Study Guide for Protozoa: Phylum Sarcomastigophora (Amoebae and Flagellates)

CHAPTER 2: Protozoa: Phylum Sarcomastigophora (Amoebae and Flagellates)

INTRODUCTION

  • This chapter discusses medically important protozoans under the phylum Sarcomastigophora, covering their:
    • Life cycle
    • Morphological characteristics
    • Epidemiology
    • Clinical diagnosis and treatment
  • Classifications include:
    • Intestinal or lumen-dwelling amoebae
    • Free-living amoebae
    • Atrial flagellates
    • Hemoflagellates
  • These organisms have a worldwide distribution; prevalence in human populations often correlates with hygiene levels.

PHYLUM SARCOMASTIGOPHORA

  • Organisms from this phylum are amoeboid and flagellated parasites under the Kingdom Protozoa, causing human infections.
  • Amoebae: classified under sub-phylum Sarcodina, recognized by foot-like projections called pseudopodia.
  • Flagellates: classified under sub-phylum Mastigophora, characterized by whip-like flagella.

THE AMOEBA

  • Characteristics of amoebae include:
    • Ameboid structure with a cortical undifferentiated ectoplasm.
    • Presence of pseudopodia used for locomotion.
Important Amoeba in Humans
  • Entamoeba
    • Six species of the genus can infect humans:
    • Commensals:
      • Entamoeba coli
      • Entamoeba hartmanni
      • Entamoeba dispar
      • Entamoeba moshkovskii
      • Entamoeba gingivalis
    • Pathogenic:
      • Entamoeba histolytica
    • Zoonotic:
      • Entamoeba polecki (pigs and monkeys) - Asymptomatic in humans, occasionally causing diarrhea.
  • Other commensals: Endolimax nana, Blastocystis hominis, and Iodamoeba bütschlii.
Genus Entamoeba
  • General Characteristics:
    • Affects invertebrate and vertebrate organisms.
    • Identifiable by a vesicular nucleus with a central karyosome surrounded by peripheral chromatin.
    • E. histolytica, E. dispar, E. moshkovskii, E. hartmanni are morphologically similar; distinguished by isoenzyme analysis or advanced methods (PCR, monoclonal antibodies).
    • E. hartmanni is smaller than E. histolytica.
  • Generalities:
    • All Entamoeba species are lumen-dwelling except for E. gingivalis.
    • All are non-pathogenic except E. histolytica.
    • Cystic stage is the infectious stage (except E. gingivalis).
Entamoeba histolytica
  • First identified by a Russian peasant named Loshchook from diarrheic stool samples.
  • Causes amoebic dysentery and liver abscess, prevalent in areas with poor sanitation.
  • Morphological Characteristics:
    • Trophozoite:
    • Size: 12 to 60 µm in diameter; invasive forms > 20 µm.
    • Active motility via pseudopodia with glass-like projections.
    • Cytoplasm with few inclusions.
    • Cyst:
    • Size: 10 to 20 µm in diameter; usually spherical.
    • Nuclei: up to 4 nuclei in mature cysts, may see chromatoidal bars.
    • Diagnostic features include presence of encapsulated red blood cells or typical cyst structure.
Life Cycle of Entamoeba histolytica
  1. Infective cyst ingested by host.
  2. Excystation occurs in the small intestine (neutral to slightly alkaline pH needed).
  3. Trophozoites inhabit the colon, undergo binary fission.
  4. Encystation occurs when the gut is dehydrated; cysts will form.

SYMPTOMS AND PATHOGENESIS

  • Symptoms vary based on tissue invasion extent:
    • Asymptomatic to severe intestinal amoebiasis characterized by diarrhea, dysentery, abdominal pain, and weight loss.
    • Severe cases called amoebic colitis; may develop systemic infections (liver abscesses).
    • Factors affecting invasion: host immunity, pathogen strain, antibiotic use, malnutrition, pregnancy can worsen prognosis.

DIAGNOSIS

  • Laboratory diagnosis involves:
    • Microscopic examination of stool/colonic aspirate for trophozoites or cysts.
    • Serological tests and stool antigen tests can confirm E. histolytica infection.
    • Molecular testing (PCR) differentiates pathogenic from non-pathogenic species.