Summary
Highlights
The lecture begins with an introduction to protozoa, highlighting that most are non-pathogenic and beneficial to the environment. However, a select few are parasitic to humans. Protozoa are unicellular eukaryotic organisms, meaning they possess a true nucleus and organelles. They are characterized by two cytoplasmic regions: an outer ectoplasm and an inner endoplasm. Locomotion is achieved through specialized organs like pseudopodia, flagella, or cilia. The infective stage for pathogenic protozoa is typically the cyst, while the vegetative or feeding stage is the trophozoite.
Protozoa are classified based on their locomotory organelles. Sarcoidina (amoeba) use pseudopodia (false feet), which are extensions of the cytoplasm. Mastigophora (flagellates), such as Giardia and Leishmania, use flagella. Ciliophora (ciliates) use cilia, with Balantidium coli being the sole pathogenic ciliate. Sporozoans, like Plasmodium and Cryptosporidium, lack a defined locomotory organelle. The most common transmission route for intestinal amoeba is fecal-oral ingestion of cysts from contaminated food or water. All medically important amoeba, except Entamoeba gingivalis, possess a cystic stage. They are typically commensals, residing in harmony with the host, except for Entamoeba histolytica, which is a true pathogen.
Key nuclear structures used for amoeba identification include the karyosome (a mass of chromatin within the nucleus), peripheral chromatin (surrounding the nucleus), chromatoidal bars (condensed RNA found in cysts), and glycogen mass (stored food in cysts). The life cycle of intestinal amoeba involves two main processes: excystation, where the cyst transforms into a trophozoite in the intestine, and encystation, where the trophozoite forms a cyst in unfavorable conditions to protect itself. Trophozoites are motile, feeding, and reproductive, thriving in wet environments like liquid stool, while cysts are non-motile and non-feeding, often found in formed stools.
Entamoeba histolytica, first described by Losh, is the only pathogenic intestinal amoeba. Its infective stage is the mature quadrinucleate cyst. Pathogenesis involves direct lysis of host cells, leading to tissue destruction, necrosis, and damage to the extracellular matrix. Trophozoites adhere to mucosal cells using virulence factors such as cysteine proteases, which degrade proteins, and Gal/GalNAc lectin, which interacts with mucin. Entamoeba histolytica also exhibits 'trogocytosis' or 'nibbling,' where it ingests host cells, particularly red blood cells, which is a diagnostic feature. Invasive strains are resistant to complement-mediated lysis, allowing them to evade the host's immune response.
Infections with Entamoeba histolytica can range from asymptomatic to severe. Asymptomatic carriers have a weak antibody titer but can still shed cysts. Symptomatic infections include amoebic dysentery and diarrhea, characterized by frequent, bloody, and mucoid stools (90% of cases). Amoebic colitis involves ulceration of the intestinal wall, causing abdominal cramping, anorexia, and fatigue. Granuloma formation, known as amoeboma, can resemble malignant tumors. Flask-shaped ulcers are characteristic microscopic findings. Extra-intestinal amoebiasis primarily affects the liver, leading to amoebic liver abscesses (ALA), which contain necrotic tissue and thick, chocolate-brown or anchovy sauce-like pus. Other sites include the lungs (pulmonary amoebiasis) and the brain (secondary amoebic meningoencephalitis). Sexual transmission can also cause genital ulcers.
Diagnosis involves standard O&P (Ova and Parasite) examination of stool samples, including direct wet mounts, concentration techniques, and permanent stains like iron hematoxylin or trichrome. Sigmoidoscopy specimens and histological examination using periodic acid-Schiff (PAS) stain can reveal trophozoites in affected tissues. For amoebic liver abscesses, aspiration and examination of liver aspirate are definitive. Culture on media like TYI-S-33 can also be performed, but it's not routine. Serological tests, such as indirect hemagglutination (IHA), indirect fluorescent antibody (IFA), latex agglutination, and ELISA, detect antibodies or antigens, proving useful in extra-intestinal cases where stool examination might be negative. ELISA tests like E. histolytica II can differentiate E. histolytica from E. dispar and E. moshkovskii using specific antigens.
Morphological characteristics are crucial for differentiating Entamoeba histolytica from non-pathogenic species like Entamoeba coli. E. histolytica cysts are 10-20 micrometers, have coffin-shaped chromatoidal bars with rounded ends, and contain up to four nuclei. The peripheral chromatin is fine and uniformly distributed, and the karyosome is small and central. Trophozoites of E. histolytica are characterized by progressive, unidirectional motility and the presence of ingested red blood cells in their cytoplasm, a key diagnostic feature. In contrast, E. coli cysts are larger, have splinter-like chromatoidal bars, up to eight nuclei, and its trophozoites ingest bacteria and detritus, not red blood cells.