Topic: introduction to medical parasitology

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Parasitology is the study of parasites and as such that does not include bacterial, fungal or viral parasites. Human parasites are separated into intestinal and blood borne parasites. For a parasite to be defined as intestinal it must have an intestinal life cycle stage, though it may have life-cycle stages in the heart, blood vessels, and lungs in the humans, other animals or the environment.

The association between two organisms may be one of the following:

MUTUALISM: mutual benefit is derived from the association.

SYMBIOSIS: mutual benefit, but the two organisms cannot live independently.

COMMENSALISM: one partner benefits (commensal) while the other (host) is unaffected. It may be called a non-pathogenic parasite.

When an animal lives on another organism from which it receives food and shelter without any compensation to it, and then this association is called parasitism. The animal, which enjoys advantages, is the parasite. All animals have parasites; hence there are more parasites than free-living animals. The habitat occupied by a parasite is very different from the environment of its free-living ancestors, hence it has either to adapt itself to this new habitat or perish.

PARASITISM: one organism (parasite) lives at the expense of the other (host). The latter usually suffers from the association with pathogenic parasite).

Parasitism is the form of mutual relations between organisms of various kinds, from which one (parasite) uses another (host) as environment for living, and from which it obtains food causing him damage (disease).

PARASITOLOGY is a complex biological science studying the phenomena of parasitism. There are mutual relations between the parasite and host, their dependence on the factors of external environment, and also diseases, caused by the parasites, and methods of fighting with them in man, animals and plants.

MEDICAL PARASITOLOGY consists of 3 parts: medical protozoology, medical helminthology and medical entomology.


Each parasite belongs to a phylum, class, order, family, genus and species; the scientific designation of a parasite is binomial, a generic name (genus) and a specific name (species).

The parasites of humans in the kingdom protozoa are now classified under 3 phyla: Sarcomastigophora (containing the amoebae and flagellates); Apicomplexa (containing the sporozoan); and Ciliophora (containing the ciliates). The important human parasites are found within these great groups.

1. Class LOBOZEA (Sarcodina) is typically amoeboid and in represented in humans by class of Entamoeba, Endolimax, lodamoeba, Naegleria, and Acanthamoeba.

2. Class ZOOMASTIGOPHORA, the flagellates, have one or more whip-like flagella and, in some cases, an undulating membrane (e.g., trypanosomes). These include intestinal and genitourinary flagellates (Giardia, Trichomonas, Dientamoeba, Chilomastix) and blood tissue flagellates (Trypanosoma, Leishmania).

3. Class SPOROZOA undergoes a complex life cycle with alternating sexual and asexual reproductive phases, usually involving two different hosts (e.g., arthropod and vertebrate, as in the blood forms). The subclass Coccidia contains the human parasites Isospora, Toxoplasma, and others. One of these, Cryptosporidium, has been implicated as a cause of intractable diarrhea among the immunosuppressed. Among the Haemosporina (blood sporozoan) are the malaria parasite (Plasmodium species) and the subclass Piroplasmia, which includes Babesia species. Pneumocystis has recently been shown to be a member of the Fungi rather than the Protozoa. It is another opportunistic parasite of immunosuppressed individuals.

4. Class LITOSTOMATEA is a complex protozoan bearing cilia distributed in rows or patches, with two kinds of nuclei in each individual. Balantidium coli, a giant intestinal ciliate of humans and pigs, is the only human parasite representative of this group.

THE PARASITIC WORMS, or helminths, of a human being, belong to two phyla:

1. PLATYHELMINTHS (flatworms) lack a true body cavity (celom) and are characteristically flat in dorsoventral section. Medically important species belong to the classes Cestoda (tapeworms) and Trematoda (flukes). The tapeworms of humans are band-like and segmented; the flukes are typically leaf-shaped, and the schistosomes are narrow and elongate. The important tissue and intestinal cestodes of humans belong to the genera Diphyllobothrium, Spirometra, Taenia, Echinococcus, Hymenolepis, and Dipylidium. Medically important trematode genera include Schistosoma, Paragonimus, Clonorchis, Opistorchis, Heterophyes, Metagonimus, Fusciolopsis, and Fasciola.

2. NEMATHELMINTHS (worm-like, separate-sexed, insegmented roundworms) include many parasitic species that infect humans.

PHYLUM ARTHROPODA includes 3 classes of medical importance:

1. CLASS CRUSTACEA: Cyclops, etc.

2. CLASS ARACHNIDA (Octapoda): scorpions, ticks and mites.

3. CLASS INSECTA (Hexapoda): mosquitoes, flies, bugs, lice and fleas.


ENDOPARASITE: lives within the body of the host (infection).

ECTOPARASITE: lives on the outside of the body of the host (infestation).

OBLIGATE PARASITE: completely dependent upon its host and cannot lead a free life.

FACULTATIVE PARASITE: capable of leading both a free-living and a parasitic existence.

INCIDENTAL PARASITE: can establish itself in a host in which it does not ordinarily live.

ACCIDENTAL PARASITE: a free-living organism that may live as a parasite in a host.

PERMANENT PARASITE: remains all or most of its life in or on its host.

TEMPORARY PARASITE (occasional or intermittent): free-living but seeks its host from time to time for food.

PERIODIC PARASITE (transitory): passes a definite part of its life cycle as a parasite.

SPECIFIC PARASITE: occurs in one particular host.

PSEUDOPARASITE: an artifact mistaken for a parasite.

COPROZOIC or SPURIOUS PARASITE: a free-living or a non-human parasite passing through the alimentary canal without infecting man or contaminating his stools after being passed.

OPPORTUNISTIC PARASITE: occurs in patients with impaired host defense (immunodeficient or immunocompromised host). Such parasite does not ordinarily cause disease in healthy immunocompetent) individuals.


1. FINAL HOST or DEFINITIVE HOST: harbours the adult or sexually mature parasite.

2. RESERVOIR HOST: an animal that harbours the same species of parasites as man and constitute a source of infection to him. Usually these animals are not affected by infection. The reservoir host serves as a potential source of human reinfection and as means of sustaining the parasite when it is not infecting man. Diseases of animals that are transmissible to man are called zoonotic diseases.

3. INTERMEDIATE HOST: harbours the immature or asexual stages of the parasite.

4. VECTOR: an arthropod that carries a parasite to its host.
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