introduction

Parasitology

Parasitology: the science that deals with parasites and there pathogenic effects.
Parasite: an organism that lives in or on other organisms (host) and obtains its food from host.
Host: an organisms which harbors parasite.
The parasites of medical importance fall into two kingdom: Protista and Animalia.
The microscopic, single celled, eukaryotic (having a true nuclear membrane) organisms which superficially resemble yeasts, in their size and simplicity are designated as protozoa and belong to the kingdom protista.
In contrast, helminthes are macroscopic, multicellular worms possessing well-differentiated tissues and complex organ systems. There vary in length from less than millimeter to more than meter and belong to the kingdom Animalia.
Protozoa:
Consist of a true membrane bound nucleus and cytoplasm. The shape, size and mode of reproduction. Type of locomotion organelle have been used to divide these into four major classes(super classes):
1- Rhizopods (amoebae): organelles of locomotion are pseudopods and the mode of reproduce is binary fission.
2- Ciliates : cilia are organelles of locomotion and reproduce by binary fission.
3- Flagellates: flagella are organelles of locomotion and reproduce by binary fission.
4- Sporozoa : it is non motile and reproduce by sporogony\schizogony.

Types of parasites:
Parasites are classified based on their interactions with their hosts and on their life cycles.
Those that live on its surface are called ectoparasites (e.g. some mites) and those that live inside the host are called endoparasites (e.g. hookworms).
Endoparasites can exist in one of two forms: intercellular (inhabiting spaces in the host’s body) or intracellular (inhabiting cells in the host’s body). Intracellular parasites, such as bacteria or viruses, tend to rely on a third organism which is generally known as the carrier or vector. The vector does the job of transmitting them to the host. An example of this interaction is the transmission of malaria, caused by a protozoan of the genus Plasmodium, to humans by the bite of an anopheles mosquito.
Most parasites are obligated parasites; they must spend at less some of their life cycle in or on host. Few parasites are facultative; they normally are free living but they can obtain their nutrients from the host, it is called as accidental parasite.
Parasite can be also classified by the duration of their association with their hosts to:
1-permanent parasites, such as tapeworms remain in or on the host once they have invaded it.
2- temporary parasites , such as many biting insects feed and leave their hosts.
3- hyper parasitism, refer to parasite itself having parasites, some mosquitoes which themselves are temporary parasite may also harbor malarial parasites.

Classes of hosts:
1- Definitive hosts: which harbor a parasite while it reproduce sexually.
2- Intermediate hosts: which harbor the parasite during some developmental stages.
3- Reservoir hosts : are infected animals that make parasites available for transmission to other host.
4- Paratenic host: is host which harbor the parasite without showing any development of parasite.
5- Natural host: which is naturally infected with certain spp. Of parasites.
6- Accidental host: which is usually not infected with parasites.
Host-parasite relationship:
The relationship between two dissimilar organisms that are adapted to living together is called symbiosis. The association may be beneficial or harmful to either of the associates.
Commensalism: is a class of relationship between two organisms where one organism benefits but the other is unaffected . Commensalism derives from the English word commensal, meaning "sharing of food"
Mutualism:The term "mutualism" describes any relationship between individuals of different species where both individuals derive a benefit.
Parasitism: is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the host.
In general, parasites are much smaller than their hosts
Attributes of pathogenic parasites:
The parasites may cause damage externally on the surface of skin at the site of invasion (hook worm, blood-fluke larvae) or internally (enterobiasis, taeniasis).different spp. of protozoa elaborate several enzymes which cause digestion and necrosis of host cells.
-parasite damage the host tissues by following mechanisms:
-direct physical and chemical damage
-damage by immunological mechanisms
-induction of neoplastic changes
Direct physical and chemical damage :
• Intracellular parasites usually kill the host cell within which they multiply.(malaria).
• Damage through inflamation as a result of rupture of parasitized cells or by immunological mechanisms.
• Some protozoa release cytolytic enzyme on contact with host cells
• Through vascular blockage as in falciparum malariae.
• Through vascular narrowing (narrowing of the pulmonary arteriols and of the hepatic radicals of the portal vein in schistomiasis.
• Intestinal obstruction.
• Mechanical pressure (hydatid cyst can compress the biliary tract by external pressure.
• Direct damage to the mucosal surface by the worm.
• Through irritant effect of large worm in the gut.
• Larval cestodes damages local tissue as they expand.
Transmission of parasitic infection
1- modes (portals) of entry the host:
• ingestion
• inoculation
• inhalation
• congenital
• venereal
• others
2- portals of exit from host:
• respiratory tract
• gastrointestinal tract
• genital tract
• biting insects

laboratory diagnosis methods for parasitic disease:
1- direct demonstration of parasites
2- microscopic examination of body fluids and tissues
3- cultural methods
4- xenodiagnosis:this imploys an intermediate host or vector within which the parasite multiplies to detect low level of parasites
5- animal inoculation
6- serology and skin reaction tests
7- histopathology