General Parasitology Chapter Notes | Microbiology - NEET PG PDF Download

Introduction

A parasite is an organism that depends on another organism, called the host, for nutrients. This relationship is harmful to the host as the parasite does not provide any benefits in return.

Parasites may be Classified as

• Ectoparasite: These parasites live on the surface of the host's body without penetrating its tissues. Examples include fleas and ticks. When ectoparasites are present, it is referred to as infestation.
• Endoparasite: Endoparasites reside within the body of the host. The presence of an endoparasite is called infection.
• Obligate parasite: These parasites must live as parasites and cannot survive without a host. An example is Plasmodium spp.
• Facultative parasite: Facultative parasites can live either as parasites or independently. Acanthamoeba is an example.
• Accidental parasite: Accidental parasites infect unusual hosts. For instance, Echinococcus granulosus can affect humans.
• Aberrant parasiteor wandering parasite: These parasites cannot develop further when they infect an unusual host. An example is larva migrans, such as Toxocara in humans.

HOST: The organism that provides nourishment and shelter for the parasite.

• Definitive host: The host where the sexual cycle of the parasite occurs.
• Intermediate host: The host where the asexual cycle of the parasite occurs.
• Reservoir host: A source of infection for other susceptible hosts.
• Paratenic host: Acts as a transport host for the parasite, where the parasite lives but cannot develop further. This host is not essential for the parasite's life cycle. An example is the freshwater prawn for Angiostrongylus cantonensis.
  

Direct/Simple life cycle: Parasites that require only one host, which can be any suitable organism, including humans.

Protozoa

• Entamoeba histolytica
• Free-living ameba
• Giardia lamblia
• Trichomonas vaginalis
• Balantidium coli
• Cryptosporidium parvum
• Cyclospora cayetanensis
• Isospora belli
• Microsporidia

Helminths

• Cestodes: Hymenolepis nana
• Nematodes:
  • Ascaris lumbricoides
  • Hookworm
  • Enterobius vermicularis
  • Trichuris trichiura
  • Strongyloides spp.

Indirect/Complex life cycle: Parasites requiring one definitive host and one intermediate host

Indirect/Complex Life Cycle with Two Intermediate Hosts

Unusual/Rare Mode of Transmission

• Parasites transmitted through sexual contact: These include Trichomonas, Entamoeba, Giardia, and Enterobius.
• Parasites transmitted via the transplacental or perinatal route: This mode of transmission includes Plasmodium, Toxoplasma, and Trypanosoma.
• Parasites spread through blood transfusion: Such parasites include Plasmodium, Babesia, Toxoplasma, Leishmania, and Trypanosoma.
• Parasites entering through the conjunctiva: An example of this is Acanthamoeba spp.

Tropical Parasitic Diseases

• Food and Waterborne Diseases: These include conditions such as Giardiasis and Amoebiasis.
• Soil-transmitted Diseases: This category includes parasites like Ancylostoma duodenale, Ancylostoma braziliense, and Ancylostoma caninum.
• Vector-borne Diseases: This includes diseases caused by Plasmodium spp., Leishmania donovani, Wuchereria bancrofti, Brugia malayi, Onchocerca volvulus, Trypanosoma brucei, and Trypanosoma cruzi.