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MI4.1-4 | Diarrhoea & Dysentery — SDL Guide (Part 3)

Laboratory Diagnosis of Diarrhoea — Practical Approach (MI4.4)

A four-panel workflow diagram shows stool sample collection, macroscopic stool patterns, microscopy findings, and culture media used in laboratory diagnosis of diarrhoea. — **Panel A:** Sample collection showing clean wide-mouth stool container, rectal swab for Shigella, and fresh warm stool transport within 30 minutes for parasites.. **Panel B:** Macroscopy comparison showing rice-water stool for cholera, bloody mucoid stool for dysentery, fatty greasy stool for Giardia, and watery non-bloody stool for ETEC, Rotavirus, or Cryptosporidium.. **Panel C:** Microscopy showing saline and iodine wet mount, cysts and trophozoites, PMNs and RBCs, and absence of leucocytes in viral or toxin-mediated diarrhoea.. **Panel D:** Culture media table showing TCBS for Vibrio cholerae, MacConkey and XLD for Shigella, MacConkey XLD and SS agar for Salmonella, and CCDA under microaerophilic conditions for Campylobacter..

Sample collection: Collect stool in a clean wide-mouth container. Rectal swabs acceptable for Shigella. For parasites: fresh warm stool within 30 minutes to the lab.

Macroscopy:
- Rice-water: cholera
- Bloody mucoid: dysentery (Shigella, E. histolytica)
- Fatty/greasy: Giardia
- Watery, non-bloody: ETEC, Rotavirus, Cryptosporidium

Microscopy: Wet mount (saline + iodine), concentration techniques (formol-ether)
- Cysts and trophozoites: parasites
- PMNs + RBCs: invasive bacterial/amoebic dysentery
- No leucocytes: viral or toxin-mediated

Culture media:

Organism	Medium	Colony appearance
Vibrio cholerae	TCBS	Yellow (sucrose+)
Shigella	MacConkey, XLD	Pale/colourless (NLF)
Salmonella	MacConkey, XLD, SS agar	Black colonies (H₂S) on XLD/SS
Campylobacter	CCDA (microaerophilic)	Non-lactose fermenter, comma rods

Rapid tests: Cholera dipstick (Crystal VC), Rotavirus antigen ELISA, C. difficile toxin EIA

Flowchart linking macroscopic stool appearance to microscopy and culture pathways for distinguishing secretory, invasive bacterial, and parasitic diarrhoea. — **Panel A:** Diagnostic pathway from fresh stool specimen to macroscopic appearance, wet mount microscopy, and culture or confirmatory testing for secretory, invasive bacterial, and parasitic diarrhoea.. **Panel B:** Secretory diarrhoea microscopy showing absent stool leucocytes, absent RBCs, watery stool, and possible cholera or viral pathway.. **Panel C:** Bacillary dysentery microscopy showing sheets of PMNs with RBCs, suggesting invasive bacterial diarrhoea such as Shigella.. **Panel D:** Amoebic dysentery microscopy showing Entamoeba histolytica trophozoite with erythrophagocytosis, fewer PMNs, and ingested RBCs..

CLINICAL PEARL

Leucocyte count in stool predicts invasion: In bacillary dysentery (Shigella), the wet mount shows sheets of PMNs with RBCs. In amoebic dysentery, there are fewer PMNs (trophozoites secrete leucolytic factors) but erythrophagocytosis is seen. In cholera or viral diarrhoea, leucocytes are absent. This single observation guides empirical treatment before culture results return.

SELF-CHECK

In the stool microscopy of a patient with dysentery, you identify motile trophozoites with ingested red blood cells. Which of the following additional features would confirm Entamoeba histolytica rather than non-pathogenic species?

A. Presence of chromatoid bodies in cysts

B. Erythrophagocytosis by the trophozoite

C. Quadrinucleate cyst on iodine mount

D. Absence of PMNs in the stool

Reveal Answer

Answer: B. Erythrophagocytosis by the trophozoite

Erythrophagocytosis — the ingestion of red blood cells by trophozoites — is the pathognomonic finding distinguishing Entamoeba histolytica from E. dispar and other Entamoeba species. Chromatoid bodies and quadrinucleate cysts are seen in both E. histolytica and E. dispar (morphologically identical). Absence of PMNs is a relative finding in amoebiasis but is not confirmatory.

Interactive practice: True / False

Interactive practice: Multiple Choice

Interactive practice: Match the Images