Intro to parasites IA %% + Flashcards Preview

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  • A parasite is an organism that lives in another organism (host) and gets its food at the expense of this host.
  • Wide range of pathogens
  • Simple organisms
  • More complex than bacteria



  • Microscopic, one-celled organisms
  • Can be free-living or parasitic in nature
  • Able to multiply in humans


  • Protozoa that live in a human's intestine ⇒ fecal-oral route
  • Protozoa that live in the blood or tissue of humans ⇒ arthropod vector (mosquitoes, flies, sand flies, lice, fleas, ticks etc)



  • Complex life cycle
  • Sporozoites injected under skin by mosquito
  • Travel through blood and enter liver
  • Mature in liver and re-enter circulation as merozoites
  • Invade red cells, multiply and lyse cells – reinvade more red cells
  • Sexual forms taken up by mosquito


Malaria control

  • Insecticide treated mosquito nets (ITNs)
  • Prophylaxis
  • Malaria Vaccine research :in progress, currently no effective malaria vaccine


Diagnostic principles

  • Identification of parasites in host tissue or excreta.

  • Microscopy of different stages:

    - parasites, cysts and ova (P, C and O) in faeces

  • Culture rarely possible
  • Blood films for malaria: Thick and thin films
  • Serology: Detection of antibodies can be very useful (e.g when parasite is located in deep tissue sites)


Regions affected

Protozoal infections are common in developing tropical and subtropical regions where sanitary conditions and control of the vectors of transmission are poor. However, with increased world travel and immigration, protozoal diseases are no longer confined to specific geographic location


Why are they difficult to treat?

  • They are eukaryotes.
  • Protozoa, like fungi, have metabolic processes closer to those of the human host than to prokaryotic bacterial pathogens.
  • Protozoal diseases are, therefore, less easily treated than bacterial infections because many antiprotozoal drugs are toxic to the human host


Main groups pic


Common protozoal diseases

  • Malaria
  • Amoebic Dysentery
  • Leishmaniasis


Malaria cause

  • Plasmodium species (4) – P. falciparum has the highest mortality
  • Mosquito vector (Anopheles mosquito)
  • Giemsa stained blood film shows infected red cells (parasitaemia)