Parasitology Review: Amebae

Intestinal Amebae

  • Characteristics

    • Live in the large intestine.

    • Possess both trophozoite (motile, feeding, binary fission) and cyst (environmentally resistant, infective, nuclear multiplication) stages.

  • Pathogens

    • Entamoeba histolytica

    • Blastocystis hominis (maybe)

  • Non-pathogens

    • Entamoeba hartmanni, Entamoeba coli, Endolimax nana, Iodamoeba butschlii

  • Life Cycle (Simple Direct Transmission)

    • Fecal-oral route via contaminated food/water; no intermediate host.

    • Ingestion of infective cyst →\to Excystation in intestinal tract →\to Trophozoites colonize cecum →\to Trophozoites reproduce by binary fission.

  • Treatment

    • Metronidazole for E. histolytica or symptomatic B. hominis (primarily non-endemic areas).

  • Identification Clues

    • Size, number of nuclei, nuclear structure (karyosome, chromatin), internal structures (glycogen vacuoles, chromatoidal bars, ingested RBCs), and motility.

    • Entamoeba histolytica, E. dispar, E. moshkovskii are morphologically identical; E. histolytica is distinguished by the presence of ingested RBCs in trophozoites.

  • Entamoeba histolytica

    • Major human pathogen (worldwide; ∼50\sim 50 million infections/year).

    • Causes invasive intestinal amebiasis and extraintestinal amebic infections (adheres, invades, disrupts mucosal barrier, induces apoptosis).

    • Symptoms: Asymptomatic (90%); Amebic dysentery (acute: abdominal pain, diarrheic stools with trophs, Charcot-Leyden crystals; chronic: amebic granuloma); Extraintestinal (hepatic/lung abscesses).

    • Lab Diagnosis:

      • Trophozoites: 10−50μm10-50 \mu m (avg. 15−25μm15-25 \mu m), directional motility, small central karyosome, even fine peripheral chromatin, ingested RBCs are diagnostic.

      • Cysts: 10−20μm10-20 \mu m, 1-4 nuclei with small central karyosome and fine chromatin, occasional cigar-shaped chromatoidal bars.

      • Clinical findings: Sigmoidoscopy shows