Protozoa: Features and Structure

• Protozoa are single-celled eukaryotic microorganisms, which perform reproduction, digestion, excretion, respiration, etc.
• The word protozoa comes from the Greek words "protos" (first) and "zoon" (animal).
• Parasitic protozoa adapt to different host species.

General Features of Protozoa

• Most protozoa are non-pathogenic.
• Protozoa that are opportunistic can cause severe disease in immunocompromised individuals.
• Some protozoa cause major diseases such as malaria.
• Protozoa range in size from 1 to 150 μm and exhibit a wide variety of shapes and structures.

Structure of Protozoa

• The protozoan cell is covered with a cell membrane and consists of cytoplasm and a nucleus.

Cytoplasm Components

• Ectoplasm is the outer homogeneous part.
• Ectoplasm is the organ for locomotion and engulfs food via pseudopodia.
• Ectoplasm aids in respiration, ingestion, and discharging waste material.
• Ectoplasm provides a sensory and protective covering for the cell.
• Endoplasm is the inner granular portion.
• Endoplasm contains the nucleus, golgi bodies, endoplasmic reticulum, and food vacuoles.
• Contractile vacuoles regulate osmotic pressure.
• Cytoplasm functions in nutrition (metabolism) and reproduction.

Nucleus Features

• The nucleus is typically single but can be double or multiple.
• Some species can have as many as 100 nuclei in a single cell.
• The nucleus contains one or more nucleoli or a central karyosome, which is a DNA-containing body.
• Chromatin may be distributed along the periphery of the nucleus.
• The nucleus regulates cell activity and reproduction.

Biology of Protozoa - Movement

• Protozoa can move by pseudopodia, cilia, or flagella with undulating membranes.
  • Amoeba move by pseudopodia.
  • Balantidium move by cilia.
  • Giardia and Leishmania move by flagella.
  • Trypanosoma move by flagella with undulating membrane.
• Apicomplexan protozoa are spore-forming, and they do not have organs of locomotion but have an apical complex used for host cell penetration, e.g., Plasmodium, Toxoplasma, and Cryptosporidium.

Respiration

• Protozoa respire by taking oxygen directly or via a metabolic process.

Nutrition

• Protozoa absorb liquid food.
• Protozoa ingest solid food via pseudopodia or cytostome, surrounded by food vacuoles.

Excretion

• Excretion occurs through osmotic pressure, contractile vacuoles, diffusion, or cytopyge.

Secretion

• Protozoa can secrete cyst walls, pigments, digestive and proteolytic enzymes, and toxic and antigenic substances.

Reproduction

Asexual Reproduction Types

• Binary fission: A parasite divides longitudinally or transversally into two or more equal parasites, e.g., Amoeba and ciliates.
• Multiple fission: The nucleus undergoes several successive divisions (schizogony, merogony), e.g., Malaria parasite (Plasmodium) in humans.
• Endodyogeny: A parasite multiplies by internal budding, resulting in two daughter cells, e.g., Toxoplasma.

Sexual Reproduction Types

• Conjugation: Two organisms join and reciprocally exchange nuclear material, e.g., Balantidium.
• Gametogony: Male and female gametocytes are produced, then after fertilization, a zygote forms, which gives rise to numerous sporozoites, e.g., the Malaria parasite in mosquitoes.

Life Cycle of Protozoa

• Simple life cycles involve protozoa needing only one host to multiply asexually, e.g., Amoeba.
• Complex life cycles involve protozoa multiplying asexually in one host and sexually in another, e.g., Malaria parasite in humans and mosquitoes.

Common Life Stages of Protozoa

• Trophozoite: The active feeding, moving, and multiplying stage of the parasite.
• Cyst: The resting or resistant stage of protozoa bounded by a tough cell wall.

Modes of Infection

• Ingestion of the infective stage via:
  • Giardia intestinalis cyst
  • Entamoeba histolytica cyst
  • Cryptosporidium parvum oocyst
  • Toxoplasma gondii oocyst
• Eating improperly cooked meat:
  • Toxoplasma gondii cyst
• Insect bites from:
  • Female Anopheles transmitting malaria
  • Female sand fly (Phlebotomus) transmitting leishmaniasis
  • Kissing bug (Reduviid or Triatomine) transmitting Chagas disease
• Sexual transmission of Trichomonas vaginalis trophozoite
• Blood transfusion of Plasmodium trophozoites and merozoites
• Organ transplantation of Leishmania donovani amastigote
• Transplacental transmission of Toxoplasma gondii tachyzoites
• Autoinfection, either internal or external (feco-oral transmission), e.g., Giardia lamblia, Entamoeba histolytica, Cryptosporidium parvum.

Prevention and Control of Protozoa

• Health education
• Environmental sanitation and personal hygiene
• Case treatment
• Pure water supply
• Safe and clean food
• Sanitary disposal of feces
• Vector control

Classification of Protozoa

• Phylum: Sarcomastigophora
  • Subphylum: Sarcodina (Amoebae), which includes parasitic and free-living amoeba.
  • Subphylum: Mastigophora (Flagellates):
    • Intestinal (Giardia)
    • Urogenital (Trichomonas)
    • Haemo-somatic (Leishmania and Trypanosoma)
• Phylum: Ciliophora (Balantidium)
• Phylum: Apicomplexa (Plasmodium, Toxoplasma & Cryptosporidium)