Parasite Evasion Mechanisms, Key Terms, and Examples

Posted on Apr 19, 2025 in Naval Architecture and Marine Engineering

Parasitology Essentials

Parasite Evasion Mechanisms

  • Antigenic Variation – Trypanosoma, Plasmodium, Giardia, Toxoplasma gondii
  • Inhibition of Macrophage Phagolysosome Fusion – Leishmania
  • Evasion of Toxic Molecules – Trypanosoma sp.
  • Modulation of Phagocytic Function (SCAP to Cytoplasm) – Leishmania
  • Adsorption of Host Antigens – Schistosoma sp., Plasmodium, Cysticercus
  • Antigenic Mimicry – Schistosoma mansoni
  • Shedding of Antigens – Trichinella sp., Schistosoma sp.
  • Intrinsic Membrane Changes – Schistosoma sp.
  • Cleavage of Antibodies by Enzymes (storytelling) – Trypanosoma cruzi, Filariae, Schistosoma sp.
  • Resistance to Complement-Mediated Lysis – Schistosoma, Trypanosoma sp., Cysticercus
  • Immunosuppression – Leishmania sp., Cysticercus

Key Parasitology Terms

  • Cestode: Flatworm with a head (scolex) and segments (proglottids), hermaphroditic.
  • Commensalism: A relationship where one organism benefits without affecting the other.
  • Ectoparasite: Found on the surface of the host.
  • Endoparasite: Invades inside the host.
  • Facultative Parasite (Optional): Can live a free life.
  • Helminth: Worms with multicellular tissues and organs.
  • Host: An organism on or within which a parasite lives.
  • Definitive Host: The host in which the parasite carries out sexual reproduction.
  • Intermediate Host: The host in which the parasite conducts larval or asexual reproduction.
  • Paratenic Host (Optional): Unnecessary to complete the life cycle, but a host through which a parasite can reach the definitive host.
  • Infection: The entry and replication of a microorganism in the host.
  • Infestation: That caused by ectoparasites.
  • Monoxenous: Requires only one host to complete its life cycle.
  • Mutualism (Symbiosis): A relationship in which both organisms benefit.
  • Nematode: Cylindrical, non-segmented worm with a coelomic cavity, bilateral symmetry, complete digestive tract, and is dioecious.
  • Obligate Parasite (Required): Needs to be parasitic to complete its life cycle.
  • Parasite: An organism that lives at the expense of another, causing damage.
  • Parasitism: A relationship in which one organism benefits and the other is damaged.
  • Platyhelminth: Flatworm with bilateral symmetry and no coelomic cavity.
  • Polyxenous: Requires more than one host to complete its life cycle.
  • Protozoan: Single-celled organism capable of replication in the host.
  • Temporary Parasite: Momentarily depends on the host.
  • Vector (Transmitter): Usually an arthropod that transfers an infectious agent from one host to another. Biological vectors (e.g., Anopheles mosquito for Plasmodium sp.) and Mechanical vectors (e.g., flies carrying E. histolytica cysts and helminth eggs).
  • Trematode: Non-segmented flatworm with a blind digestive tract, usually hermaphroditic.

Examples of Parasites

Intracellular Parasites

  • Plasmodium – Transmission: Anopheles mosquito, blood transfusion, transplacental. Stages: Sporozoites (hepatocytes), Merozoites (erythrocytes).
  • Leishmania – Stages: Promastigote, Amastigote. Vector: Lutzomyia. Hosts: Mammals (skin), Rodents.
  • Toxoplasma gondii – Transmission: Raw meat (cysts), Oocysts (feces). Location: Nervous system, central. Hosts: Cats, Mammals, Birds.
  • Trypanosoma cruzi – Vector: Triatoma. Stages: Trypomastigote (blood), Amastigote (heart). Hosts: Armadillos. Transmission: Vector, blood transfusion, transplacental (metacyclic trypomastigotes).

Extracellular Parasites

  • Giardia – Stage: Cysts. Location: Duodenum, Jejunum. Transmission: Fecal-oral. Host: Humans.
  • Entamoeba histolytica – Stage: Cysts. Location: Large Intestine. Transmission: Fecal-oral. Host: Humans.

Helminths (Worms)

Nematodes (Roundworms)
  • Intestinal:
    • Strongyloides – Stage: Filariform larvae. Location: Duodenum, Jejunum. Transmission: Fecal-cutaneous. Host: Humans.
    • Hookworms – Stage: Filariform larvae. Location: Duodenum. Transmission: Fecal-cutaneous. Host: Humans.
    • Trichuris – Stage: Eggs (latent). Location: Large Intestine. Transmission: Fecal-oral. Host: Humans.
    • Ascaris – Stage: Eggs (latent). Location: Small Intestine. Transmission: Fecal-oral. Host: Humans.
    • Enterobius – Stage: Eggs (latent). Location: Intestine, Perianal region. Transmission: Fecal-oral. Host: Humans.
  • Tissue:
    • Trichinella – Stage: Encysted larvae. Location: Striated muscle. Transmission: Consumption of raw/undercooked meat. Hosts: Humans, Pigs.
    • Onchocerca – Vector: Simulium. Stage: Larvae. Location: Skin, Corneas. Host: Humans.
Cestodes (Tapeworms)
  • Intestinal:
    • Taenia saginata – Stage: Metacestode (Cysticercus). Location: Small Intestine. Transmission: Consumption of raw beef. Host: Humans.
    • Taenia solium – Stage: Metacestode (Cysticercus). Location: Small Intestine. Transmission: Consumption of raw pork. Host: Humans.
    • Hymenolepis nana – Stage: Eggs. Location: Small Intestine. Transmission: Fecal-oral. Host: Humans.
  • Tissue:
    • Taenia solium – Stage: Larvae (Oncospheres). Location: Muscle, Brain (larval phase). Transmission: Fecal-oral (ingestion of eggs). Host: Humans.
    • Echinococcus granulosus – Stage: Oncospheres (Hydatid cysts). Location: Liver, Lung (larval phase). Transmission: Fecal-oral (ingestion of eggs). Hosts: Carnivores (Dogs), Humans.
Trematodes (Flukes)
  • Tissue:
    • Fasciola hepatica – Stage: Metacercaria. Location: Biliary channels, Liver (intrahepatic). Transmission: Ingestion (e.g., contaminated water/plants). Hosts: Humans, Bovine.
    • Paragonimus – Stage: Metacercaria. Location: Lung. Transmission: Consumption of raw/undercooked crustaceans (prawns, river crabs). Hosts: Humans, Opossums.