PARASITOLOGY

• Symbiosis


  Relationship where unlike organisms exist together
• Types of symbiotic relationships


  • Commensalism
  • Mutualism
  • Parasitism
• Commensalism


  Symbiotic relationship where two species live together and one species benefits from the other without harming or benefitting the other
• Mutualism


  Symbiotic relationship where two organisms mutually benefit from each other
• Parasitism


  Symbiotic relationship where one party or symbiont (the parasite) benefits to the detriment of the other (the host)
• Elements in parasitism


  • Parasite
  • Host

  
• Classification of parasites based on habitat

  • Ectoparasites
  • Endoparasites

  
• Ectoparasites


  Parasites that live outside the host's body

  
• Endoparasites


  Parasites that live inside the body of the host

  
• Classification of parasites based on ability to live independently of the host

  • Facultative parasites
  • Obligate parasites

  
• Facultative parasites


  Parasites that can live independently of the host (i.e. free living)

  
• Obligate parasites


  Parasites that must live inside a host

  
• Classification of parasites based on mode of living

  • Permanent parasites
  • Intermittent parasites
  • Incidental parasites
  • Transitory parasites
  • Erratic parasites

  
• Permanent parasites


  Parasites that remain in a host from early life to maturity

  
• Intermittent parasites


  Parasites that simply visit the host during feeding time

  
• Incidental parasites


  Parasites that occur in an unusual host

  
• Transitory parasites


  Parasites whose larva develops in a host while the adult is free living

  
• Erratic parasites


  Parasites that are seen in an unusual organ, different from that which it ordinarily parasitizes

  
• Types of hosts


  • Definitive hosts
  • Intermediate hosts
  • Reservoir hosts
  • Paratenic hosts
• Definitive hosts

  Hosts that harbor the adult stage of the parasite or where the sexual stage or sexual phase of the life cycle of the parasite occurs

  
• Intermediate hosts


  Hosts that harbor the larval stage of the parasite or where the asexual stage of the life cycle of the parasite occurs

  
• Reservoir hosts


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

  
• Paratenic hosts


  Hosts that serve as a means of transport for the parasite 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
  • One's self (auto infection)

  
• The most common source of exposure to infection is soil contaminated or polluted with human feces
• Water may be the source of the viable cysts of the parasitic amoebae and intestinal flagellates, the larvae of the blood flukes, and the eggs of the pork tapeworm Taenia solium

  
• Freshwater fish serve as the source for the fish tapeworm Diphyllobothrium latum, as well as other intestinal and liver flukes

  
• Raw pork is the source of Trichinella spiralis and Taenia solium, while improperly cooked or raw beef is the source of Taenia saginata
• Blood sucking insects may serve as source for certain parasites - the female Anopheles mosquito for the malaria parasite Plasmodium; sand fly for leishmaniasis; tsetse fly and reduviid bug for trypanosomes; and the Culex and Mansonia mosquitoes for filariasis

  
• Dogs are the direct source of infection with the hydatid cyst of the dog tapeworm Echinococcus granulosus
• Other human beings are directly responsible for a considerable amount of infection with the pathogenic amoeba Entamoeba histolytica, the pinworm Enterobius vermicularis, and the dwarf tapeworm Hymenolepis nana

  
• Auto infection accounts for some of the infections and some re-infections with Hymenolepis nana, Enterobius vermicularis, and Strongyloides stercoralis
• Modes of transmission


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

  
• Ingestion of contaminated food and water (fecal-oral transmission) is the most common mode of transmission of most intestinal parasites

  
• Some parasites actively enter the body through penetration of the skin from the soil or from contaminated water

  
• Other modes of transmission include bite of blood sucking insect vectors, inhalation of eggs, transplacental or congenital infection, transmammary (mother's milk) infection, and through sexual intercourse
• Portals of exit of parasites

  • Anus
  • Urine
  • Sputum
• The most common portal of exit of parasites is through the anus
• Urine may serve as the portal of exit for some parasites, and sputum may be the portal of exit for others
• 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