Question
Entamoeba histolytica.
Answer
Entamoeba histolytica is an intestinal protozoan parasite, the cause of amoebiasis, ranging from asymptomatic colonization to invasive intestinal (amoebic dysentery) and extraintestinal (amoebic liver abscess) disease.
Morphology: exists as a motile trophozoite and a resistant, infective cyst.
- Trophozoite: actively motile via pseudopodia, ranges 10–60 μm, characteristically shows ingested (phagocytosed) red blood cells in its cytoplasm when invasive/pathogenic — the classical diagnostic hallmark distinguishing it from non-pathogenic Entamoeba species (e.g., E. dispar, morphologically identical but lacking RBC ingestion and invasive potential).
- Cyst: round, up to four nuclei when mature, contains chromatoid bodies (rod-shaped structures with rounded ends) — the infective, environmentally resistant form.
Transmission: faecal-oral route, ingestion of the mature (quadrinucleate) cyst in contaminated food/water.
Pathogenesis: cysts excyst in the small intestine, releasing trophozoites that colonize the colon; in invasive disease, trophozoites adhere to and lyse colonic epithelial cells (via the Gal/GalNAc lectin for adhesion and amoebapores/cysteine proteases for cytolysis), penetrating the mucosa and undermining it to form characteristic flask-shaped ulcers; trophozoites can enter the portal circulation and disseminate to the liver, causing amoebic liver abscess.
Clinical features: intestinal amoebiasis — amoebic dysentery (gradual onset, blood-streaked “anchovy sauce”-like stool, less fever/tenesmus than bacillary dysentery); extraintestinal amoebiasis — amoebic liver abscess (fever, right upper quadrant pain, tender hepatomegaly).
Laboratory diagnosis: stool microscopy for trophozoites (with ingested RBCs, in loose stool) and cysts (in formed stool); stool antigen ELISA; serology (highly sensitive for liver abscess, since stool exam is often negative in extraintestinal disease); imaging (ultrasound/CT) for liver abscess.
Treatment: metronidazole (tissue amoebicide) followed by a luminal agent (e.g., paromomycin) to eliminate residual cyst carriage.