Lesson 4

• Symbiosis


  A relationship where unlike microorganisms exist together
• Types of Symbiotic Relationships

  • Commensalism
  • Mutualism
  • Parasitism
• Commensalism


  A symbiotic relationship in which two species live together and one species benefits from the other without harming or benefiting the other
• Mutualism


  A symbiotic relationship in which two organisms mutually benefit from each other
• Parasitism


  A form of symbiotic relationship where one party or symbiont (i.e. parasite) benefits to the detriment of the other (the host). In most cases the parasite deprives the host of essential nutrients and produce disease in the host.
• Two Important Elements of Parasitism

  • The Parasite
  • The Host
• Ectoparasites


  Parasites that live outside the host's body (e.g., fleas, lice). Invasion of the body by ectoparasites is called infestation
• Endoparasites


  Parasites that live inside the body of the host (e.g., helminths or worms). Invasion of the body by endoparasites is called infection.
• Facultative parasites

  Parasites that can live independently of the host (i.e. free living). These parasites do not have to live inside a host to complete their life cycle. Ex. Strongyloides stercoralis
• Obligate parasites


  Parasites that must live inside a host to complete the life cycle (e.g., Plasmodium, Leishmania, hookworms). Majority of the parasites that infect humans are obligate parasites.
• Permanent parasites


  Parasites that remain in a host from early life to maturity. (e.g., Plasmodium)
• Intermittent parasites


  Parasites that simply visit the host during feeding time (e.g., non-pathogenic parasites).
• Incidental parasites


  Parasites that occur in an unusual host (e.g., dog tapeworm in humans).
• Transitory parasites


  Parasites whose larva develops in a host while the adult is free-living. (e.g., Echinococcus granulosus or dog tapeworm).
• Erratic parasites


  Parasites that are seen in an unusual organ, different from that which it ordinarily parasitizes (e.g., Entamoeba histolytica in the liver or lung of humans)
• Types of Hosts

  • Definitive Host
  • Intermediate Host
  • Reservoir Host
  • Paratenic Host
• Definitive Host


  Hosts that harbor the adult stage of the parasite or where the sexual stage or sexual phase of the life cycle of the parasite occur.
• Intermediate Host


  Hosts that harbor the larval stage of the parasite or where the asexual stage of the life cycle of the parasite occurs.
• Reservoir Host


  Vertebrate hosts that harbor the parasite and may act as additional source of infection in man.
• Paratenic Host


  Hosts that serve as means of transport for the parasite (e.g., insect vectors) so that the infective stage of a certain parasite may reach its final host.
• Sources of Exposure to Infection or Infestation

  • Contaminated soil or water
  • Food containing the parasite's infective stage
  • A blood-sucking insect
  • A domestic or wild animal harboring the parasite
  • Another person and his or her clothing, bedding, or the immediate environment he or she has contaminated
• Modes of Transmission

  • Ingestion of contaminated food and water (fecal-oral transmission)
  • Penetration of the skin from the soil or contaminated water
  • Bite of blood-sucking insect vectors
  • Inhalation of eggs
  • Transplacental or congenital infection
  • Transmammary (mother's milk) infection
  • Through sexual intercourse
• Portals of Exit

  • Anus
  • Urine
  • Sputum
• Mechanisms of Disease Production by Parasites

  • Trauma or physical damage
  • Lytic necrosis
  • Stimulation of host tissue reaction
  • Toxic and allergic phenomena
  • Opening of pathways for entry of other pathogens into the tissues
• Classification of Parasites

  • Protozoa
  • Helminths
• Protozoa

  • Amoebae and flagellates
  • Sporozoa
  • Ciliates

  
• Helminths

  • Nemathelminthes (roundworms)
  • Platyhelminthes (flatworms)
• Platyhelminthes

  • Trematoda (flukes)
  • Cestoda (tapeworms)