Introduction to Parasitology

• a relationship between unlike organisms which exist together

  SYMBIOSIS
• one species benefits while the other is not harmed


  COMMENSALISM
• two organisms mutually benefit each other.

  MUTUALISM
• one benefits and the other is harmed

  PARASITISM
• = live outside the host’s body

  = fleas, lice

  Ectoparasites
• = live inside the body of the host

  = helminths or worms

  Endoparasites
• = live independently of the host e. free living

  = don’t have to live inside a host to complete their life cycle

  Facultative parasites
• = must live inside a host eg. Plasmodium, hookworms

  = majority of parasites that infect humans

  Obligate parasites
• = remains in host from early life to maturity

  = ex. Plasmodium

  Permanent parasites
• = simple visit the host during feeding

  = ex. non-pathogenic parasites

  Intermittent parasites
• = remains in an unusual host

  = ex. dog tapeworm

  Incidental parasites
• = larva develops in a host while adult is free-living

  = ex. dog tapeworm

  Transitory parasites
• = seen in an unusual organ

  = different from that which it ordinarily parasitizes

  = ex. Ascaris lumbricoides in lungs or kidneys

  Erratic parasites
• • harbor the adult stage of parasite

  ex. humans for the intestinal worms


  DEFINITIVE HOSTS
• • harbor the larval stage of parasites

  • where the asexual stage of the life cycle of the parasite

  INTERMEDIATE HOSTS
• • vertebrate hosts that harbor the parasite and may act as additional source of infection in man

  • migratory birds serve as the reservoir host of the parasite

  • ex. Capillaria philippinensis which people normally get from contaminated fresh water

  RESERVOIR HOSTS
• • those that serve as a means of transport for the parasite so that the infective stage of a certain parasite may reach its final host

  • ex. insect vectors

  PARATENIC HOSTS
• SOURCES OF EXPOSURE TO INFECTION

  1. contaminated soil or water
  2. food containing the parasite’s infective stage
  3. a blood-sucking insect
  4. a domestic or wild animal harboring the parasite
  5. Another person and his or her clothing, bedding, or the immediate environment he or she has contaminated
  6. one’s self (auto-infection)
• MODES of TRANSMISSION

  1. Fecal-Oral
  2. Penetration of the skin from the soil or contaminated water
  3. Bite of blood-sucking insect vectors
  4. Transplacental or Congenital Infection
  5. Transmammary (mother's milk) infection
  6. Sexual Intercourse
• the most common portal of exit of parasites is through the ANUS
• MECHANISM of DISEASE PRODUCTION BY PARASITES

  1. Physical Trauma
  2. Lytic Necrosis
  3. Stimulation of Host Tissue Reaction
  4. Toxic and Allergic Phenomena (Immunopathology)
  5. Opening of Pathways for entry of other Pathogens into the Tissues
• stage that is present in the parasite’s source of infection

  Infective stage
• stage that enters the host

  Infective Stage
• non-motile cyst


  Infective Stage
• responsible for producing the disease in the host leading to the clinical manifestations

  Pathogenic stage
• motile trophozoite


  Pathogenic Stage
• stage when they are absorbing and feeding nutrients from the host and reproducing

  Pathogenic Stage
• process by which trophozoites differentiate into cyst forms

  Encystation
• process by which cysts differentiate into trophozoite forms

  Excystation
• only known pathogenic intestinal ameba

  Entamoeba histolytica
• "old man facies”


  Giardia lamblia
• Water is the major source of infection

  Entamoeba histolytica
• "Traveler’s diarrhea"


  Giardia lamblia
• Largest protozoan to infect human

  Balantidium coli
• Reservoir is pig


  Balantidium coli
• transmission is through Sexual intercourse


  Trichomonas vaginalis
• Treats BOTH partners to prevent “ping pong” infections

  Trichomonas vaginalis
• Infection in immunocompromised


  Acanthamoeba castellani
• Disease: Granulomatous amebic encephalitis


  Acanthamoeba castellani
• Disease: Trichomoniasis


  Trichomonas vaginalis