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Epidemiology - PowerPoint

VIEWS: 61 PAGES: 22

									      Pathogenesis
Populations at Risk for Infection
          or Disease

  Bob L. Larson, DVM, PhD
    University of Missouri
   Commercial Agriculture,
     Beef Focus Team
                              1.   Measurement of disease events
                              2.   Infectious disease epidemiology


Infectious disease epidemiology
•   Involves two populations
    1. Animal
    2. Infectious agent
        Has its own life cycle, immunology, and evolution
•   Involves additional considerations
       Reservoir
       Transmission
       Contact and mixing patterns of infective and
        susceptible hosts
       Degree and duration of infectiousness
                              1.   Measurement of disease events
                              2.   Infectious disease epidemiology


Infectiousness
• Time lines of infection
   Time line of infectiousness
      Latent period
        Time from infection to development of infectiousness
      Period of infectiousness
        Time from development of infectiousness to end of time
          when the host can infect another host
      Noninfectious period
        Removed from population - harvested
        Dead
        Recovered
                             1.   Measurement of disease events
                             2.   Infectious disease epidemiology


Infectiousness
• Time lines of infection
   Time line of infectiousness
   Time line of disease
      Incubation period
        Time from infection to development of symptomatic
          disease
                             1.   Measurement of disease events
                             2.   Infectious disease epidemiology


Infectiousness
• Time lines of infection
   Time line of infectiousness
   Time line of disease
      Incubation period
      Symptomatic period
        Infection without symptoms = inapparent case
           Can be infectious
        Pathogenicity = probability of developing
         symptoms after being infected
                            1.   Measurement of disease events
                            2.   Infectious disease epidemiology


Infectiousness
• Time lines of infection
   Time line of infectiousness
   Time line of disease
      Incubation period
      Symptomatic period
      Nondiseased
        Recovery from symptoms
           Recovered
           Immune
           Carrier – recovers from symptoms but remains
              infectious
        Death
    • Time lines of infection

                            Time Line of Infectiousness
                                                Noninfectious
                                                    • Removed
               Latent             Infectious        • Dead
Susceptible    Period               Period          • Recovered

                                 Time Line of Disease
                                                        Nondiseased
                                                          • Dead
                                                          • Recovered
                    Incubation        Symptomatic         • Immune
Susceptible           Period            Period            • Carrier

 Time
         Time of           Development           End of
        infection          of Symptoms         Symptoms
    • Time lines of infection

                            Time Line of Infectiousness
                                                    Noninfectious
                                                       • Removed
               Latent           Infectious             • Dead
Susceptible    Period             Period               • Recovered
                               Time Line of Disease
                                                    Nondiseased
                                                      • Dead
                                                      • Recovered
                    Incubation   Symptomatic          • Immune
Susceptible           Period       Period             • Carrier

 Time
         Time of       Development         End of
        infection      of Symptoms       Symptoms
    • Time lines of infection




   Important to Patient and Clinician
                                 Time Line of Disease
                                                     Nondiseased
                                                      • Dead
                                                      • Recovered
                    Incubation    Symptomatic         • Immune
Susceptible           Period        Period            • Carrier

 Time
         Time of       Development          End of
        infection      of Symptoms        Symptoms
    • Time lines of infection

                       Time Line of Infectiousness
                                               Noninfectious
                                                  • Removed
              Latent       Infectious             • Dead
Susceptible   Period         Period               • Recovered




   Important to Infectious Agent and
       Production Medicine Vet
 Time
                        1.   Measurement of disease events
                        2.   Infectious disease epidemiology


Transmission probability
• The probability that, given contact
  between an infective source and a
  susceptible host, successful transfer of
  the parasite will occur so that the
  susceptible host becomes infected
    Depends on:
     The characteristics of the infective source
     The parasite
     The susceptible host
     The type and definition of contact
                        1.   Measurement of disease events
                        2.   Infectious disease epidemiology


Transmission probability
• The probability that, given contact
  between an infective source and a
  susceptible host, successful transfer of
  the parasite will occur so that the
  susceptible host becomes infected
    Depends on:
     The characteristics of the infective source
       Another animal
       Insect vector
       Inanimate object: water, feed, fomites
                        1.   Measurement of disease events
                        2.   Infectious disease epidemiology


Transmission probability
• The probability that, given contact
  between an infective source and a
  susceptible host, successful transfer of
  the parasite will occur so that the
  susceptible host becomes infected
    Depends on:
     The type and definition of contact
       The mode of transmission determines
         what types of contact are potentially
         infectious
                           1.   Measurement of disease events
                           2.   Infectious disease epidemiology


Contact pattern – contagious disease
• Some diseases – contact pattern is determined
  by density
   When confined in smaller areas, contacts are more
    frequent
   Important for diseases spread by casual contact
 Other diseases – contact pattern is determined
  by pathogen or animal characteristics
   Sexually transmitted diseases will only affect
    sexually active adults, not calves, pregnant animals,
    or anestrous animals
                         1.   Measurement of disease events
                         2.   Infectious disease epidemiology


Contact pattern – non-contagious disease
 • Vertically transmitted diseases
    Dam to offspring in utero
  Vector borne diseases
    Snail
    Biting insects / arthropods
                                     1.   Measurement of disease events
                                     2.   Infectious disease epidemiology


 Basic reproductive number (R0)
          Number of         Transmission
                                                    Duration of
R0   =   contacts per      probability per      infectiousness    = cpd
           unit time           contact



R0 is a complex parameter that summarizes
many of the important aspects of an
infectious agent in a host population
                                         1.   Measurement of disease events
                                         2.   Infectious disease epidemiology




                        Epidemic, low R0
Number of individuals




                           Susceptible
                                                   Immune




                                              Infected
                                                                New Cases
                              Time
                                        1.   Measurement of disease events
                                        2.   Infectious disease epidemiology




                        Epidemic, high R0
Number of individuals




                         Susceptible               Immune




                                             Infected
                                                              New Cases

                                 Time
                                       1.   Measurement of disease events
                                       2.   Infectious disease epidemiology




                        Endemic, low R0
Number of individuals




                         Susceptible




                                             Immune
                                                              New Cases
                                             Infected


                               Time
                                        1.   Measurement of disease events
                                        2.   Infectious disease epidemiology




                        Endemic, high R0
Number of individuals




                          Susceptible



                                              Immune
                                                               New Cases
                                                   Infected


                                Time
                   1.   Measurement of disease events
                   2.   Infectious disease epidemiology


Epidemic, low R0              Epidemic, high R0




Endemic, low R0               Endemic, high R0
Historical photo courtesy of the K-State Research and Extension archives

								
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