Protozoa: Types, Features, and Reproduction

• Protozoa is a classification of single-celled eukaryotic microorganisms that belong to the kingdom Protista.
• Protozoa is derived from the Greek words "protos" (first) and "zoon" (animal).
• Protozoa include Amoebae, Flagellates, Sporozoa, and Ciliates.

Examples of Protozoa:

• Amoebae: Entamoeba and Nagleria
• Flagellates: Gardia, Trichomonas
• Sporozoa: Plasmodium and Toxoplasma
• Ciliates: Blantidium

General Features of Protozoa:

• Protozoa are single-celled and perform all functions within that single cell.
• Most protozoa are nonpathogenic; however, some cause major diseases like malaria, leishmaniasis, and sleeping sickness.
• Protozoa like Cryptosporidium parvum and Toxoplasma gondii are opportunistic pathogens, especially in HIV patients or those undergoing immunosuppressive therapy.
• Protozoa range in size from 1 to 150 µm and have varied shapes and structures.

Reproduction in General:

• Protozoa can reproduce asexually via binary fission, multiple fission, or endodyogeny.
• Sexual reproduction via conjugation or gametogony/syngamy can occur in ciliates and sporozoans.
• Asexual reproduction is the most common type of reproduction in protozoans.

Life Cycle:

• Some protozoa require a single host to complete their life cycle.
• Intestinal protozoans uses only one host, multiply asexually, and transfer between hosts by cystic forms.
• Some protozoa require two hosts.
• Plasmodium species uses a human host (man) for asexual reproduction and a mosquito host for sexual reproduction.

Classification of Protozoa:

• Protozoa are classified under the kingdom Protista and subkingdom Protozoa
• Protozoa are further divided into four phyla: Arcomastigophora, Apicomplexa, Microspora, and Ciliophora.

Amoeba

• Amoeba classification:
  • Kingdom: Protista
  • Subkingdom: Protozoa
  • Phylum: Sarcomastigophora
  • Subphylum: Sarcodina
  • Superclass: Rhizopoda
  • Order: Amebida

Classification of Amoebae:

• Intestinal Amoebae: e.g. Entamoeba histolytica, Entamoeba dispar, Entamoeba coli, Entamoeba hartmanni, Entamoeba gingivalis
  • All intestinal amebae are nonpathogenic, except Entamoeba histolytica
• Free-living Amoebae: e.g. Naegleria fowleri, Acanthamoeba spp.
  • All free-living amebae are opportunistic pathogens

General Characteristics of Amoebae:

• Cytoplasm is bounded by a membrane that differentiates into outer ectoplasm and endoplasm.
• Amoebae use pseudopodia for locomotion and to engulf food through phagocytosis.
• Reproduction occurs through binary fission.
• Cysts form in unfavorable conditions
• Cysts are the infective form for vertebrate hosts.
• Amebae are classified as either free-living or intestinal.
• Some free-living amebae can act as human pathogens
• The pathogens can cause meningoencephalitis and other infections, e.g., Naegleria

Entamoeba Histolytica - Morphology:

• E. histolytica occurs in three forms: Trophozoite, Precyst, and Cyst.

General Characteristics of Amoebae:

• Word origin: "Ameba" comes from the Greek word "amibe," meaning change.
• Amebae are structurally simple protozoans with no fixed shape.
• Geographical Distribution: Cosmopolitan, with E. histolytica found worldwide.

Common diseases:

• Intestinal amebiasis

• Amoebic colitis

• Amoebic dysentery

• Extraintestinal amoebiasis.

• The habitat for Amebae are caecum and sigmoido-rectal region of man

• Amebae reproduce either by fission or budding

• Amebae are classified as free-living or endoparasites

Precystic Stage:

• During the precystic stage, trophozoites undergo encystment in the intestinal lumen.
• Encystment does not occur in the tissues or in feces outside the body.

Mode of Infection:

• Heteroinfection:
  • Eating raw vegetables (salad)
  • Consuming contaminated food and water
  • Transmission via flies and food handlers (cyst passers)
• Autoinfection:
  • Faecal-oral routes

Process of Excystation:

• The cyst wall resists gastric juice, allowing the cysts to pass through the stomach unharmed and enter the small intestine.
• When the cyst reaches the cecum or lower ileum, the alkaline medium damages the cyst wall with the assistance of trypsin
• This leads to Excystation.

Life Cycle: E. histolytica

• The life cycle is completed in a single host (man).
• Infection occurs by ingesting cysts in contaminated food or water.

Pathology, Pathogenesis, and Clinical Manifestations:

• E. histolytica causes intestinal and extraintestinal amebiasis.

Incubation Period:

• The incubation period is variable, ranging from 4 days to 4 months.

Symptom display:

• Amebiasis can present in different forms depending on the organ affected and the extent of damage.

Pathogenesis:

• Adherence: Mediated by amoebic lectins that bind to glycogen receptors of colonic mucosa.
• Cytolysis: Metacystic trophozoites penetrate colon cells with amebic cysteine proteases degrading the extracellular matrix.
• Tissue Necrosis: Caused by inflammatory cell lysosomal enzymes surrounding the trophozoites
• These include proinflammatory cytokines like interleukin-8 (IL-8) and tumor necrosis factor-a (TNF-a)
• Mucosal Penetration: This produces discrete, pinhead-centered, crater-like ulcers forming flask-shaped ulcers.

Symptoms:

• Acute Amebiasis: Frequent dysentery with necrotic mucosa and abdominal pain.
• Chronic Amebiasis:
  • Recurrent episodes of dysentery with blood and mucus in feces.
  • Intervening gastrointestinal disturbances and constipation.
  • Cysts are present in the stool.
  • The organism may invade the liver, lung, and brain, leading to abscesses, liver dysfunction, pneumonitis, and encephalitis.
• Extraintestinal Amoebiasis:
• Amoebic hepatitis
• Amoebic liver abscess
• Metastatic amebiasis
• Cutaneous amebiasis

Amoebic Liver Abscess:

• Amoebae may enter the bloodstream and travel to the liver.
• They can form an abscess primarily in the right lobe of the liver.
• Characteristics of Abscess: Liver is affected in roughly 5–10% of individuals with intestinal amebiasis
  • Liver abscess contains thick chocolate brown pus (anchovy sauce pus).
• Clinical features include pain and tenderness in the liver region and fever with chills and night sweats.
• Patients with large or multiple abscesses may experience jaundice and anaemia.

Diagnosis (Intestinal Amebiasis):

• Clinical Specimen: Stool

• Direct Demonstration:

  • Trophozoites are found in unformed stool, while cysts are found in formed stool.
  • Saline Wet Mount: Trophozoite motility well seen in fresh stool specimen

• This is observed via microscope

• Iodine Wet Mount: Cyst wall is well stained

• Can be observed using microscope

• Permanent Stain:

  • With iron hematoxylin or trichrome.

• Concentration Techniques for Cysts:

  • Zinc sulfate floatation
  • Formal ether conc.

Diagnosis:

• Serological Methods:

  • In chronic amebiasis, copro-antigen detection uses monoclonal antibodies in a patient's stool.

• Molecular Techniques:

  • PCR technology: Conventional or real-time PCR testing
  • Used to differentiate between species Note:

• Differentiation between pathogenic E. histolytica and commensal E. dispar requires species-specific monoclonal antibodies or PCR.

• Examination: Examination of aspirate from lung or liver abscesses can identify trophozoites.

• Serology:

  • Serological tests can identify antibodies or antigens in the patient's serum.

• Molecular testing:

  • PCR for definitive diagnosis

Pathogenic Free-Living Amoebae:

• Naegleria Fowleri: This is the only species of genus Naegleria that infects humans.
• Primary Amoebic Meningoencephalitis (PAM): It is a brain infection that leads to the destruction of brain tissue.
• -fowleri occurs in 3 forms: Cyst, Amoeboid trophozoite, and Flagellate trophozoite.

Acanthamoeba culbertsoni

• Morphology: Exists as trophozoite and cyst forms.
• Infection: Infections typically occur in individuals with immunodeficiency that were exposed to infected contact lens fluids.
• Clinical Features:
  • Causes Granulomatous Amoebic Encephalitis (GAE)
  • Trophozoites can be found in the CSF
  • Acanthamoeba keratitis: Cysts can be found in the corneal tissues

Balamuthia mandrillaris

Morphology: Has irregular or branching shape trophozoites with finger-shaped pseudopodia. Its cysts are spherical, with sizes ranging from 6 to 30µm. The cysts also have three layered wall Disease: Causes Granulomatous Amoebic Encephalitis (GAE) Diagnosis: Trophozoites and cystic forms can be seen in the CSF