Introduction to Parasitology in Biology

Parasitology

• Parasitology is the area of biology concerned with the phenomenon of dependence of one living organism on another.
• Medical parasitology deals with the parasites that infect humans, the diseases they produce, and the response generated by humans against them, as well as methods of diagnosis, prevention, and treatment.

Parasites

• A parasite is an organism that is entirely dependent on another organism, referred to as its host, for all or part of its life cycle and metabolic requirements.
• Microparasites: small, unicellular, and multiply within their vertebrate host, often inside cells (e.g., protozoa).
• Macroparasites: large, multicellular, and have no direct reproduction within their vertebrate host (e.g., helminths).

Classification of Parasites

• Ectoparasites: live on the surface of the body (e.g., human louse, Pediculus humanus).
• Endoparasites: live within the body of the host (e.g., all protozoan and helminthic parasites of humans).

Types of Endoparasites

• Obligate parasites: organisms that cannot exist without a host (e.g., Toxoplasma gondii).
• Facultative parasites: organisms that can live either a parasitic or free-living existence under unfavorable circumstances (e.g., Naegleria fowleri).
• Accidental parasites: organisms that attack an unusual host (e.g., Echinococcus granulosus in humans).
• Aberrant parasites: organisms that attack a host where they cannot live or develop further (e.g., Toxocara canis in humans).

Hosts

• Host: an organism that harbors the parasite and provides nourishment and shelter.
• Definitive host: harbors the adult parasite, where the parasite replicates sexually.
• Intermediate host: the host that alternates with the definitive host and harbors the larval or asexual stages of a parasite.
• Paratenic host: a host in which the larval stage of a parasite survives but does not develop further.
• Reservoir host: can harbor a pathogen indefinitely with no ill effects.
• Compromised host: one in whom normal defense mechanisms are impaired (e.g., AIDS).

Host-Parasite Relationships

• Symbiosis: an association in which both host and parasite are dependent on each other, with no harm to either partner.
• Commensalism: an association in which only the parasite benefits, with no detectable damage to the host.
• Parasitism: one partner (the parasite) harms or lives on the expense of the other (host).

Classification of Animal Parasites and Vectors

• Taxonomic classification: Subphylum, Class, Order, Family, Genus, Species.
• Species: a population with essentially the same genetic characters, capable of continued reproduction, but usually cannot interbreed with other species.