures. Accessed September 27, 2026. https://www.cdc.gov/dpdx/diagnosticprocedures/stool/staining.html

Watch one

Read each decision and its explanation.

Liquid stool from a man with bloody diarrhea was placed in PVA 20 minutes after passage. The trichrome smear shows trophozoites about 22 µm across. Each has one nucleus with a small central karyosome and fine, even peripheral chromatin. Several contain ingested red cells. What do you report?

  1. Check the specimen: liquid stool, fixed within 20 minutes.

    Trophozoites degenerate soon after passage, so prompt fixation preserves reliable nuclear detail.

  2. Measure with the calibrated ocular micrometer: about 22 µm.

    Size separates look-alikes. The complex's trophozoites measure 10 to 60 µm, and E. hartmanni measures 12 µm or less.

  3. Read the nucleus: a small central karyosome and fine, even peripheral chromatin.

    E. coli has coarse, irregular peripheral chromatin and a large eccentric karyosome. Endolimax nana has no peripheral chromatin.

  4. Read the cytoplasm: ingested red cells.

    Ingested red cells favor E. histolytica. They are not species-definitive, so they are recorded as an observation.

  5. Choose the species test: an E. histolytica-specific antigen assay or NAAT on a specimen the assay accepts.

    PVA is unsuitable for many antigen and molecular methods, so the species test may need a fresh specimen.

Report Entamoeba histolytica/E. dispar complex trophozoites with ingested red cells, and state that a species-specific test is needed to identify E. histolytica.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A permanent stained smear shows cysts 12 to 15 µm with four nuclei and blunt-ended chromatoid bodies. What can be reported?

Incorrect. E. dispar, E. moshkovskii, and E. bangladeshi are indistinguishable from E. histolytica by light microscopy.

Correct. Only E. histolytica is an established invasive member of the complex, and its cysts cannot be told from those of E. dispar, E. moshkovskii, and E. bangladeshi by light microscopy, so a species-level result comes from a validated E. histolytica-specific antigen assay or NAAT.

Incorrect. E. hartmanni cysts also have 4 nuclei and measure 5 to 10 µm, so cysts of 12 to 15 µm fit the E. histolytica complex.

Hint
  1. Measure first. Which amebae make 4-nucleus cysts of this size?
  2. Ask whether light microscopy can separate the members of that group.

Review Intestinal amebae and look-alikes

Problem 2 of 3 · Hints available

A routine ova and parasite (O&P) examination with trichrome stain is negative in a patient with advanced HIV and prolonged watery diarrhea. Which additional testing does the laboratory need to look for Cryptosporidium, Cyclospora, and Cystoisospora?

Incorrect. Routine trichrome is inadequate for these oocysts regardless of stool consistency, so a dedicated stain or assay is needed.

Incorrect. Flotation only concentrates the specimen. The oocysts still need a dedicated stain or assay to be seen.

Correct. Coccidian oocysts need modified acid-fast staining of concentrated sediment, with hot safranin or autofluorescence useful for Cyclospora. Many gastrointestinal panels omit Cystoisospora, so a NAAT detects only the targets it includes. DFA, antigen testing, and NAAT are more sensitive than acid-fast microscopy for Cryptosporidium.

Hint
  1. Ask what a trichrome stain is designed to show.
  2. Consider which methods are designed to show oocysts and which targets a panel may omit.

Review Ova-and-parasite examination

Problem 3 of 3 · On your own

A clinic sends stool in a PVA vial with an order for a Giardia and Cryptosporidium antigen enzyme immunoassay. The kit instructions list fresh stool and formalin-preserved stool. What do you do?

PVA is unsuitable for many antigen methods, and this kit does not list it. A negative result could be falsely negative.

The antigen test is valid only on the specimen types its instructions accept. Record why the PVA specimen was not tested.

A trichrome smear is a different test. Routine trichrome also misses Cryptosporidium.

Mixing preservatives produces a specimen type no kit is validated for.

Review Stool specimens and test selection

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Winnie Spindler, 47, MRN 5520917, is a kidney transplant recipient who keeps a meticulous food diary.
  • She has had 3 weeks of watery diarrhea.
  • A gastrointestinal NAAT panel last week was negative. Its parasite targets are Giardia, Cryptosporidium, E. histolytica and Cyclospora.
  • Today's order is an O&P. The stool arrived in a formalin vial and a PVA vial.
Decision 1 of 3

The trichrome smear from the PVA vial shows no parasites. What else does this workup need?

Routine trichrome does not reliably show Cryptosporidium, Cyclospora or Cystoisospora, and her panel had no Cystoisospora target.

Assumed a routine O&P excludes intestinal coccidia

Routine trichrome staining is inadequate for Cryptosporidium, Cyclospora, and Cystoisospora, and many multiplex panels omit Cystoisospora. Reporting a negative ova and parasite (O&P) exam as excluding coccidia can miss them. Modified acid-fast staining, direct fluorescent antibody (DFA) or antigen testing, or a NAAT whose panel includes the target detects them.

The acid-fast stain shows coccidian oocysts in concentrated sediment, including Cystoisospora, which her panel did not cover.

The panel detects only its listed targets. A repeat still leaves Cystoisospora untested.

Review Coccidia and microsporidia

Decision 2 of 3

The acid-fast stain shows ellipsoidal, variably acid-fast oocysts about 28 by 14 µm, each with one sporoblast. What are they?

Cryptosporidium oocysts are round and 4 to 5 µm.

Ellipsoidal oocysts about 25 to 30 µm long, usually immature with one sporoblast, fit C. belli.

Cyclospora oocysts are round and 7.5 to 10 µm.

Yeast can mimic small round Cryptosporidium oocysts. These are large ellipsoidal oocysts with an internal sporoblast.

Review Coccidia and microsporidia

Decision 3 of 3

The clinician asks the reference laboratory to confirm with a Cystoisospora PCR. Which specimen goes?

Formalin can inhibit amplification, so the PCR could be falsely negative.

Sent a formalin-fixed stool for NAAT

Formalin suits concentration and inhibits many NAATs. Polyvinyl alcohol (PVA) suits permanent stains and is unsuitable for antigen or molecular methods. A specimen in the wrong preservative can give a false-negative result, so the collection has to match the assay requested.

PVA is unsuitable for molecular methods.

Fresh refrigerated or frozen stool, or a molecular transport medium the assay accepts, is suitable for the PCR.

Review Stool specimens and test selection

The key finding is the ellipsoidal oocyst about 28 µm long, seen only on the modified acid-fast stain. Her panel had no Cystoisospora target, and the trichrome does not show it. The acid-fast stain detected it, and the PCR needs a new specimen in a container suitable for molecular testing.

For reference

Sources checked