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Introduction of Medical Parasitology
Mustafa HA. Rasool
MLT Dep.(2015-2016)
Hawler Health Technical College
Definitions :
Medical Parasitology \ Is the study of the parasites which cause disease in
human , means the study of relationship, geographical distribution, habitat,
morphology, lifecycle, mode of infection, disease manifestations, host response,
treatment and control.
Parasite \ a live organism living in, or on , and having some metabolic dependence
on another organism known as a host, It may be a pathogen causing disease in a
host or it may be nonpathogenic, Parasites may be simple unicellular protozoa or
complex multicellular metazoan.
Host \ the host is the organism which harbors the parasite, the parasite does take
nourishment and other benefits from host .
Different kinds of parasite:
1 -Ectoparasite: parasite that lives on the outside of the host (infestation).
-Example: tick, mite.
2- Endoparasite: parasite that lives within the body of the host (infection).
-Example: Plasmodium sp.
3-Temporary parasite: is free living during part of its existence and seeks its host
intermittently to obtain nourishment .
4- Facultative parasite: parasites that capable of leading both a free and a parasitic
existence.
5- Obligate parasite: parasites that take up a permanent residence in and are
completely dependent upon the host. Example: Plasmodium sp.
6- Pseudoparasite: an artifact mistaken for a parasite.
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7- Coprozoic parasite: a foreign species that has passed through the alimentary tract
without infecting the host.
8- Incidental parasite: parasite that establishes itself in a host which it does not
ordinarily live .
9- Permanent parasite: remains on or in the body of the host from early life until
maturity, sometimes for its entire life.
10- Pathogenic parasite: parasite that cause injury to the host by its mechanical,
traumatic, or toxic activities.
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Different kinds of hosts
1- Definitive host: The final host harbors the adult or sexual stage of the parasite.
Example: human for the Schistosome sp.
2- Intermediate host: Host that part or all of the larval or asexual stage may take
place in another animal.
Example: snail for the schistosome.
It can be, two intermediate hosts for a parasite life cycle: primary and secondary
intermediate host.
3- Incidental host: The situation in which the infected individual is not necessary for
the parasite’s survival or development.
4- Paratenic host : Harbors the parasite in an arrested state of development;
However the parasite is capable of continuing its cycle in a subsequent suitable host.
5- Dead-end host : Human or incidental host is called as dead-end host if the cycle
for transmission of the parasite is such as that it cannot be transmitted further.
6- Reservoir host: Animal that harbors the some parasite. These hosts ensure
continuity of the parasite’s life cycle and act as additional sources of human
infection.
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THE NATURE OF HOST-PARASITE INTERACTIONS
These interactions or associations may be as follows:
1- Symbiosis \ Is an association in which both are so dependent upon each other,
that one cannot live without the help of other and none of the partners suffers any
harm the association.
2- Commensalism \ is an association in which the parasite is gain benefits without
causing injures to the host, one partner is benefited, the other is unaffected.
3- Mutualism \ is an association where both of the parasite and host are benefited;
it is another form of symbiosis.
4- Parasitism \ Is an association where parasite gets benefit and the host gets
nothing, in return always suffer some injury.
-any reciprocal association in which a species depends upon another for its
existence.
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Mechanism of disease production by parasites
A. The effect of parasites on their host:
The damage which pathogenic parasites produce in the tissue of the host may be
affected in any of the following ways :
1- Mechanical injury \ produce of pressure as it grow larger.
E.g\ hydated syst … by blockage of ducts such as blood vessels.
2- Deleterious action of toxic substances \ E.g\ producing of tissue dissolving
enzymes by En.hystolytica which cause lesions in human gut.
3- Deprivation of nutrients, fluids and metabolites \ Parasites my produce disease
by competing with the host for food.
4- Introduction of pathogenic microorganism's \ bacterial infection of lesions
produced initially by parasites which sometimes become lethal.
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B. The effect of the host on parasites \
May produce different types of host reaction on the parasites:
1- Immunological response.
2- The diet or nutritional status of the host.
High-protein diet ~ intestinal protozoa
Carbohydrate in the diet ~ tapeworms
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Geographic distribution
1- Tropical countries are most favorable places for the survival, larval development,
and transmission of parasites. Optimal conditions of temperature and humidity are
present.
2- Short summer season prevents the development of many species that required
high temperatures during their larval stages.
3- Intense dry heat / direct sunlight may destroy the larval forms.
4- Low temperatures arrest the development of eggs and larvae and may even
destroy them.
5- Moisture is essential for the development of free-living larvae and propagation of
intermediate hosts, arthropods, snails and fishes .
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Life cycle
The parasitic life cycle is the obligatory of parasites for their growth, development
and multiplication that may be accomplished in their host (or hosts) or environment .
1- Directly –simple, only need a host.
2- Indirectly –complicated, need more than a host .
The life cycle depends on the type of parasites.
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classification
The parasitic organisms that are importance for human health are eukaryotes - they
have a well-defined chromosome in a nuclear membrane.
Parasites are classified into 2 sub-kingdoms :
A. Protozoa (unicellular(.
B. Metazoan (multicellular.(
Medically important parasites generally include:
1- Medical Protozoology.
2- Medical Helminthology.
3- Medical Entomology.
Sub-kingdom \ Protozoa:
1- Class \ Sarcodina (Rhizopoda(.
2- Class \ Mastigophora (Flagellata(.
3- Class \ Sporozoa.
4- Class \ Cilliata
Sub-kingdum \ Metazoa:
1- Class \ Trematoda.
2- Class \ Nematoda.
3- Class \ Cestoda.
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