Intestinal Protozoans - Part 3

Overview of Intestinal Flagellates

  • Definition of Flagellates: Organisms with a specialized locomotor organelle called flagella, which are long, thin cytoplasmic extensions that vary in number and position on the organism.

  • Importance: Study of intestinal flagellates is critical due to their potential pathogenicity.

  • Major Intestinal Flagellates: Four main species discussed are:

    • Giardia duodenalis (pathogenic)

    • Dientamoeba fragilis (pathogenic)

    • Chylomastix mesnilii (non-pathogenic)

    • Pentatrichomonas hominis (non-pathogenic)

Key Terminology

  • Sucking Disc:

    • Structure aiding in adherence to body parts (intestines).

    • Functions similarly to a mouth but does not represent a true mouth.

  • Axonemes:

    • Extracellular parts of the flagella.

  • Axostyle:

    • A group of microtubules starting at the base of a flagellum, extending the length of the protozoan (variable by protozoan).

  • Cytostome:

    • Specialized structure for phagocytosis, referred to as a cell mouth, but not a true mouth.

  • Undulating Membrane:

    • Fin-like extension of the cytoplasmic membrane with a flagellar sheath;

    • Appears like a sheath attached to the organism.

    • Reference: Figure 47-18, page 663 of the textbook for visual representation.

Life Cycle of Intestinal Flagellates

  • General Life Cycle:

    • Ingestion of infective cyst occurs via food or water.

    • Passage through the gastrointestinal tract.

    • Asexual reproduction occurs.

    • Presence of trophozoite in liquid or soft stool, and cyst in formed stool.

  • Importance of Stages for Diagnosis: Different stages signify different phases in the life cycle which are critical for diagnosis.

Giardia duodenalis

  • Commonality: One of the most frequent protozoans in the United States, alongside Blastocystis species.

  • Pathology: Causes giardiasis or traveler's diarrhea.

  • Transmission:

    • Found in lakes, streams, contaminated food, and water.

    • Most common transmission is via drinking water or recreational water.

  • Forms and Structure:

    • Trophozoite:

    • Shaped like a teardrop;

    • Attaches to the intestines using a sucking disc.

    • Divides by longitudinal binary fission.

    • Usually located in the crypts of the duodenum, termed the intestinal dwelling stage.

    • Detaches and sloughs off every 72 hours due to intestinal mucosa shedding.

    • Cyst:

    • Develops in the jejunum; contents double for fission.

    • Infective when cysts are ingested.

  • Transmission Routes:

    • Through contaminated food and drinks.

    • Close contact with an infected individual, particularly prevalent in children and groups with poor sanitation facilities.

  • Pathogenesis:

    • Can be asymptomatic, or cause intestinal infection after a 12-20 day incubation period.

    • Symptoms mimic viral enteritis or bacterial food poisoning, resulting in loose, foul-smelling stools and other gastrointestinal distress.

  • Symptoms:

    • Nausea, anorexia, low-grade fever, explosive watery diarrhea, epigastric pain, flatulence, and malabsorption leading to fatty stools.

  • Diagnosis:

    • Routine stool examinations (recovery may require several samples).

    • Utilization of Entero Test (string test) for adherent forms.

    • Antigen detection, fluorescent methods, and histology if tissue involvement suspected.

  • Microscopic Characteristics:

    • Trophozoite:

    • Pear-shaped with a teardrop appearance.

    • Motility resembles falling leaf motion.

    • Notable feature: sucking disc visible in side view.

    • Cyst:

    • Oval to round, immature cysts.

    • 2 to 4 nuclei;

    • Deeply stained median bodies and possible interior flagellar structures.

Chylomastix mesnilii

  • Pathogenicity: Non-pathogenic, cosmopolitan distribution, more common in warmer climates.

  • Trophozoite Features:

    • Pear-shaped with a single nucleus.

    • Cytostome near nucleus; distinct oral groove.

  • Cyst Features:

    • Pear or lemon shape with a knob-like structure and curved fibril (the Shepherd's crook).

  • Transmission: Similar to other flagellates via ingestion of infective cysts.

  • Motility: Stiff or rotary-like in movement, three anterior flagella and one near the cytostome.

Dientamoeba fragilis

  • Pathogenicity: Pathogen with worldwide distribution, higher incidence in certain populations (e.g., mental institutions, Native Americans in Arizona).

  • Transmission: Possibly via helminth eggs or fecal-oral routes.

  • Symptoms: Diarrhea, intermittent diarrhea, fatigue, abdominal pain, nausea, anorexia, and unexplained eosinophilia.

  • Microscopy:

    • Trophozoites resemble amoeba, broad lobes with no progressive pseudopodia.

    • Cytoplasm may show ingested debris.

  • Cyst Stage Characteristics:

    • Oval to round with irregular cyst walls.

    • Large clear space between the outer and inner wall; 1-2 nuclei.

    • Chromatin granules appear as clumps resembling ink blots.

Pentatrichomonas hominis

  • Pathogenicity: Non-pathogenic; common flagellate in both warm and temperate climates.

  • Cyst Stage: No known cysts; exists solely in the trophozoite form.

  • Habitat: Inserts in the cecum and feeds on bacteria.

  • Trophozoite Description:

    • Pyriform shape with axostyle and a full-length undulating membrane.

    • Distinction made with Trichomonas vaginalis, which has a shorter undulating membrane.

  • Transmission: Via ingestion of trophozoite form only.

  • Microscopic Characteristics:

    • Pear shape, jerky motility observed.

    • Possesses three to five anterior flagella and one posterior flagellum, along with a full-length undulating membrane and a conical cytostome.