Medical Parasitology of Parasite and Host

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Medical Parasitology of Parasite and Host

Parasites are living organisms, which depend on living host for their nourishment and survival. They multiply or undergo development in the host. Host is defined as an organism, which harbors the parasite, provides nourishment and gives shelter to parasite. Host is relatively larger than the parasite.

Association between Host and Parasite

The relationship between host and the parasite can be of the following types:

  • Symbiosis
  • Commensalism, and
  • Parasitism.

Flowchart 8.1 describes the types of host – parasite relationshipMedical Parasitology of Parasite and Host img 1

Types and Classification of Parasite

According to the nature of the host – parasite interaction and the environmental factors, the parasite may be one of the following,


These parasites live on the outer surface or in the superficial tissues of the host (Example: Lice). The infection by these parasites is called infestation.


The parasite which lives within the host is called Endoparasite. Invasion by the parasite is called Infection. Most of the protozoan and helminthic parasites causing human diseases are endoparasites.

Endoparasites can be further classified as:

Obligate parasite:
This parasite is completely dependent on its host and cannot survive without it.


Facultative parasite:
This parasite may either live as free living form or as a parasite when the opportunity arises.

Naegleria fowleri.

Opportunistic parasite:
This parasite is capable of producing disease in an immune deficient host (like AIDS and cancer patients).

Example: Toxoplasma gondii.

Zoonotic Parasite:
This parasite primarily infects animals and is transmittable to humans.

Fasciola species.

Accidental parasite:
This parasite infect an unusual host are known as accidental parasites.

Echinococcus granulosus infects man accidentally.

Wandering or Aberrant parasites:

Parasites which infect a host migrate to the site where it cannot live or develop further are called aberrant parasites.

Dog roundworm infecting humans.

Types of Host

Definite host:

The host which harbour the adult parasites or in which parasites undergo sexual method of reproduction is referred to as a definite host. The definite host may be a human or any other living organism. Example: Mosquito acts as a definite host for Plasmodium spp. in Malaria.

Intermediate host:

The host in which the larval stages of the parasite live or in which asexual reproduction of parasite takes place is called the intermediate host.

Man acts as an intermediate host for Plasmodium spp. in Malaria.

Reservoir host:

The host which harbour the parasite and acts has an important source of infection to other susceptible hosts is known as reservoir host. It is also called temporary host.

Dog is the reservoir host for disease kala azar.

Natural host:
The host which is naturally infected with a certain species of parasite, is called natural host.

Pig is the natural host of Balantidium coli.

Paratenic host or transport host:

some parasites enter a host in which they do not undergo any development but remains alive till they gain entry into the definitive host or intermediate host. Such a host is termed as paratenic host or transport host or carrier host.

Classification of Medical Parasitology

The most acceptable taxonomic classification of human parasites includes Endoparasites and Ectoparasite. Endoparasites are subclassified into protozoan parasite (unicellular organisms) and helminthic parasite (multicellular organism). Parasites of medical Importance come under the Kingdom called Protista and Animalia. Protista includes the microscopic single – celled eukaryotes known as protozoa.

In contrast, helminths are microscopic, multicellular worms possessing well differentiated tissues and complex organs belonging to the kingdom Animalia. Classification of medically important parasites is given in Flowchart 8.2.

Life Cycle of Parasites

Direct life cycle

The life cycle of parasite that requires only single host to complete its development, is called direct life cycle.

Entamoeba histolytica requires only human host to complete its life cycle.

Indirect life cycle

The life cycle of parasite that requires two or more species of hosts to complete its development, the life cycle is called as indirect life cycle.

Malarial parasite (Plasmodium spp.) requires both human host and mosquito to complete its life cycle.

Transmission of Parasites

It depends upon Source or reservoir of infection, and mode of transmission.

1. Sources of infection

A. Human:

Human is the source or reservoir for a majority of parasitic infection. The condition in which the infection is transmitted from one infected human to another human is called anthroponoses.

B. Animals:

Animals act as the source of infection in many parasitic diseases. The condition where infection is transmitted from animals to humans is called zoonoses.

2. Mode of transmission

A. Oral transmission:

This is through ingestion of contaminated food, water, vegetables, soiled fingers or fomites contaminated by faeces that contain the infective stage of parasite. This mode of transmission is referred to as faecal-oral route. Example: Cysts of Entamoeba histolytica.

B. Skin transmission:

This is another important route. The infective larvae of hookworm enter the skin of persons walking bare footed on contaminated soil.

C. Vector transmission:

It could be a biological or a mechanical means. Many parasitic diseases are transmitted by insect bite.

sandfly is vector for Leishmania.

D. Direct transmission by person to person contact. Frequently, Entamoeba, Giardia andTrichomonas are transmitted by sexual contact among homosexuals.

E. Vertical transmission:

It is the transmission from mother to fetus.


So far, we have learnt about the general introduction and classification of parasites. Now, let us learn a few important human parasites in detail.

Introduction to Protozoa

General characteristics of protozoa:

  1. They are microscopic unicellular eukaryotes.
  2. The single cell has a relatively complex internal structure and it performs various complex metabolic activities such as digestion, reproduction, respiration and excretion.
  3. Each cell consists of nucleus and cytoplasm.
  4. A protozoa parasite during its life cycle may exist in two stages such as trophozoite and cyst.


Amoebae are structurally simple protozoans which have no fixed shape. The cytoplasm of amoeba is bounded by a membrane and can be differentiated into an outer ectoplasm and inner endoplasm. Pseudopodia (false foot) are formed by the amoebae by throwing out ectoplasm followed by endoplasm. These are employed for locomotion and engulfment of food by phagocytosis.

Reproduction occurs by fission and budding. Amoebae are classified as either free living or intestinal amoebae.

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