Entamoeba histolytica Life Cycle and Pathogenesis Quiz

Protozoa

• Single-celled animals that perform all necessary functions of life, with most being free-living.
• Classified under four phyla: Sarcomastigophora, Apicomplexa, Ciliophora, and Microspora.

Sarcomastigophora Phylum

• Subdivided into two subphyla: Sarcodina and Mastigophora.
• Sarcodina (Amoebae): Use pseudopodia for locomotion and feeding.

Entamoeba Histolytica

• A type of amoeba that is medically important.
• Exists in three forms: trophozoite, precyst, and cyst.
• Trophozoites:
  • 10-60 µm in size.
  • Have unidirectional motility and a single pseudopodium.
  • Ingest erythrocytes.
  • Have granular cytoplasm, a small central karyosome, and beaded chromatin.
  • Reside in the mucosa and submucosa of the large intestine of humans.
• Life cycle:
  • Infection occurs through ingesting mature cysts in contaminated food, water, or hands.
  • Excystation occurs in the small intestine, releasing trophozoites that migrate to the large intestine.
  • Trophozoites multiply by binary fission and produce cysts.
  • Both stages are passed in the feces.

Encystation

• Occurs in the intestinal lumen.
• Chromatin is concentrated into bars or grape-like clusters (chromatoidal bodies) in the cytoplasm of the cyst.
• The nucleus of the cyst divides into two, then each daughter nucleus divides again, resulting in a mature cyst with four nuclei.
• Diffuse glycogen becomes concentrated in a mass with a hazy margin.

Excystation

• Occurs only after a mature cyst has been ingested and reaches a suitable level of the intestine.
• The amoeba becomes active, ruptures the cyst membrane, and starts to multiply by binary fission and colonization inside the host's digestive tract.

Pathogenesis

• Intestinal amoebiasis:
  • Amoeba invades colonic mucosa, producing flask-shaped ulcers and profuse bloody diarrhea (amoebic dysentery).
  • Ulcers may be deep or superficial.
• Extraintestinal amoebiasis:
  • Amoeba multiply in the liver, leading to cytolytic action and small abscesses that merge to form large liver abscesses.
  • Abscesses can grow in various directions, entering the general circulation and involving the lungs, brain, spleen, skin, etc.

Laboratory Diagnosis

• Diagnosis involves differentiation from other intestinal non-pathogenic amoebae.
• Microscopic identification of cysts and trophozoites in the stool is the common method for diagnosing E. histolytica.
• Other methods include blood examination, serological tests, histology, and molecular methods.

Treatment

• Asymptomatic infections: paromomycin or iodoquinol.
• Symptomatic intestinal or extraintestinal infections: metronidazole or tinidazole, followed by paromomycin or iodoquinol.
• Failure of metronidazole therapy may require surgical intervention.

Non-Pathogenic Amoebae

• Entamoeba dispar: Morphologically identical to E. histolytica, but can be differentiated by isoenzymatic, immunologic, or molecular analyses.
• Entamoeba hartmanni: Smaller in size than E. histolytica.
• Entamoeba coli: Distinguished from E. histolytica by having an eccentric endosome and mature cysts with 8 nuclei.
• Endolimax nana: Small amoeba with a large, eccentric endosome and thin nuclear envelope.
• Iodamoeba butschlii: Trophozoite and cyst have one nucleus with a large endosome, and the cyst contains a large glycogen vacuole.

Pathogenic and Opportunistic Free-Living Amoebae

• Naegleria fowleri: Causes acute and fulminating meningoencephalitis in immunocompetent children and young adults.
• Found in warm freshwater and soil near industrial plants, and in unchlorinated swimming pools.
• Can be seen in either an amoeboid or temporary flagellate stage.
• N. fowleri is inhaled through the nose, entering the nasal and olfactory nerve tissue and travelling to the brain.