Case Analysis: Bloody Diarrhea with Mucus, Trophozoites with Ingested RBCs

A] Provisional Diagnosis and Likely Pathogen [2 marks]

• Bloody diarrhea with mucus (dysentery)
• Abdominal pain and fever
• Trophozoites with pseudopodia and ingested RBCs on stool microscopy

• Medical Microbiology 9e, p. 801

B] Pathogenesis [5 marks]

Life Cycle Overview

Mechanism of Intestinal Invasion

1. Attachment: Trophozoites attach to colonic epithelial cells via a galactose-inhibitable adherence lectin (Gal/GalNAc lectin). This attachment is a prerequisite for cytolysis and tissue necrosis.
2. Cytolysis: Attachment is followed by a lethal alteration of host cell membrane permeability, causing an irreversible rise in intracellular calcium, leading to cell death. Colonic epithelial cells, neutrophils, lymphocytes, and monocytes are all lysed. Released toxic neutrophil constituents amplify tissue destruction.
3. Ulcer Formation: Trophozoites invade the intestinal mucosa, producing discrete ulcers with a pinhead-sized center and raised edges, from which mucus, necrotic cells, and amoebae are shed. Amebae multiply above the muscularis mucosae and spread laterally, undermining the mucosa and producing the characteristic "flask-shaped" ulcer - a narrow neck through the mucosa opening into an expanded necrotic area in the submucosa.
4. Spread: Coalescence of colonies can undermine large areas. Trophozoites may penetrate through the muscle layers and occasionally the serosa, causing perforation.
5. Extraintestinal spread: Trophozoites entering portal blood are carried to the liver (most common extraintestinal site), producing amoebic liver abscess containing characteristic "anchovy paste" (necrotic, bacteriologically sterile material). Less commonly, lungs, brain, and spleen may be involved.
6. Favored sites: Cecum, appendix, ascending colon (primary); sigmoid colon and rectum (later lesions). Occasionally a chronic granulomatous mass called an ameboma forms on the intestinal wall.

The basis for tissue destruction includes: (i) cytotoxin production, (ii) lectin-mediated adherence, and (iii) trophozoite-mediated lysis with secondary neutrophil-mediated injury.

• Medical Microbiology 9e, pp. 800-801; Jawetz Melnick & Adelberg's Medical Microbiology 28e, pp. 733-734

C] Laboratory Diagnosis [4 marks]

1. Microscopic Examination of Stool (Definitive)

• Size: 12-50 μm
• Motile with unidirectional pseudopodia (directional movement distinguishes from non-pathogenic amoebae)
• Cytoplasm: hyaline outer ectoplasm, granular endoplasm
• Single nucleus with fine, evenly-distributed peripheral chromatin and a small, central karyosome
• Ingested RBCs in cytoplasm - pathognomonic

• Size: 10-20 μm
• 1-4 nuclei (mature cyst: 4 nuclei)
• Chromatoidal bars with bluntly rounded ends (vs. splintered ends in E. coli)
• May contain glycogen vacuole

2. Staining Methods

• Trichrome stain or iron-haematoxylin stain: for permanent smears, better nuclear detail
• Iodine wet mount: stains cysts (glycogen appears brown), nuclei visible
• Saline wet mount: shows motile trophozoites in fresh specimens

3. Concentration Techniques

• Formol-ether (formalin-ethyl acetate) sedimentation: concentrates cysts and eggs from formed stool
• Zinc sulphate flotation: for cyst recovery

4. Proctoscopy/Colonoscopy with Biopsy

• Reveals characteristic flask-shaped ulcers
• Biopsy of ulcer edges shows trophozoites on histopathology (H&E stain)

5. Antigen Detection (ELISA/EIA)

• Stool antigen tests detect E. histolytica-specific antigens
• Distinguishes E. histolytica from E. dispar (which is morphologically identical but non-pathogenic) - important since standard microscopy cannot differentiate these species

6. Serology (Indirect Haemagglutination, ELISA)

• Useful for extraintestinal amoebiasis (liver abscess), where stool microscopy is frequently negative

• 80% of intestinal disease cases are seropositive at clinical presentation

• Less useful in endemic areas (high background seropositivity)

7. PCR / DNA-Based Assays

• Most sensitive and specific; can differentiate E. histolytica from E. dispar, E. moshkovskii, and E. bangladeshi
• Used when morphological distinction is insufficient

8. Imaging (for Extraintestinal Disease)

• Ultrasound, CT scan, MRI, or radionuclide scan for amoebic liver abscess
• Aspirated "anchovy paste" (sterile, chocolate-brown pus) is characteristic; trophozoites are at the margins of the abscess wall, not in the necrotic center
• Medical Microbiology 9e, pp. 801-802; Henry's Clinical Diagnosis and Management by Laboratory Methods, pp. 1518-1519

D] Treatment and Prevention [4 marks]

Treatment

• Metronidazole is the drug of choice for invasive and extraintestinal disease; it is a tissue amebicide that is inadequate alone because it does not reliably eliminate luminal cysts.
• The subsequent luminal agent (diloxanide furoate / iodoquinol / paromomycin) is mandatory to prevent relapse and break the transmission cycle.
• Tinidazole or secnidazole are alternatives to metronidazole with similar efficacy.

Prevention

• Boil or treat drinking water (chlorination alone does not reliably kill cysts - filtration is also needed)
• Thoroughly wash and peel fruits and vegetables before consumption
• Avoid raw vegetables and salads in endemic areas
• Practice careful hand hygiene, especially after defecation and before food handling
• Travelers to endemic regions should be advised to avoid ice cubes, unpeeled fruits, and tap water

• Introduce adequate sanitation infrastructure (sewage disposal, latrines)
• Prevent use of human feces (night soil) as agricultural fertilizer
• Safe water supply: chlorination + filtration of municipal water
• Control of mechanical vectors (flies, cockroaches) that transport cysts
• Identify and treat asymptomatic cyst carriers, especially in healthcare, food service, and institutional settings
• Health education regarding fecal-oral transmission routes
• Medical Microbiology 9e, p. 802; Jawetz Melnick & Adelberg's Medical Microbiology 28e, p. 834