Parasite Classification and Basic Terminology

Unicellular eukaryotes, the smallest of the parasite groups covered here.

• Trophozoite: the actively feeding, motile, replicating form, typically the form causing tissue damage and symptomatic disease.
• Cyst: a dormant, environmentally resistant form with a protective wall, typically the transmissible/infective stage for organisms with a faecal-oral cycle.
• Intracellular versus extracellular forms: some protozoa live within host cells (e.g. Plasmodium within red blood cells and hepatocytes, Toxoplasma within nucleated host cells), others remain extracellular within a lumen or tissue space (e.g. Giardia within the small intestinal lumen, Trichomonas within the vaginal lumen). Intracellular location has direct implications for immune evasion and for which arm of the immune response is required for clearance.

Major protozoal groups:

Multicellular worms, subdivided into three major groups.

Nematodes (roundworms):

• Cylindrical, elongated, unsegmented body with a complete digestive tract and separate sexes.
• Examples: Ascaris lumbricoides, hookworms, Enterobius vermicularis, Strongyloides stercoralis, filarial worms (organism-specific detail in Parasitology).

Cestodes (tapeworms):

• Flat, segmented body (a chain of proglottids) with no digestive tract of their own, absorb nutrients directly across their surface.
• Examples: Taenia solium, Taenia saginata, Echinococcus granulosus, Diphyllobothrium latum.

Trematodes (flukes):

• Flat, leaf-shaped, unsegmented body, most are hermaphroditic (schistosomes are a notable exception, having separate sexes).
• Examples: Schistosoma species, Fasciola hepatica, Paragonimus westermani, Clonorchis sinensis.

• Organisms that live on, rather than within, the host, typically feeding on blood or skin.
• Lice: obligate ectoparasites, species-specific (head lice, body lice, pubic lice), transmit some bacterial pathogens (e.g. body lice transmit Rickettsia prowazekii, the cause of epidemic typhus).
• Mites: cause direct infestation (e.g. Sarcoptes scabiei, scabies) and transmit certain rickettsial and viral infections via bite.
• Fleas: feed intermittently, vectors for plague (Yersinia pestis) and murine typhus, and intermediate hosts for some tapeworms.

• These terms recur constantly throughout Parasitology: correctly identifying the definitive host, intermediate host, vector, infective stage, and diagnostic stage for a given organism is the standard framework for understanding both its transmission and its laboratory diagnosis.