Disease detectives Notes
Epidemiology- study of how disease is distrubuted is population’s abd the factors that
inluence or determine this distrubution
Objectives:
-identify cause and risk factors of a disease
-how is it transmitted
-determine extent of disease in community
-study natural history and prognosis of disease
-identify high risk groups: find specific factors/change
-Primary Prevention: (ultimate goal) prevent in healthy people beofre
Secondary Prevention: already developed disease in early stages intervene
Population based approach: preventative measure applied to population
High-risk approach: applied to high-risk group of people
-Diagnosis and prognosis are often population based- “what’s going around” (helpful- not
always occur)
1) Determne whether association is b/w factor (ie environmental exposure) or
Characterisitic and development of disease in question=study characteristic of
groups and individual
a) find differences- make sure c comparable data and try to find why (high
risk people? Ethnicity?) disease is almost always based on interactiony,
even if genetic.
-Clinical disease- characterized by signs and symptoms
-Host must be susceptible
-Disease tranmitted directly (person to person) or indirectly (water supply)
-Non-clinical disease:
prechricail disease- not yet clinically apparent
subclinical disease- not clinically apparent and never will be
presistant (chronic)- persists for years
-Latent: infection with no active mult. Of virus
-Horizontal transmission: person to person
-Vertical tansmission: generation to generation (usually genetic)
-Host: ie person
Agent: bacteria
Environment: contaminating water supply
Vector: ie mosquito
-Carrier state: person harbors organism but is not know to be infected and can infect
others (usually not very many)
-Endemic: habitual presence of disease in given geographic area
Epidemic: occurance in a community or region of a group of illnesses of a similar nature
-Pandemic: worldwide epidemic
-Single or multiple exposures
-Periodic or continuous contamination
-Common vehicle exposure: ie people sick from eating same food
-Single exposure, common vehicle outbreak- explosive (all increase at once) and limited
to people sharing exposure
-Determinants of disease outbreaks: depends on the numble of people susceptible or
number immune
-Herd immunity: resistance of a group to an attack by disease because a large proportion
of group members are immune- if a large percentage of the population is immune,
everyone is usually protected not just the immune because disease spreads from person
to person (must have random mixing of people, constant mixing, can’t have reservoir
outside human host for disease
-Incubation period: interval from receipt of infection to onset of the clinical illness
-Epidemic circle: distribution of the times of onset of the disease
-When did the exposure take place?
-When did the disease begin?
-What was the incubation period?
-attack rate = # at risk that develop illness
# at risk
-Primary case: person aquires disease from exposure
-Secondary case: person aquires disease from primary case
-Sources of data:
-interviews
-physician records
-hospital records
-Incidence: number of new cases of disease that occur during specified period of time in
population at risk (measure of risk)
-Cumulative incidence: when entire population is at risk
-Determines incidence:
# of new cases occuring in a population
during a specified period of time
x 1,000 = incidence per 1,000
# of people at risk of developing it
during that time
-Incidence rate: denominator is sum of different times individuals were at risk
-Prevalence (per 1,000): determines the number that have the disease
-point (specific point in time) vs. period (any time in a certain period) prevalence
-prevalence = incidence x duration of disease
number of affected people in population at a specific time
number of people in population at that time
x 1,000
-Spot map: map, each point is case
-Annual mortality rate (per 1,000 in population) =
total # of death from --x 1,000 (-can make it age, disease,
# of people at risk at a specific time
cause, etc. specific)
-Case fatality rate (%) =
# of people dying during a specific time after the disease onset/ diagnosis
# of people with disease
x 1,000
-Proportionate mortality =
# of death from ---- to ---- x 100
total # of deaths from ---- to ----Mortality rate is a good reflection of incidence rate when: case/ fatality rate is high and
duration of the disease is short.
-Stardardized mortality ratios =
observed # of deaths per year
expected # of deaths per year
-Cohort: group of people who share the same experience
-Mortality changes may be due to: quality of life
-Median survival time: length of time that ½ the study population survives
-Relative survival rate =
observed survival in people with disease
expected survival if disease was absent
-Randomized Trial:
-Case study: need a control
-Historical controls: look at records of past patients treated differently
-Simultaneous non-randomized controls, randomization
-Cohort studies: group of exposed and group of non-exposed individuals and follows up
to compare incidence of the disease, incidence of exposed: a/a + b not: c/c + d
-Concurrent prospective study: investigator identifies original population at the beginning
and follows through time… takes at least 20 years retrospecitve cohort study- use
historical data
-Must have an idea of possible cause / exposure to do cohort- easier when time b/w exp.
and outcome is short
-Case-control study: people with disease and people without disease determine proportion
exp. and not (& controls) a/a + c > b/b + d
-Group matching: proportion of controls of certain characteristics are identical to
proportion of cases with same
-Individual matching: control selected of similar variables of concern (age, race, etc.)
-Problem: limitation of recall and recall bias cross-sectional studies- both disease and
exp. are determined simultaneously (snapshot of population)
-Outbreak: more cases of a disease than expected in an area, group of people or period of
time (epidemic = large # of people and large area)
-Cluster: aggregation of cases
public health surveillance
investigate for control and prevention, severity and risk to others research and training
if cases continue occurring- control cases extent and pop’n @ risk to prevent
if almost over- investigate to ID source
Steps:
1. prepare for field work
2. establish existence
3. verify diagnosis
4. define identify cases
5. orient data: time, place, person
6. develop hypotheses
7. evaluate hypotheses
8. refine hypotheses and carry out additional studies
9. control and prevention measures
10. communicate findings
#1: Research disease and gather equip, and supplies, make arrangements, consult all
parties for role in invest. And local contacts
#2. determine expected # of cases for area in the time frame (compare current and
previous comparable data) health dept. surveillance records local sources (death records
etc.) estimates from neighboring states/ national data or telephone survey # reported cases
greater than expected may not mean an out break (change in reporting case def. nat. local
awareness population size)
#3. Ensure problem was properly diagnosed, review clinical findings and lab results and
lab tech, look at techniques being used, visit ill people and ask questions
#4. establish case der (standard criteria) includes: clinical information, characteristics of
affected people, info about location/places, specification of time when it occurred;
confirmed, probable or possible c= lab verification prob= clinical
identify and count cases, determine size and geographic extent, look at health care
facilities (calling/ visiting = actual surveillance), alert public through media, conduct
survey of population, collect info for line listing (11= table, each column representing
variable), identifying info, demographic info, clinical info, risk factor info
#5. Descriptive epidemiology- describe what has occurred in population under study
characterizing by time: epidemic curve (tells where you are in course of epidemic, maybe
future, maybe estimate time period of exposure, infer: common source exposure? Person
to person spread? Must know: time of onset of illness for each person
-consider overall shape, period of time people are exposed, unit ¼ to 1/3 incubation
period, minimum, average, max incubation period
-steep slope and gradual climb = point/ single source epidemic (all cases with incubation
period) if exp is prolonged = continuous common source epidemic = plateau
- person to person spread = series of progressively taller peaks 1 incubation period apart,
outliers and information!
Characterizing by place: “spot map” of area- clustering indicates focal source or personto-person spread; scattering = common source (can be misleading- # distribution, use
attack rate)
Characterizing by person: use characteristics (age and sex esp) of population , summarize
#6. hypothesize- source of agent, mode (vehicle/vector) of transmission, exposures
causing disease – should be able to be tested – talk to a few people, use descriptions,
epidemiology