6. Epidemiology of Infectious Disease and the Host-Parasite Relationship

  1. parasitism
    one organism (parasite) gains benefit while the other (host) is harmed
  2. commensalism
    one organism benefits and the other is not affected
  3. mutualism
    both organisms gain benefit
  4. parasite
    organism dependent on another organism for its existence
  5. host
    the organims the parasite depends on
  6. infection
    establishment of a host-parasite relationship by growth and multiplication of the parasite in or on the host
  7. disease
    an injury or abnormal finding in a living organism which results in recognizable effects
  8. epidemiology
    the study of the distribution and determinants of disease in a population
  9. infectious disease
    one that results from an infection with an organism causing morbidity
  10. non-infectious disease
    one that does not involve infection with an organism
  11. reservoir
    host or place where the parasitic organism usually lives and mulitiplies
  12. 3 main reservoirs for organisms that cause disease in humans
    • soil
    • animals
    • humans
  13. soil organims
    • saprophytes - free-living
    • utilize dead organic materials for nutrients
    • only become parasitic on occasion
  14. animals
    • always parasitic
    • organisms are called zoophilic
    • different species of animals will be hosts for different microorganisms
    • diseases caused by these organisms are called zoonoses
  15. humans
    • only found in humans
    • organisms are always parasitic
  16. portal of entry
    • place where the parasitic organism enters the host
    • mouth for GI tract infections
    • nose and mouth for respiratory tract infections
  17. portal of exit
    place where microorganism leaves host
  18. transmission
    • the way in which an organism gets from its reservoir to its host
    • may or mat not involve intermediate vehicle
  19. vector
    • arthropod that carries organism from reservoir to host
    • biological - multiplication in vector
    • mechanical - no mulitiplcation in vector
  20. fomites
    inanimate objects that carry organisms from reservoir to host
  21. 4 main routes of transmission
    • respiratory route or droplet infection
    • fecal-oral route
    • blood-borne route
    • contact route
  22. respiratory route
    • reservoir - humans or animals
    • portal of entry - respiratory tract
    • port of exit - respiratory tract (droplets produced through sneezing and coughing)
    • fomites (hands) often involved
  23. fecal-oral route
    • reservoir - humans or animals
    • portal of entry - mouth
    • portal of exit - anus and mouth (in feces and vomit)
    • often spread through contaminated food and water
    • fomites frequently involved
  24. blood-borne route
    • reservoir - humans or animals
    • portal of entry - broken skin, insect bite
    • portal of exit - broken skin, insect bite
    • intermediate vehicles essential - arthropod vectors, needles, blood transfusions
    • - not needed when transmitted through sexual contact
  25. contact route
    • reservoir - humans, animals, soil
    • portal of entry - abrasion, mucous membrane
    • portal of exit - lesion, sore, contaminated secretion (none for soil organisms)
    • fomites can be involved
    • sexual transmission is through contact route unless blood-borne
  26. communicability
    the ability of an infective organism to pass from one host to another
  27. communicable disease
    • one which can be transmitted from one human or animal to another (contagious)
    • most diseases
  28. non-communicable disease
    • those which cannot be transmitted
    • diseases caused by soil organisms
    • - each human must acqurie anew
  29. endemic
    a disease that is constantly present in an area but invovles relatively few persons
  30. sporadic
    a disease that occurs only occasionally and involves only a few individuals
  31. epidemic
    a disease taht involves a large number of persons for a limited period of time
  32. pandemic
    describes a series of epidemics that involve a large part of the world
  33. emerging infectious diseases
    an infectious disease whose incidence has increased recently or has been newly discovered
  34. 4 types of emergin infectious diseases
    • new disease, new organism
    • old disease, newly discovered to have infectious origin
    • new manifestation of known organism
    • re-emergin - once common, decreased in prevalence, now coming back strong
  35. transiency
    in or on host, no multiplication
  36. colonization
    • multiplication in host without causing any reaction from host
    • - balanced relationship
    • - situation fro normal flora
  37. subclinical/asyptomatic
    multiplication in host eliciting immune reaction, but no disease
  38. clinical/symptomatic
    multiplication in host with clinically evident disease
  39. iceberg concept of infection
    • what you see above water is just a small part of the whole
    • most get exposure but no infection or infection but no symptoms
  40. pathogen
    an organism that is capable of causing disease
  41. non-pathogen
    an organism that is not capable of causing disease
  42. opportunist
    an organism that only causes disease when the host is compromised
  43. what determines the outcome of an imbalance in the host-parasite relationship?
    • exposure dose - how many individual microogranisms is the host exposed to
    • properties of microorganism
    • - pathogenicity = ability to cause disease
    • - virulence = extent of disease
  44. invasiveness
    • ability to invade through host tissues and reach secondary sites
    • - can be through direct extension
    • - or via the blood and lymph systems
  45. septicemia
    microorganisms in the blood
  46. strategies of invasiveness
    • antiphagocytic (interfering with phagocytosis)
    • extracellular substances break down tissue
    • alternation of surface to avoid immune system
    • genetic integration - lysogenic viruses
    • flagella (motility) and pili (attachment)
  47. toxigenicity
    ability ot produce potent toxic substances
  48. toxins
    • potent poisons produced by microorganisms
    • - destroy host cells
    • - interfere with normal function
    • 2 types = exotoxins and endotoxins
    • many toxins are phage encoded
    • - lysogenic conversion = when an avirulent strain becomes virulent in this manner
  49. exotoxin vs. endotoxin
    • exotoxin:
    • gram-postive and gram-negative species
    • synthesized in the cytoplasm, may or may not be secreted
    • protein
    • can generally form toxoid
    • inactivated by heat
    • generally very potent

    • endotoxin:
    • gram-negative species only
    • component of the outer membrane
    • lipopolysaccharide
    • cannot form a toxoid
    • heat-stable
    • not very toxix
  50. immune response
    • necessary to control and clear infections
    • many disease manifestations are immune mediated
  51. host factors
    • genetics
    • age
    • nutritional status
    • hormonal balance
    • integrity of organ system
    • integrity of immune system
    • immune status
  52. evolution of host-parasite relationship
    • parasite does not want to kill host
    • better to cause minor or no disease
    • endogenous flora are highly adapted parasites
  53. advantageous adaptations of parasites
    • survive longer between hosts
    • use a vector for transmission
    • use alternative hosts
    • develop long incubation period
    • increase virulence
    • decrease virulence
  54. host adaptation
    exposure to infectious disease will select for human hosts that are more resistant
  55. history of host-parasite relationship
    • orignal humans hunters/gatherers
    • european colonization successful due to infectious diseases
    • diseases originate from hunter/gatherers - HIV, Ebola
Card Set
6. Epidemiology of Infectious Disease and the Host-Parasite Relationship
public health microbiology lecture 6