Pathogenic Protozoa: Diseases, Stages, and Identification

Pathogenic Eukaryotes

Protozoa: Characteristics and Diseases

This section describes the diseases caused, modes of locomotion, trophozoite/cyst stages, and infective and diagnostic stages for the following protozoan parasites:

• Entamoeba histolytica: Trophozoites and Cysts

  • Locomotion: This protozoan moves with the help of pseudopods (false feet).
  • Stages: The trophozoite form has a single nucleus, while the cyst contains a maximum of four nuclei and is typically round.
  • Infection & Development: During ingestion, the cell wall of a mature quadrinucleated cyst disintegrates, and its nuclei divide, producing eight active trophozoites. These trophozoites move to the colon, where they establish infection.
  • Trophozoite Morphology: Trophozoites are variable in shape. The nucleus has a discrete nuclear membrane with a central karyosome and peripheral chromatin granules. The cytoplasm is clear, and sometimes ingested red blood cells may be present.
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• Giardia lamblia: Trophozoites and Cysts

  • Disease Caused: This flagellated protozoan is responsible for abdominal discomfort and severe diarrhea (Giardiasis).
  • Diagnosis: Diagnosis is made by finding cysts and trophozoites in the stool (see Figure 13.2).
  • Trophozoite Morphology & Locomotion: The trophozoites are pear-shaped with concave sucking discs. They possess two bilaterally located nuclei with a large central karyosome that resemble eyes. Four to five flagella may be present, aiding in locomotion.
  • Cyst Morphology: The cysts are oval to ellipsoidal in shape with two to four nuclei (see Figure 13.2). Cysts also contain a cyst wall, axostyle, and parabasal body.
  • Epidemiology: Outbreaks of giardiasis have occurred in daycare centers in Arizona and many other states.
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• Trypanosoma Species: Trophozoites

  • Disease Caused & Locomotion: This hemoflagellate organism, with an undulating (wavelike) membrane, causes various forms of African sleeping sickness. Trypanosoma travels with a wavy, spiral motion produced by its contractile flagellum and undulating membrane.
  • Life Cycle: The protozoan requires two hosts to complete its life cycle. Humans host the sexually maturing forms, while the tsetse fly hosts the developmental forms.
  • Trophozoite Morphology: The trophozoite is crescent-shaped with a large, central, and polymorphic nucleus located in the granular cytoplasm.
  • Pathology & Symptoms: The parasites multiply at the site of infection. They then invade the lymphatic tissue, causing symptoms such as fever, headache, nausea, and vomiting. They can also affect the central nervous system, potentially leading to coma and death.
  • Cyst Stage: There is no cyst stage for this parasite.
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