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The Epidemiology of Infectious Disease. Chapter 37 Todd Kitten, Ph.D. Epidemiology. The science that evaluates occurrence, determinants, distribution, and control of health and disease in a defined human population

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the epidemiology of infectious disease

The Epidemiology of Infectious Disease

Chapter 37

Todd Kitten, Ph.D.

epidemiology
Epidemiology
  • The science that evaluates occurrence, determinants, distribution, and control of health and disease in a defined human population
  • Concerned primarily with the occurrence of disease as categorized by time, place, persons, and causes.
epidemiology3
Epidemiology
  • Diseases investigated can be infectious or non-infectious, and can concern injury and fatality not caused by diseases.
epidemiology5
Epidemiology
  • Diseases investigated can be infectious or non-infectious, and can concern injury and fatality not caused by diseases.
  • Often concerns associations
types of associations
Types of Associations
  • Not statistically associated
  • Statistically associated
    • Non-causal
    • Causal
      • Indirectly causal
      • Directly causal
epidemiology7
Epidemiology
  • Diseases investigated can be infectious or non-infectious, and can concern injury and fatality not caused by diseases.
  • Often concerns associations
  • Can concern associations that are beneficial rather than harmful
    • Alcohol and CHD
    • Fluoride
fluoride and dental caries
Fluoride and dental caries

Lilienfeld, D.E. and Stolley, P.D. 1994. Foundations of Epidemiology, 3rd Ed., Oxford Press.

epidemiology9
Epidemiology
  • Diseases investigated can be infectious or non-infectious, and can concern injury and fatality not caused by diseases.
  • Often concerns associations
  • Can concern associations that are beneficial rather than harmful
    • Alcohol and CHD
    • Fluoride
  • Can concern experiments rather than just observation
fluoride and dental caries10
Fluoride and dental caries

Lilienfeld, D.E. and Stolley, P.D. 1994. Foundations of Epidemiology, 3rd Ed., Oxford Press.

epidemiology terminology
Epidemiology Terminology
  • sporadic disease
    • occurs occasionally and at irregular intervals (typhoid fever)
  • endemic disease
    • maintains a relatively steady low-level frequency at a moderately regular interval (common cold)
  • hyperendemic diseases
    • gradually increase in occurrence frequency above endemic level but not to epidemic level (common cold during the winter)
  • epidemic
    • sudden increase in frequency above expected number
    • index case – first case in an epidemic
more terms
More terms…
  • outbreak
    • sudden, unexpected occurrence of disease
    • usually focal or in a limited segment of population
    • often synonymous with “epidemic”
  • pandemic
    • increase in disease occurrence within large population over wide region (usually worldwide)
animal diseases
Animal diseases
  • epizootiology
    • deals with factors that influence frequency of a disease in an animal population
  • enzootic
    • moderate prevalence of disease
  • epizootic
    • sudden outbreak of disease
  • panzootic
    • wide dissemination
  • zoonoses
    • diseases of animals that can be transmitted to humans
measuring frequency
Measuring Frequency
  • three important statistical measures of disease frequency
    • morbidity rate
    • prevalence rate
    • mortality rate
morbidity rate
Morbidity rate
  • an incidence rate
  • number of new cases in a specific time period per unit of population

# new cases during a specific time

# individuals in population

prevalence rate
Prevalence rate
  • total number of individuals infected at any moment in time
  • depends both on incidence rate and duration of illness
mortality rate
Mortality rate
  • number of deaths from a disease per number of cases of the disease

# deaths due to given disease

size of total population with disease

If 500 people in a town of 100,000 become infected with HIV and 100 die, the mortality rate is…

infectious disease epidemiology
Infectious Disease Epidemiology
  • tries to determine:
    • causative agent
    • source and/or reservoir of disease agent
    • mechanism of transmission
    • host and environmental factors that facilitate development of disease within a defined population
    • best control measures
recognition of an infectious disease in a population
Recognition of an Infectious Disease in a Population
  • involves use of surveillance methods
  • cases of a disease recognized by its characteristic disease syndrome
    • set of signs and symptoms characteristic of a disease
    • signs
      • objective changes in body that can be directly observed
    • symptoms
      • subjective changes experienced by patient
some surveillance methods
Some surveillance methods
  • review of death certificates
  • field investigation of epidemics
  • investigation of actual cases
investigation of a gi illness outbreak
Investigation of a GI illness outbreak

http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

investigation of a gi illness outbreak23
Investigation of a GI illness outbreak

http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

investigation of a gi illness outbreak24
Investigation of a GI illness outbreak

If you were to administer a questionnaire to the church supper participants, what information would you collect?

  • What did you eat?
  • How much did you eat?
  • How long after you ate did you begin to feel sick?
  • How long did it last?
  • Did anyone at the supper show illness prior to the supper?
  • What did you drink?
  • How was the food prepared/ stored?

http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

investigation of a gi illness outbreak25
Investigation of a GI illness outbreak

http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

investigation of a gi illness outbreak26
Investigation of a GI illness outbreak

http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

investigation of a gi illness outbreak27
Investigation of a GI illness outbreak

Without having clinical isolates from the infected individuals for examination, how might you tentatively identify the causative agent?

  • Temperature of storage vs organism?
  • Food vs organism?
  • Homemade vs commercial?
  • Symptoms
  • Culture ice cream.

It the outbreak has already ended, what is the value of working up the case?

http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

course of infectious disease
Course of infectious disease
  • incubation period
    • period after pathogen entry but before signs and symptoms appear
  • prodromal stage
    • onset of signs and symptoms
    • not clear enough for diagnosis
  • period of illness
    • disease is most severe and has characteristic signs and symptoms
  • convalescence
    • signs and symptoms begin to disappear
recognition of an epidemic
Recognition of an Epidemic
  • two types of epidemics
    • common source epidemic
    • propagated epidemic
herd immunity
Herd immunity
  • resistance of a population to infection and to spread of an infectious organism because of the immunity of a large percentage of the population
slide33

Figure

37.4

herd immunity35
Herd immunity
  • resistance of a population to infection and to spread of an infectious organism because of the immunity of a large percentage of the population
  • level can be altered by introduction of new susceptible individuals into population
  • level can be altered by changes in pathogen
    • antigenic shift – major change in antigenic character of pathogen
    • antigenic drift – smaller antigenic changes
what pathogen caused the disease
What Pathogen Caused the Disease?
  • Koch’s postulate (or modifications of them) are used if possible
  • clinical microbiology lab
    • plays important role in isolation and identification of pathogen
  • communicable disease
    • can be transmitted from one host to another
what was the source and or reservoir of the pathogen
What was the Source and/or Reservoir of the Pathogen?
  • source
    • location from which pathogen is transmitted to host
  • period of infectivity
    • time during which source is infectious or is disseminating the organism
  • reservoir
    • site or natural environmental location in which pathogen is normally found
    • sometimes functions as source of pathogen
human sources reservoirs
Human sources/reservoirs
  • carrier
    • infected host
    • can be casual (acute or transient) carrier – short time
    • can be chronic carrier – long time
types of carriers
Types of carriers
  • incubatory carrier
    • harbors pathogen but is not yet ill
  • active carrier
    • has overt clinical case of disease
  • convalescent carrier
    • has recovered from disease but continues to harbor large numbers of pathogen
  • healthy carrier
    • harbors pathogen but is not ill
animal reservoirs
Animal reservoirs
  • numerous diseases are zoonoses
  • transmission to human can be direct or indirect
  • vectors
    • organisms that spread disease from one host to another
how was the pathogen transmitted
How Was the Pathogen Transmitted?
  • Vertical transmission
  • Horizontal transmission:
    • airborne
    • contact
    • vehicle
    • vector-borne
airborne transmission
Airborne Transmission
  • pathogen suspended in air and travels  1 meter
  • droplet nuclei
    • small particles (1-4 mm diameter)
    • can remain airborne for long time
    • can travel long distances
    • usually propelled from respiratory tract of source organisms by sneezing, coughing, or vocalization
  • dust particles also important route of airborne transmission
contact transmission
Contact Transmission
  • coming together or touching of source/reservoir and host
  • direct contact (person-to-person)
    • physical interaction between source/reservoir and host
    • e.g., kissing, touching, and sexual contact
  • indirect contact
    • involves an intermediate (usually inanimate)
    • e.g., eating utensils, bedding
  • droplet spread
    • large particles (>5 mm) that travel < 1 meter
vehicle transmission
Vehicle Transmission
  • vehicles
    • inanimate materials or objects involved in pathogen transmission
  • common vehicle transmission
    • single vehicle spreads pathogen to multiple hosts
    • e.g., water and food
  • fomites
    • common vehicles such as surgical instruments, bedding and eating utensils
vector borne transmission
Vector-Borne Transmission
  • external (mechanical) transmission
    • passive carriage of pathogen on body of vector
    • no growth of pathogen during transmission
  • internal transmission
    • carried within vector
    • harborage transmission – pathogen does not undergo changes within vector
    • biologic transmission – pathogen undergoes changes within vector
why was the host susceptible to the pathogen
Why Was the Host Susceptible to the Pathogen?
  • two main factors
    • defense mechanisms of host
      • Can be related to age, sex, genetic factors, immune status, nutrition, environment
    • pathogenicity of pathogen
how did the pathogen leave the host
How Did the Pathogen Leave the Host?
  • active escape
    • movement of pathogen to portal of exit
  • passive escape
    • excretion in feces, urine, droplets, saliva, or desquamated cells
virulence and the mode of transmission
Virulence and the Mode of Transmission
  • evidence suggests correlation between mode of transmission and degree of virulence
    • direct contact  less virulent
    • vector-borne  highly virulent in human host; relatively benign in vector
    • greater ability to survive outside host  more virulent
slide54

more virulent strain

less virulent strain

Viral load

Time

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