Introduction to Parasitology

• Symbiosis


  A relationship where unlike organisms 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 benefitting the other
• Mutualism


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


  A symbiotic relationship where one party or symbiotic (i.e., the parasite) benefits to the detriment of the other (the host)
• Elements in parasitism


  • Parasite
  • 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, which is the result of entry and multiplication of the parasite within the host
• 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
• Obligate parasites


  • Parasites that must live inside a host (e.g., Plasmodium, Leishmania, hookworms)
  • Majority of the parasites that infect humans are obligate parasites
• Types of parasites based on mode of living

  • Permanent parasites
  • Intermittent parasites
  • Incidental parasites
  • Transitory 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., Ascaris lumbricoides in the lungs or kidneys)
• Types of hosts


  • Definitive Host
  • Intermediate Host
  • Reservoir Host
  • Paratenic Host
• 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 (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/her clothing, bedding, or the immediate environment he/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 parasitic amoebae and intestinal flagellates, larvae of the blood flukes, and eggs of the pork tapeworm (Taenia solium)
• Freshwater fish serves 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 T. solium, while improperly cooked or raw beef is the source of T. 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 animals that may serve as sources for parasites include pigs, cows, and birds
• Other human beings are directly responsible for a considerable amount of infection und the pathogenic amoeba Entamoeba histolytica, the pin worm Enterbius vermicularis, and the dwarf tapeworm Hymenolepis nana
• Auto-infection accounts for some of the infections and some re-infections with H. nana, E. vermicularis, and Strongyloides
• 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
• Ingestion of contaminated food and water (fecal-oral transmission) is the most common mode of transmission of most intestinal parasites
• Parasites that are transmitted by ingestion of contaminated water include the intestinal protozoa (cyst stage), and the embryonated egg stage of the intestinal roundworms (e.g., Ascaris lumbricoides, Trichuris trichiura)
• Trichinella spiralis, Taenia solium, Taenia saginata, Diphyllobothrium latum, intestinal flukes, and the lung flukes are transmitted by eating food containing the larval stage of the parasites
• Some parasites actively enter the body through penetration of the skin from the soil (e.g, hookworms and Strongyloides) or from contaminated water (e.g., blood fluke)
• Other modes of transmission

  • Bite of blood-sucking insect vectors (e.g., malaria, leishmaniasis, trypanosomiasis, and filariasis)
  • Inhalation of eggs (pinworm or Enterobius vermicularis)
  • Transplacental or congenital infection (Toxoplasma gondi and occasionally Plasmodium)
  • Transmammary (mother's milk) infection (Strongyloides, Ancylostoma)
  • Through sexual intercourse (Trichomonas vaginalis)
• Portals of exit of parasites

  • Anus
  • Urine
  • Sputum
  • Vaginal discharge
• Pathogenesis


  The dynamics of any disease process
• 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