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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 l.jpg

The Epidemiology of Infectious Disease

Chapter 37

Todd Kitten, Ph.D.


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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.


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Epidemiology

  • Diseases investigated can be infectious or non-infectious, and can concern injury and fatality not caused by diseases.



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Epidemiology

  • Diseases investigated can be infectious or non-infectious, and can concern injury and fatality not caused by diseases.

  • Often concerns associations


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Types of Associations

  • Not statistically associated

  • Statistically associated

    • Non-causal

    • Causal

      • Indirectly causal

      • Directly causal


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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


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Fluoride and dental caries

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


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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


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Fluoride and dental caries

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


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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



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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)


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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


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Measuring Frequency

  • three important statistical measures of disease frequency

    • morbidity rate

    • prevalence rate

    • mortality rate


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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


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Prevalence rate

  • total number of individuals infected at any moment in time

  • depends both on incidence rate and duration of illness


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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…


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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


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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


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Some surveillance methods

  • review of death certificates

  • field investigation of epidemics

  • investigation of actual cases


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Investigation of a GI illness outbreak

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


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Investigation of a GI illness outbreak

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


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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


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Investigation of a GI illness outbreak

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


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Investigation of a GI illness outbreak

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


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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


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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


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Recognition of an Epidemic

  • two types of epidemics

    • common source epidemic

    • propagated epidemic




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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


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Figure

37.4



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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



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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


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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


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Human sources/reservoirs

  • carrier

    • infected host

    • can be casual (acute or transient) carrier – short time

    • can be chronic carrier – long time


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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


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Animal reservoirs

  • numerous diseases are zoonoses

  • transmission to human can be direct or indirect

  • vectors

    • organisms that spread disease from one host to another



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How Was the Pathogen Transmitted?

  • Vertical transmission

  • Horizontal transmission:

    • airborne

    • contact

    • vehicle

    • vector-borne


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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


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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


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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


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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



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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



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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



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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


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more virulent strain

less virulent strain

Viral load

Time