How to distinguish Entamoeba coli from Entamoeba histolytica through microscopic examination of feces in a routine feces examination?

Distinguishing Entamoeba coli from Entamoeba histolytica

Do not rely on microscopy alone to distinguish E. histolytica from E. coli in routine feces examination, as morphologic differentiation is unreliable and E. histolytica requires species-specific immunoassay or NAAT for definitive identification. 1, 2

Primary Diagnostic Approach

Microscopic Examination Limitations

• Microscopic examination lacks sensitivity, is time-consuming, and requires highly trained personnel for detection and interpretation. 1
• E. coli can be differentiated morphologically from E. histolytica in theory, but diagnostic morphologic features overlap significantly, creating issues for differential diagnosis. 3, 4
• Microscopy cannot distinguish E. histolytica from the morphologically identical non-pathogenic species E. dispar and E. moshkovskii, which are far more common. 5, 4
• Fresh diarrheal stool samples maximize diagnostic yield, as delays in processing cause degradation of trophozoites. 2

Key Morphologic Features (When Microscopy is Performed)

Trophozoite characteristics:

• E. histolytica trophozoites: 12-60 μm in diameter, contain ingested red blood cells (pathognomonic when present), single nucleus with fine peripheral chromatin and small central karyosome, unidirectional progressive motility. 4
• E. coli trophozoites: 15-50 μm in diameter, larger and more sluggish than E. histolytica, nucleus with coarse irregular peripheral chromatin and large eccentric karyosome, no ingested RBCs. 4

Cyst characteristics:

• E. histolytica cysts: 10-20 μm, mature cysts contain 4 nuclei with fine peripheral chromatin, may contain chromatoid bodies with rounded ends. 4
• E. coli cysts: 10-35 μm (larger), mature cysts contain 8 nuclei (occasionally up to 16), coarse irregular nuclear chromatin, chromatoid bodies with splintered or pointed ends when present. 4

Critical pitfall: Care must be taken to distinguish large white cells (nonspecific indicator of dysentery) from trophozoites, as amebic dysentery tends to be misdiagnosed. 1

Recommended Definitive Testing

For E. histolytica Identification

Use E. histolytica species-specific immunoassay or NAAT on stool to distinguish from non-pathogenic E. dispar and other Entamoeba species. 1, 2

• Species-specific antigen detection tests and molecular diagnostic tests (conventional and real-time PCR) have been developed for detection and differentiation of E. histolytica from E. dispar and E. moshkovskii. 4
• Real-time PCR methods show approximately 98% agreement with DNA sequencing for E. histolytica and E. dispar differentiation. 5
• PCR with DNA sequencing of 18S rRNA gene regions can differentiate all Entamoeba species commonly found in human stools, including E. coli. 5, 3

When to Order Specific Testing

• Order species-specific testing when Entamoeba cysts or trophozoites are identified on microscopy to determine if pathogenic E. histolytica is present. 1, 2
• Testing is particularly important in patients with persistent diarrhea, bloody stools, or those requiring treatment decisions. 1
• In immunocompromised patients with diarrhea, broad parasitic workup including specific E. histolytica testing is required. 2

Clinical Decision Algorithm

1. If microscopy shows Entamoeba organisms: Order E. histolytica-specific immunoassay or NAAT to confirm species identity before treating. 1, 2

2. If bloody diarrhea without microscopy available: Treat empirically for bacterial dysentery (Shigella) first, as this is more common; if no clinical response occurs within 2 days, refer for stool microscopy and E. histolytica-specific testing. 1

3. If definite trophozoites are not seen on microscopy: Persons with bloody diarrhea should be treated initially for shigellosis rather than presumed amebiasis. 1

Common Pitfalls to Avoid

• Never diagnose E. histolytica based on microscopy alone, as this leads to unnecessary treatment of patients with non-pathogenic E. dispar (which is 10 times more common) or E. coli. 4
• Do not assume all Entamoeba species seen on microscopy are pathogenic—only E. histolytica causes disease; E. coli, E. dispar, E. hartmanni, and E. moshkovskii are non-pathogenic commensals. 5, 4
• Concentrated stool microscopy with permanent stained smears (O&P examination) remains the gold standard for detecting intestinal helminths but has significant limitations for Entamoeba species differentiation. 2
• Multiplex molecular panels can detect multiple parasites simultaneously but may detect colonization rather than infection, requiring clinical correlation. 1, 2