Epidemiology Flashcards by Denisse Tiangco

Study (scientific, schematic, data-driven) of the distribution (frequency and pattern) and determinants (causes, risk factors) of health-related stages or events in specified populations and the application to the control of health problems

Epidemiology

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Epidemiology is concerned with two key factors of health events:

Frequency

Pattern

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Number of health events (such as number of cases of meningitis or diabetes) but also to the relationship of that number to the size of the population

The rate allows epidemiologists to compare disease occurence across different populations

Frequency

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Occurence of health related events by time, place and person

May be annual, seasonal, weekly, daily, hourly, weekday vs any other breakdown of time that may influence disease or injury occurence (time pattern)

Geographical variation, urban/rural differences, location of work site or schools (place pattern)

Pattern

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Characterizing health events by time, place and person

Descriptive epidemiology

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Any factor, whether event, characteristic or other definable entity, that brings about a change in health condition or other defined characteristic

Causes and other factors that influence the occurence of disease and other health-related events

Determinant

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Anything that affects the well-being of a population

Health-related states or events

ex. disease

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Concerned about collective health of the people in a community or population

Epidemiologist

Focuses on identifying the exposure or source that caused illness, number of persons similarly exposed; the potential for further spread in the community and interventions to prevent additional cases or recurrence

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Explained disease from rarional rather than supernatural point of view

“On airs, waters and places”

Environmental and host factors such as behavior might influence development of disease

Hippocrates

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Haberdasher and councilman who published a landmark analysis of mortality data in 1662

First to quantify patterns of birth, death and disease occurence noting disparities between males and females, high infant mortality, urban/rural differences and seasonal variations

John Graunt 1662

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Father of modern vital statistics and surveillance

William Farr 1800

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Anesthesiologist

Father of field epidemiology

Studied cholera outbreaks discovering cause and prevention of disease

Descriptive epi -> hypothesis generation -> hypothesis testing (analytic) -> application

John Snow

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John Snow developed this tool showing the geographic distribution of cases

Spot map

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Rotavirus vaccine SE

intusucception

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Set of standard criterua for classifying whether a person has a particularly disease, syndrome or other health condition

Case definition

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Graphing annual cases or rate of a disease over period of years

Long term

Secular trend

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Disease occurence can be graphed by week or month over the course of a year or more to show its seasonal pattern

Seasonality

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Suggests hypothesis about the time and source of exposure, the mode of transmission and causative agent

Uses histogram

Epidemic period

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Single most important “person” attribute

Age

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Key feature of analytic epidemiology

Comparison group

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Quantifies the association between exposures and outcomes and to test hypotheses about casual relationships

Analytic epidemiology

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Two kinds of analytic epidemiology

Experimental

Observational

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Investigator determines through a controlled process the exposure for each indivdual (clinical trial) or community (community trial) and then tracks the individuals or communities over time to detect the effects of the exposure

Experimental studies

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Epidemiologist simply observes the exposure and disease status of each participant

Observational studies

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Observational study types:

Cohort Case-control Cross-sectional

Epidemiologist records whether each study participant is exposed or not and tracks to see if they develop disease Investigator observes rather than determines participant’s exposure status (not randomly assigned) After a period of time, the investigator compares the disease rate in rhe exposed group with the disease rate in unexposed If the disease rare is substantively different in the exposed group compared to the unexposed group, the exposure is said to be associated with illness

Cohort study

Well known cohort study that established the rates and risk factors of heart disease

Framingham study

Participants enrolled as the study begins and are then followed prospectively over time to identify occurence of the outcomes of interest

Follow-up/Prospective Cohort

Both the exposure and the outcome have already occured The investigator calculates and compares rates of disease in the exposed and unexposed groups Used in investigations of disease in groups if easily identified people such as workers at a participar factor or attendees at a wedding

Retrospective cohort study

Investigators start by enrolling a group of people with disease and a comparison group (without disease) Investigators compare previous exposures between the two groups The control group provides an estimate of the baseline or expected amount of exposure in the population. If the amount of exposure among the case group is substantially higher than the amount you would expect based on the control, then illness is said to be ASSOCIATED with that exposure

Case-Control

Sample of persons from a population is enrolled and their exposures and health outcomes are measured simultaneously Assess the PREVALENCE of health outcome at that point of time without regard to duration Weaker than either because it cannot disentangle risk factors for occurence of disease (incidence) from risk factors for survival with the disease Synonymous to

Cross-sectional study Survey Prevalance rather than incidence

Subjects are enrolled or grouped on the basis of their exposure, then are followed to document occurence of disease Difference in disease RATES between the exposed and unexposed groups lead investigators to conlcude that exposure is associated with disease

Cohort study

Subjects are enrolled according to whether they have the disease or not, then are questioned or tested to determine prior exposure Difference in EXPOSURE prevalence between the case and control allows investigators to conclude that the exposure is associated with the disease

Case-control study

Measure exposure and disease status at the same time and are better suited to descriptive epidemiology than causation

Cross-sectional study

Simplest model for causation Traditional model for infectious disease

Epidemiologic Triad

Epidemiologic triad

External agent Susceptible host Environment

Chemical contaminant responsible for eosinophilia-myalgia

L-tryptophan

Attempt to account for the multifactorial nature of causation An individual factor that contributes to cause disease is shown as a piece of pie After all the pieces of a pie fall into place, the pie is complete and disease occurs

Causal pie

The individual factors in causal pies

Component causes

The complete pie which might be considered a casual pathway is called

Sufficient cause

A component that appears in every pie or pathway is called without it, disease does not occur

Necessary cause

Hypertension/Stroke Cause

Component Cause

Treponema pallidum/Syphilis Cause?

Necessary cause

Type A personality/Heart disease Cause?

Component cause

Skin contact with a strong acid/Burn Cause?

Sufficient cause

3 forms of route of anthrax infection

Cutaneous (95%) Inhalation - fatal; mail related Intestinal

Progression of a disease process in an individual over time in the absence of treatment

Natural history of disease

Stages of Natural History of Disease

Stage of susceptibility Stage of subclinical disease Stage of clinical disease Stage of recovery: disability or death

Stage of subclinical disease extending from the time of exposure to onset of disease symptoms Asymptomatic or inapparent Some pathologic change detectable with laboratory, radiographic, or other screening methods

Incubation period - infectious disease | Latency period - chronic disease

Paralytic shellfish poisoning (tingling, numbness around lips and fingertips, giddiness, incoherent speech, respiratory paralysis, sometimes death) Incubation period:

Saxitoxin and similar toxins from shellfish Few minutes to 30 minutes

Incubation period of radium (watch dial painters) to developing bone cancer

8-40 years

Most screening programs attempt to identify the disease process during this phase of its natural history since intervention at this early stage is likely to be more effective than treatment given after the disease has progressed and become symptomatic

Incubation period

The onset of symptom marks the transition from subclinical to clinical disease Most diagnoses are made during this stage

Stage of clinical disease

Illness that ranges from mild to severe or fatal

Spectrum of disease

End of disease process

Recovery Disability Death

Proportion of exposed persons who become infected

Infectivity

Proportion of infected individuals who develop clinically apparent disease

Pathogenecity

Proportion of clinically apparent cases that are severe or fatal

Virulence

Agent leaves its reservoir or host through a portal of exit, conveyed by some mode of transmission and enters through an appropriate portal of entry to infect a susceptible host

Chain of infections

Habitat in which the agent normally lives, grows and multiplies May or may not be the source from which agent is transferred to a host

Reservoir

Those who can transmit the agent during the incubation period before clinical illness begins

Incubatory carrier

Those who have recovered from their illness but remain capable of transmitting to others

Convalescent carriers

Continue to harbor a pathogen such as Hepatits B virus or Salmonella typhi for months or years after their initial infection

Chronic carrier

Infectious disease that is transmissible under natural conditions from vertebrate animals to humans

Zoonosis

Types of direct transmissio

Direct contact | Droplet spread

Types of Indirect transmission

Airborne Vehicleborne Vectorborne (mechanical or biologic)

Final link in the chain of infection

Susceptible host

If a high enough proportion of individuals in a population are resistant to an agent, then those few who are susceptible will be protected by the resistant majority

Herd immunity

Increase, often sudden, in the number of cases of a disease above what is normally expected in that population i that area

Epidemic

Epidemic in a more limited geographic area

Outbreak

Aggregation of cases grouped in place and time that are suspected to be greater than the number expected even though the expected number may not be known

Cluster

Epidemic that had spread over several countries or continents usually affecting a large number of people

Pandemic

Persistent, high levels of disease occurence

Hyperendemic

Epidemic and their manner of spread through a population

Common-source Propagated Mixed Other

Group of person are all exposed to an infectious agent or a toxin from the same source

Common-source outbreak

If the group is exposed over a relatively bried period so that everyone who becomes ill does so within one incubation period

Point source (common source) outbreak

Point-source outbreak plot

Steep upslope more gradual downslope | Log-normal distribution

Pattern reflecting the intermittent nature of exposure

Intermittent common source outbreak

Transmission from one person to another by direct person-to-person contact, vehicleborne, vectorborne

Propagated outbreaks

Common source outbreak followed by secondary person-to-person spread

Mixed epidmeic

Neither common-source nor propagated from person to person

Epidemic not caused by point, intermittent or propagated source