Mechanisms of infection

A parasitic infection may occur by various routes, depending on the nature of both parasite and host

• Infection by passive entry.
  • By feeding and drinking . Cysts and eggs can be tranported by water or by insects such as the housefly. Examples are:
    • Giardia, an intestinal parasite of beavers may contaminate fresh water lakes and rivers in the US. The drinking of this water may lead to intestinal infections in humans.

      Giardia trophozoites in the stool (39 kb)

      
      
      
      
      
    • Entamoeba, an intestinal parasite of humans is transmitted in the form of cysts by the house fly.
      
      

      Entamoeba trophozoites in the stool (59 kb) Visit the Entamoeba home page

      
      
      

       

    • Naegleria, present in surface water may enter via the nose when swimming and lead to a fatal amoebal meningoencephalitis.

      Naegleria fowleri on dark field

    • Acanthamoeba, a facultative parasite may contaminate soft contact lenses and cause eye infections.
      
      

      Acanthamoeba trophozoites in culture (39 kb)

      By direct contact such as sexual intercourse
    • Trichomonas, is transmitted via sexual intercourse and is the causative agent of the most common sexually transmitted disease amongts humans
    • Giardia is widespread amongst the San Francisco gay community.
    • Trypanosoma equinum is a parasite of equines (horses) and is transmitted via sexual intercourse
• Active entry
  • By biting (haematophagous) insects that serve as the active vectors in the transmission of disease:
    • mosquitoes transmit malaria and filariasis
    • black flies transmit onchocerciasis or river blindness
    • tsetse flies transmit sleeping sickness in Africa
    • sandfies transmit leishmaniasis
  • By highly specialized developmental forms in the life cycle of the parasite
    • Schistosoma cercaria actively penetrate human skin

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