Introduction :
Medical 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 lifecycle 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).
Parasites may be classified as according to livening :
- Ecto-parasite: They inhabit the surface of the body of the host without penetrating into the tissues. They are important vectors transmitting the pathogenic microbes. The infection by these parasites is called as infestation . (e.g ., fleas or ticks )
- Endo-parasite: They live within the body of the host (e.g., Leishmania). Invasion by the endo- parasite is called as infection.
The endo-parasites are of following types :
- Obligate parasite: They cannot exist without a parasitic life in the host (e.g., Plasmodium species)
- Facultative parasite: They can live a parasitic life or free-living life, when the opportunity arises (e.g., Acanthamoeba)
- Accidental parasite: They infect an unusual host (e.g., Echinococcus granulosus infect humans accidentally)
- Aberrant parasite or wandering parasite: They infect a host where they cannot live or develop further (e.g., Toxocara in humans).
Host
Host is defined as an organism, which harbors the parasite and provides nourishment and shelter.
Hosts may be of the following types:
- Definitive host or final host : The host in which the adult parasites replicate sexually (e.g., anopheles species), is called as definitive host.
The definitive hosts may be human or non- human living things.
- Intermediate host: The host in which the parasite under goes asexual multiplication is called as intermediate host. (e.g., in malaria parasite life cycle, humans are the intermediate hosts)
- Intermediate hosts are essential for the completion of the life cycle for some parasites .
- Some parasites require two intermediate hosts to complete their different larval stages.
These are known as the first and second intermediate hosts respectively (e.g., Amphibian snails are the first intermediate host and aquatic plants are the second inter mediate host for Fasciola hepatica .)
Classification of Parasites
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 phylum protozoa are now classified under three subphyla: 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.
- Subphylum (Sarcodina) is typically amoeboid and in represented in humans by class of Entamoeba, Endolimax, lodamoeba, Naegleria, and Acanthamoeba.
2. Subphylum Zoo-mastigophora , the flagellates, have one or more whip-like flagella and, in some cases, an undulating membrane (e.g., trypanosomes).
These include intestinal and genito-urinary flagellates (Giardia, Trichomonas, Dientamoeba, Chilomastix ) and blood tissue flagellates (Trypanosoma, Leishmania ).
3. Subphylum 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 sp.) 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.
- Subphylum Ciliata : 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 helminthes , of a human being, belong to two Subphyla:
- Subphylum Platyhelminths (flatworms) lack a true body cavity (celom) and are characteristically flat in dorso-ventral 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.
- Subphylum Nemathelminths ( Nematodes ) (worm-like, separate-sexed, in-segmented roundworms) include many parasitic species that infect humans.
