Entamoeba histolytica: Pathogenicity and Intestinal Lesions

Cysts of Entamoeba histolytica (E. Hy) have been detected in the droppings of cockroaches.
- Cockroaches therefore act as mechanical vectors and reservoirs of human infection.
Feco-oral spread remains the principal mode; cockroach contamination adds an additional domestic/restaurant risk.

Highly variable in humans.
Typical range: $4\text{–}5\,\text{days}$ after ingestion of mature cysts.
- Longer or shorter incubation may occur depending on host immunity, parasite load, and co-morbidities.

Amoebiasis (clinical term)
- Encompasses all tissue manifestations caused by E. histolytica—intestinal or extra-intestinal.
Amoebic dysentery
- A subset of amoebiasis restricted to the large intestine.
- Hallmark: passage of blood and mucus in stool.
- NOT synonymous with intestinal amoebiasis; many intestinal cases are non-dysenteric.
Spectrum of presentations
- Acute colitis.
- Chronic colitis.
- Asymptomatic carrier state (often sheds cysts of both non-pathogenic and pathogenic strains).

Primary / Intestinal lesions
- Confined to the large intestine (initial colonisation site).
Secondary / Metastatic lesions (trophozoite dissemination)
- Liver (most common extra-intestinal site).
- Lungs.
- Brain.

Metacystic trophozoites emerge after excystation in the ileo-caecal lumen.
Entry route: through crypts of Lieberkühn → penetrate columnar epithelium.
- Mechanisms: amoeboid motility + secretion of proteolytic enzymes → cytolysis of epithelial cells.
Submucosal phase
- Rapid multiplication; colonies form; surrounding tissue digested and utilised as food.
- Progressive lateral spread undermines overlying mucosa → formation of flask-shaped abscess.
Rupture of abscess → classic ulcer with overhanging edges.

Lesions restricted to large intestine.
Two patterns:
1. Generalised: entire colon down to internal anal sphincter.
2. Localised (two foci):
- Ileo-caecal region (caecum, ascending colon, ileo-caecal valve, appendix).
- Sigmoido-rectal region (sigmoid colon & rectum).
  - Incidence ratio: ileo-caecal : sigmoido-rectal ≈ $2:1$ .

Scattered, small, hyperaemic nodules with central pin-point openings (Fig. 12 equivalent).
Oedematous margins; incision yields brownish-yellow necrotic exudate containing trophozoites.

Size: pin-point to $\ge 1\,\text{inch}$ diameter.
Shape: round/oval; transverse when multiple ulcers coalesce.
Margin: ragged, undermined; overhanging mucosa produces “flask” silhouette on vertical section.
Base: muscular coat exposed; filled with yellowish–black necrotic slough (Fig. 13).

Superficial ulcers: confined above muscularis mucosae.
Deep ulcers: reach submucosa → may laterally fuse.
If invasion extends into muscular or serosal layers, complications include:
- Local or generalised peritonitis.
- Severe haemorrhage.
- Intestinal perforation.
- Pericaecal/pericolic abscesses.
- Sloughing & gangrene of colon.

Central zone: coagulative necrosis; few/no amoebae.
Peripheral zone: numerous trophozoites in inter-glandular spaces; minimal host inflammatory infiltrate.

Destruction of crypt epithelium & basement membrane evident; parasites “march” downward.

Trophozoites migrate great distances from ulcer edge → inter-muscular planes and peritoneal coat.
Vascular invasion
- Amoebae within lumina of venous radicles; alongside RBCs & leukocytes.
- Endothelial hyperplasia & thrombosis; trophozoites sometimes found inside thrombi.

Post-slough separation: granulation tissue covers ulcer floor.
Small superficial ulcers:
- Complete mucosal regeneration → site later almost invisible.
Large/deep ulcers:
- Scar tissue forms; mucosal epithelium fails to regrow fully.
- Detectable as smooth depressed scars; may show pigmentation.
- Excessive fibrosis → strictures, luminal narrowing, or diffuse thickening.

Multiple small mucosal ulcers.
Extensive superficial ulceration with surrounding hyperaemia.
Prominent scarring → thinning, dilatation, sacculation.
Broad serosal adhesions to adjacent viscera.
Localised bowel wall thickening causing luminal narrowing.
Diffuse thickened colon palpable/visible in emaciated patients.
Amoebic granuloma (amoeboma)
- Tumour-like mass of exuberant granulation tissue.
- Clinically mimics carcinoma; definitive Dx requires demonstration of trophozoites in biopsy/autopsy sections.

Low-pathogenic strains
- May remain confined superficially within crypts of Lieberkühn.
- Exhibit anaerobic metabolism; live symbiotically with intestinal bacteria—feed on mucus, not on bacteria.
- Assigned a “low pathogenic index” but still capable of cyst shedding.

Asymptomatic carriers constitute a hidden reservoir; stool microscopy essential for public-health surveillance.
In endemic regions, differentiation between amoebic dysentery and bacillary dysentery is critical for therapy (metronidazole vs antibiotics).
Complication awareness (e.g., amoebic liver abscess) should prompt screening in patients with right-upper-quadrant pain and history of intestinal symptoms.

Incubation: $\approx 4\text{–}5\,\text{days}$ .
Ulcer diameter: <1\,\text{mm} (pin-head) → >25\,\text{mm} ( $1\,\text{inch}$ ).
Regional prevalence of ulceration: ileo-caecal twice sigmoido-rectal (ratio $2:1$ ).

Household pests (cockroaches) are not mere nuisances but potential disease vectors; sanitation and pest control are ethical obligations in communal living.
Carrier detection and treatment reduces transmission—especially important among food handlers, daycare staff, and immunocompromised contacts.
Misdiagnosis of amoeboma as carcinoma may lead to unnecessary surgery; mandates pathologist-clinician communication and adequate tissue sampling.