Hosts & Vectors

Hosts and Vectors

I. Host

• Definition:
  A host is an organism that provides shelter, nutrition, and a conducive environment for a parasite to survive, grow, and often reproduce.
• Role of Host:
  • Supports different stages of the parasite’s life cycle.
  • May experience varying degrees of harm depending on the parasite-host interaction.
  • Hosts can be classified based on the parasite’s life cycle stage occurring within them.

II. Types of Hosts

1. Definitive Host

• Definition:
  The host in which the parasite attains sexual maturity and undergoes sexual reproduction.
• Key Features:
  • Final host in the life cycle.
  • Often shows full manifestation of disease.
• Examples:
  • Humans for Taenia solium (adult tapeworm).
  • Anopheles mosquito for Plasmodium spp. (sexual reproduction occurs in mosquito)

2. Intermediate Host

• Definition:
  The host in which the parasite undergoes developmental stages but does not reach sexual maturity.
• Key Features:
  • Important for transmission of infection.
  • May involve morphological changes or multiplication.
• Examples:
  • Snails for Fasciola hepatica (liver fluke).
  • Humans for Plasmodium spp. (asexual reproduction in humans).

3. Paratenic (Transport) Host

• Definition:
  A host that harbors the parasite without any further development.
• Key Features:
  • Acts as a temporary shelter.
  • Helps in bridging the ecological gap between intermediate and definitive hosts.
  • No biological change in the parasite occurs inside this host.
• Examples:
  • Frogs serving as paratenic hosts for some nematodes like Dioctophyma renale.
  • Rodents as paratenic hosts for Toxocara canis larvae.

4. Reservoir Host

• Definition:
  A host that harbors the parasite and serves as a continual source of infection but does not necessarily suffer from the disease.
• Key Features:
  • Important in maintaining the parasite in nature.
  • Acts as a hidden source of infection for susceptible hosts.
• Examples:
  • Wild canids as reservoir hosts for Leishmania infantum.
  • Rodents as reservoir hosts for Trypanosoma cruzi.

III. Vector

• Definition:
  A vector is an organism, typically an arthropod, that transmits parasites from one host to another and may either serve merely as a mechanical carrier or also allow biological development.

IV. Types of Vectors

1. Mechanical Vector

• Definition:
  A vector that transmits parasites without any biological change (multiplication or development) in the vector.
• Key Features:
  • Acts like a “flying syringe” or contaminated instrument.
  • Transmission is usually external (on the body surface, mouthparts, etc.).
  • Short-term carriage.
• Examples:
  • Houseflies (Musca domestica) transmitting cysts of Entamoeba histolytica.
  • Cockroaches carrying eggs of parasitic worms.

2. Biological Vector

• Definition:
  A vector that permits the multiplication or development of the parasite, thereby making it infective to the next host.
• Key Features:
  • Parasite undergoes essential life cycle stages inside the vector.
  • Transmission is active and usually long-term.
• Examples:
  • Female Anopheles mosquitoes for Plasmodium spp. (malaria).
  • Tsetse flies (Glossina spp.) for Trypanosoma brucei (sleeping sickness).
  • Sandflies (Phlebotomus spp.) for Leishmania spp.

Summary