Introduction to parasitology

 1.1.         Parasitology and parasitism

·       Parasitology: “a science  deal with parasites and parasitism

·       Medical Parasitology: studies parasites of man and relation with human beings.

·       Parasite: an organism depends on the host for nutrition and/or shelter, growth, reproduction.

·       Parasites infect billions mainly in the tropics and subtropics

1.2.         The need to study parasitology?

o   The intimate interaction of human with parasites of different infectious disease–health welfare of human

o   The emergence of “new” disease agents drug resistant parasite and

o   Insecticide resistance vectors imposed health problem

1.2.1.    Host- parasite relationships

·        Any close association relation that more or less permanent is called a symbiosis

¨     Mutualism

·        An obligatory association where both partners benefit. Interdependent r/n ship

·        The mutuals are metabolically dependent on one another;

 

§  Commensalism

·       relation that one benefited of food, shelter, transport, but the other is not harmed

·       Partners help each other but also capable of independent life.

§  Parasitism

·       One organism (the parasite) depends on another organism (the host) in an obligatory relationship.

·       The parasite is benefited from the host. The host receives no benefit at all-even it is harmed

1.3.         Effect of parasites on the host

§  Obstruction of passages

§  Damage of host tissue

§  Invasion and destruction of host cells

§  Destruction of bone marrow

§  Allergy developments

§  secondary infection

1.4.    Habitats and environments of parasites

·       Hosts-

·       Organisms which harbor the parasites.-Partner providing food and /or protection

·       Types of Hosts

§  Definitive hosts

·       Harbors the adult stage of a parasite or, Sexually mature stages of a parasite

§  Intermediate hosts

o   Hosts harboring sexually immature or larval stage of a parasite 

§  Resevior host                  

·       A wild or domestic animal that harbors a parasite and acts as sources of infection to humans- Without showing sign or symptom of disease.

§  Carrier hosts

·       Harboring and disseminating a parasite but exhibiting no clinical signs and symptoms.

§  Accidental (incidental) hosts

·       Infection of a host other than the normal host species.

§  Paratenic hosts

·       A host that harbors the parasite in a dormant state of development.

§  Vectors

·       Any arthropod or other living carrier which transports a pathogenic parasite from infected to non-infected host.

§  Biological vectors- which are essential to complete the life cycle of a parasite.

o   Mechanical vectors – in this type of vector multiplication, development and morphological change does not takes place. The vector is Passive carriers of parasites and not essential in the life cycle.

 

1.5. Relevance of Parasitology in developing Countries

i.                 High top leading morbidity and mortality causing infections are prevalent in developing countries

 

o   More than 500 million people in the world are affected by diseases from developing countries.

·       Half of humanity lives in the tropical regions

·       and is therefore at risk from those diseases

 

ii.                Some are ecologically restricted to developing region the world.

       

iii.             Many parasitic infections are associated with:

·       contaminated food and water

·       under nutrition & other poverty-related factors

1.6.         Prevalence factors for development of parasitic diseases in Ethiopia

o   Increased density of population  ;Low socioeconomic conditions and the warm climate /tropical country/

1.7.Taxonomic classification èbelongs to kingdom Animalia

v In general parasites classified as protozoans and metazoans.

-        Most of parasites are members of three phyla:

·       Phylum protozoa

·       Phylum Platyhelminths

·       Phylum Nemathelminths, and

·       Arthropoda*

1.8.          Parasites Classification

A.   According to their size

1.     Microparasite – is small, unicellular and multiplies within its host, often inside cells.

E.g. Protozoa

         2.  Macroparasite – is large, multicellular and has no direct reproduction within its host.

E.g. Helminths

   B.    According to their habitat/location

     1.   Ectoparasites:

¨     Parasites living on the outer surface of the host and inhabit the body surface only, without penetrating into the tissues.

  E.g. Lice, ticks, fleas, mites- vectors transmitting pathogenic microbes.

     2. Endoparasites:

¨     Parasites living within the body of the host.

     C.   According to their dependence and their location in the host

       1.  Permanent (obligate) parasites

         - Complete dependency upon its host for metabolites, shelter and transportation.

       2. Temporary (facultative) parasites

          - Parasites capable of independent existence in addition to parasitic life

       3.Aberrant /Erratic/ parasites

           

          -    Parasite found in a location in which it does not normally live.  Eg. Toxocara canis (a dog Ascaris) in human body. 

–        Parasites that wanders into an organ in which it is not usually found. 

     4.   Accidental parasites

     -    Parasites that attack unusual host

    5.    Opportunistic parasites

  - Parasites cause no or mild disease in healthy individuals.

        - Cause severe disease in immuno-deficient hosts

      -Free – living but pathogenic parasites but Cause diseases when they are introduced to the host

                       

1.9.          Major types of life

        Life cycle:  a route followed by the parasite in the host from entry to exit, and including extracorporeal (outside host) life.

·       Study of life cycles helps to understand:

§  Pathogenesis and pathology of parasites in the host.

§  Sympatology of the parasitic diseases.

§  Method of diagnosis and the selection of appropriate medication.

§  Epidemiology, prevention, and control of the parasitic infection

§  Depending on how many different hosts are involved to complete parasites life cycle we can classify life cycle in to two main types that are direct and indirect.

A.   Direct / Simple / monoxenous  life-cycle

§  One host involved- require only a single host to complete its cycle.

§  Does not require intermediate hosts.

§  Transmission through direct contact or used mechanical vectors.

B.    Indirect / Complex /heteroxenous  life-cycle

Ø   Parasites require different types of hosts for various stages of their life cycle.

·       The parasite requires:

Ø  One definitive host,

Ø  One or more intermediate hosts for transmission

Ø  Not easily spread and controlled

 

 

                            

1.10.      Sources of exposure to parasitic infections

§  Contaminated soil , water ,food

§  . Blood sucking arthropods

§  Animals harboring the parasite

§  Sexual intercourse, Eg. Trichomonas vaginalis

§  Autoinfection, Eg.  Strogyloides stercoralis, Enterobius vermicularis, Taenia solium, etc.

1.11.        Pathogenesis and pathology

v Depends on the complex interaction of a variety of host and parasite factors.

                 i.     Host factors:-

               ii.      Genetic factors

o   Those that lack Duffy antigen ,Possession of sickle cell trait  and Glucose-6-phosphate dehydrogenase deficient

              iii.     Age and status natural immunity at the time of infection.

Eg. Very old or very young are more vulnerable to intestinal parasites

              iv.     Intensity and frequency of infections

vi.             Sex-Females - hygiene related infections

           - Males –vector borne parasitic disease.

1.12. .Prevention and control of parasitic infections

v Based on integrated actions of:-

·       Chemotherapeutic intervention,

·       Vector, intermediate, and reservoir  host control,

·       Preventing  zoonotic diseases transmission,

·       Personal and environmental hygiene,

·       Socioeconomic developments,

·       Public health awareness through education, etc.

Ø  Control of parasitic infections should be included as a tool in the combat of HIV infection.