Entamoeba histolytica: Morphology

Amoebiasis

• Amoebas are primitive unicellular microorganisms.
• Their life cycle consists of two stages: trophozoite and cyst.
• Reproduction occurs through binary fission or the development of numerous trophozoites within the mature multinucleated cyst.
• Motility is facilitated by the extension of pseudopodia ("false foot").

Entamoeba histolytica: Morphological Features

Trophozoites

• Viable trophozoites range in size from 10-60µm in diameter.
• Motility is rapid, progressive, and unidirectional, achieved through pseudopods.
• The nucleus has evenly arranged chromatin on the nuclear membrane and a small, compact, centrally located karyosome.
• The cytoplasm is finely granular with few ingested bacteria or debris in vacuoles.
• In dysentery cases, red blood cells (RBCs) may be visible in the cytoplasm, which is diagnostic for E. histolytica.

Cysts

• Cysts range in size from 10-20µm.
• Immature cysts contain inclusions such as glycogen mass and chromatoidal bars.
• As the cyst matures, glycogen disappears, and chromatoidals may be absent in the mature cyst.

Life Cycle and Infection

• Intestinal infections occur through the ingestion of a mature quadrinucleate infective cyst via contaminated food, drink, or hand-to-mouth contact.
• The cyst passes unaltered through the stomach into the terminal ileum, where excystation occurs in the alkaline pH.
• Trophozoites invade tissues and lodge in the submucous layer of the large bowel.
• They grow and multiply by binary fission, producing lesions in amoebiasis.
• Invasion of blood vessels can lead to secondary extraintestinal lesions.
• The parasite's effect on the host is toned down with increased host tolerance.
• Trophozoites transform into pre-cyst forms, which secrete a cyst wall and become uninucleate cysts.
• Mature quadrinucleate cysts form, which are infective.
• Both mature and immature cysts can be passed in feces, and immature cysts can mature and become infective in external environments.

Pathogenesis

• Trophozoites divide and cause extensive local necrosis in the large intestine.
• Invasion into the deeper mucosa can extend into the peritoneal cavity, leading to secondary involvement of organs like the liver, lungs, brain, and heart.
• Extraintestinal amebiasis is associated with trophozoites.
• Amoebas multiply rapidly in anaerobic environments, as trophozoites are killed by ambient oxygen concentration.

Clinical Features

• Infection outcomes include a carrier state, intestinal amebiasis, or extraintestinal amebiasis.
• Symptomatic patients report diarrhea, flatulence, and cramping.
• Severe disease involves passing numerous bloody stools daily.
• Systemic signs of infection (fever, leukocytosis, rigors) are present in patients with extraintestinal amebiasis.
• The liver is primarily involved due to trophozoites being removed from the blood by the portal veins.
• The right lobe is most commonly affected, causing pain over the liver with hepatomegaly.