Host-parasite Relationship

October 11, 2026 , ,

 Introduction to Host-parasite Relationship

  • The science which deals the occurrence, diseases caused, mode of transmission, prevention of the parasite and its cure is called Parasitology. Parasitism is an association in which two organisms of different species live together where one enjoys all benefits like nourishment, Shelter, one enjoys all benefits like nourishment, Shelter, reproduction etc. at the expense of the other. The benefited organism is called the parasite and the organism harboring the parasite is called the host.
  • Due to close association, the responsive reactions and resistance displayed by a host to its parasite and the protective mechanism adopted by a parasite in their respective environments are called interactions. 
  • The effects of parasitism on the hosts are associated on several factors, such as age, diet, genetic factors, susceptibility of the hosts, the size, number and virulence of the parasites, their mortality, migration, and method of feeding. 

Effects of Parasites on Hosts: 

  1. Destruction of Host’s Tissues - Some parasites injure the host’s tissue during the process of entry, e.g. Ancylostoma duodenale, whose infective larvae causes extensive damage tissue while penetrating the host’s skin. Some causes tissue damage after they have entered, e.g., larvae of Ascaris lumbricoides while passing through lungs causes pneumonia. Entamoeba histolytica destroy the host’s large intestine and liver.
  2. Necrosis - parasite causes persistent cell degeneration 
  3. Hyperplasia: an increased rate of cell division.
  4. Hypertrophy: increase in cell size.
  5. Metaplasia: changing of one type of tissue into another 5. Metaplasia: changing of one type of tissue into another.
  6. Neoplasia: it is the growth of cells to a tumour.
  7. Competition for host’s nutrients.
  8. Mechanical interferences - parasite cause injuries to hosts, e.g. elephantiasis or filariasis.
  9. Effects of toxins, poisons and secretions - P. vivax produces toxic effect in infected persons and the patients suffer from periodic effect of high fever with chilliness and shivering. 
  10. Sex reversals: Gonads of parasitized hosts may change, leading to sex reversals e.g. crab when parasitized by Sacculina-display sex reversals.

HOST 

Host: A person or other animal, including birds and arthropods that affords or give shelter to an infectious agent under natural conditions are called host. Ex: Man, Mosquito 

Definitive host: Hosts in which the parasite attains Definitive host: Hosts in which the parasite attains maturity or passes its sexual stage are called definitive host or Primary host. Ex –Man is the primary host of Tape worm.

Intermediate host: Hosts in which the parasite is in larval or in asexual state are called intermediate host or secondary host. Ex: Pig is the intermediate host of Tapeworm.

Symbiosis: Mutualism is a symbiosis in which both members benefit from the relationship. Commensalism, -Commensalism is a relationship in which one member benefits, and the other one neither benefits nor is harmed. 

Reservoir: A reservoir is defined as any person, animal, arthropod, plant, soil or any substance in which an infectious agent lives and multiplies , on which it depends primarily for survival and reproduce and finally transmitted to a susceptible host. 

The reservoir may of 3 types: 

  1. Human reservoir: The most important reservoir of infection for human is man himself. Sick human -- Acute illness. Ex: influenza, chicken pox, cholera, strep throat. Carrier human -- subclinical infection or chronic infection. Ex: tuberculosis, syphilis, HIV infection, Hepatitis B 
  2. Animal reservoir: wild animal ,domestic animals or birds .
  3. Non-living reservoir: Soil harbors organisms that cause a variety of infectious diseases. Ex: tetanus, anthrax, Pseudomonas infections, a variety of fungi. Water is often contaminated and can serve as a vehicle of infection. Ex. Typhoid, cholera etc.