Parasitology is the study of parasites and their relationship with their host.


Parasitism is a type of symbiotic relationship between two heterospecific organisms in which one organism called the parasite is metabolically dependent on the other called the host. The parasite benefits at the expense of the host and is also usually smaller in size than its host.

Parasitic relationship is said to obligatory as normally the parasite cannot survive without its host.

Parasite derives its nourishment directly from its host.

Antibodies are usually synthesized in host’s body against the parasite.

The parasitic relationship may be:

  1. Permanent – e.g. Tapeworms in gut.
  2. Temporary – e.g. Mosquitoes sucking human blood.

Types of Parasites:

  1. Obligatory Parasite: The parasite is completely dependent on the host either for a segment of its life cycle or throughout its life cycle. Example: All viruses are obligatory parasites.
  2. Facultative Parasite: (facultative means optional) – An organism which is not completely dependent on the parasitic way of life but have capability of adapting to parasitic way of life if placed under such conditions, otherwise can be free living, is called facultative parasite. Example: Naegleria fowleri (Brain eating amoeba).
  3. Accidental/Incidental Parasite: An organism which accidentally acquires unnatural host & survives there is termed accidental/incidental parasite. Example: Angiostrongylus cantonensis is an incidental parasite of humans.
  4. Erratic Parasite: (Erratic means unpredictable) – A parasite which wanders/ is found in an organ where it is not normally found i.e. the organ which it generally does not invade, is called erratic parasite.
  5. Periodic/Sporadic Parasite: A parasite which visit its host periodically to obtain required nutrition. Example: Ticks, Fleas, Mosquitoes etc.
  6. Pathogenic Parasite: A parasite which causes disease in its host. The disease can be chronic or acute. Example: Entamoeba histolytica.
  7. Endoparasite: A parasite which lives inside the body of its host. Example: Tapeworm, Liver fluke etc.
  8. Ectoparasite: A parasite which lives outside the body of its host. Example: Ticks, fleas etc.
  9. Stenoxenous Parasite: A parasite which have a narrow range of hosts. Example: Hookworm, Coccidia
  10. Euryxenous Parasite: A parasite which have a wide range of hosts. Example: Toxoplasma gondi.
  11. Heteroxenous Parasite: Parasite whose development occurs in more than one host. They have indirect life cycle. Example: Plasmodium etc.
  12. Homo/Monoxenous Parasite: Parasite whose development occurs in one host only. They have direct life cycle. Example: Trypanosomatids etc.

Types of Hosts

  1. Definitive/Primary/Final Host: The host in which a parasite attains sexual maturity & reproduces itself. Example: Anopheles mosquito is primary host of Plasmodium.
  2. Secondary/Intermediate Host: The host which serves as essential environment for completion of a parasite’s life cycle. Usually, the parasite completes its asexual/sexually immature phase of life inside its intermediate host. Example: Humans are secondary host of Plasmodium.
  3. Transfer/Paratenic Host: The host which serves as a ‘refuge’ & ‘vehicle’ for a parasite to reach its obligatory host. The parasite usually does not undergo any developmental stage inside its transfer/paratenic host. Example: Snake is paratenic host for Alaria americana.
  4. Vector: Organism which serves as a carrier of a disease causing parasite. The vector can act as transfer agent only or it may have itself been infected by the parasite. Though Arthropods comprise a major group of vectors but some invertebrates too act as vectors. Example of vector: Anopheles mosquito acts as vector for Plasmodium.
  5. Dead end/Incidental/Accidental Host: A host which is unnatural host to a parasite, it acquires the parasite accidently and generally does not allow the transmission of the parasite to its natural host. Example: Human is dead end host for West Nile Virus.
  6. Reservoir Host: It is a type of host in which a parasite normally lives, grows and reproduces but that host may or may not act as agent of infection for the parasite to reach its other host. Example: Filarial worm – Brugia malayi causing filariasis, is transmitted from human to human through bite of mosquito but it might also be transmitted to human from cat/monkey via mosquito. Cat & Monkey acting as reservoir hosts from which infection in humans is possible.

Some other parasite related terms:

  1. Zoonosis: Infectious disease caused by a pathogen which can be transmitted from other animals to humans. Example: Ebola Virus Disease.
  2. Hyper-parasite: Organism which is a parasite of another parasite. Example: Nosema dollfusi is a hyperparasite – parasitizing another parasite Bucephalus cuculus and Bucephalus cuculus is itself parasite of American Oyster (host).

Overview of Parasitic Life Cycles: (Species to Species variation can occur)

  • Direct Life Cycle: There is only one host in such life cycle. Example: Coccidea, Monogenea etc.
  • Indirect Life Cycle:

i) Two hosts – Passage through both hosts is mandatory for parasite’s life cycle completion. Example: Sporozoa, Flagellates etc.

ii) Three hosts- Example: Strigeoidea.

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