Epidemiology Flashcards
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
Epidemiology is concerned with two key factors of health events:
Frequency
Pattern
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
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
Characterizing health events by time, place and person
Descriptive epidemiology
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
Anything that affects the well-being of a population
Health-related states or events
ex. disease
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
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
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
Father of modern vital statistics and surveillance
William Farr 1800
Anesthesiologist
Father of field epidemiology
Studied cholera outbreaks discovering cause and prevention of disease
Descriptive epi -> hypothesis generation -> hypothesis testing (analytic) -> application
John Snow
John Snow developed this tool showing the geographic distribution of cases
Spot map
Rotavirus vaccine SE
intusucception
Set of standard criterua for classifying whether a person has a particularly disease, syndrome or other health condition
Case definition
Graphing annual cases or rate of a disease over period of years
Long term
Secular trend
Disease occurence can be graphed by week or month over the course of a year or more to show its seasonal pattern
Seasonality
Suggests hypothesis about the time and source of exposure, the mode of transmission and causative agent
Uses histogram
Epidemic period
Single most important “person” attribute
Age
Key feature of analytic epidemiology
Comparison group
Quantifies the association between exposures and outcomes and to test hypotheses about casual relationships
Analytic epidemiology
Two kinds of analytic epidemiology
Experimental
Observational
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
Epidemiologist simply observes the exposure and disease status of each participant
Observational studies
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
