Epidemiology The study of the distribution and determinants of disease in populations and its application to the control of health problems
DEFINITIONS OF HEALTH AND DISEASE REQUIRE
DEFINITIONS OF NORMALITY:
• Common (frequent)
• Statistical (within a rangę)
• Pragmatic (related to risks)
• Practical (related to benefits)
DEFINITIONS ARE BASED ON
• Signs
• Symptoms
• Results of tests
MEASUREMENT OF HEALTH AND DISEASE IS
REGUIRED FOR:
• Preventing disease
• Promoting health
• Planning health services
THE CENTRAL TOOL OF
EPIDEMIOLOGY IS THE COMPARISON
OF RATES:
Rate = Numerator
Denominator
Defining health
• The absence of disease
• A feeling of well-being
• Freedom from the risk of
disease and untimely death
• ‘A state of complete
physical, mental and
social well being and not
merely the absence of
disease or infirmity’
(WHO)
– The Definition has not
been amended since
1948
• Many definitions
Population Health
• Conceptual framework
for thinking why some
people are healthier than
others
• Policy development for
resource allocation
– Maintenance of health
– Health research
• Social structures and
processes that contribute
to health
• Epidemiology addresses
health at the population
level
Uses of Epidemiology
• Identifying the cause of disease
– Legionnaires’ disease, Bird Flu
• Determining the effectiveness of
clinical and preventive measures
– Clinical trials, mammograms
• Identifying new symptoms
– Varieties of hepatitis, new
salmonella strains,
• Monitoring the health of a
community, region, or nation
– Surveillance, accident reports
• Identifying risks in terms of
probability statements
– Down Syndrome at increasing
age
• Studying trends over time to make
predictions for the future
– Estimating health services
needs
Epidemiologist’s
checklist
• Define (verify) disease –
what?
– Accuracy of diagnosis
• Describe historical trend of
disease – when?
• Define extent of the problem
– how much?
• Identify determinants of
disease distribution – why?
• Formulate hypotheses about
causation
– how?
• Determine modes of
transmission
• Develop and evaluate
interventions for control and
prevention
Define disease Case definition is the set
of criteria that
differentiate normal (non-
diseased) from abnormal
(diseased).
Case definition may be
based on symptoms
– sometimes the absences
of symptoms
Case definition may be
complicated because there:
– Are no tests;
– Tests are expensive; or
– Test are impractical.
It is important that case
definitions are frequently
reviewed.
Case definition Case definition should
consider how cases will be
detected.
Things to consider:
– Is there a registry of
case?
– Are all cases likely to be
recorded on the
registry?
– Are cases seen by a
medical practitioner?
– Are all cases likely to be
seen by a medical
practitioner?
Defining the
population
Populations may
be defined by:
–Location
–Age
–Sex
–School
– Study
population
the population of
individuals
selected to
participate in the
study (regardless
of whether they
participate)
– Target
population
Target population
= population to
which the results
will apply
Prevalence
Proportion of
population at a defined
point in time that are
suffering from the disease.
Case becomes an
prevalent case until
recovery, death or it leaves
the population
Calculated as the:
P = Number of individuals with attribute at a specified point in time x 10n
Total population at the given point in time
Incidence rate
Where
I = number of cases
PT = Person time a risk
Person time at risk
An individual contributes time at
risk until
– Loss to follow-up
– Death from another cause
– Change in risk status (e.g.
hysterectomy eliminates risk of
uterine cancer
What is person time at risk?
Person-time at risk is the
denominator for rates of disease
1000 person-years at risk =
PT
IIR
– 1000 people followed for 1 year
– 500 people followed for 2 years
– 10,000 people followed for 1/10
of a year
– 100 people followed for 10
years
Calculating
person time at risk
Exact method
– Sum up the total person time
at risk
Cumulative incidence
Where:
– I = Number of cases during
a time period
– N0 = Disease free people at
time zero
Probability that an individual will
become diseased over specified
period
0N
ICI
Case fatality rate:
The proportion of individuals contracting a
disease that die of that
disease.
ROUTINE INFORMATION
MORTALITY (DEATH) DATA: (Coded according to the ICD)
CRUDE MORTALITY RATE (CMR)
Total # people dying CMR = ____________
Total # people
*AGE-SPECIFIC RATES *SEX-SPECIFIC RATES *CAUSE-SPECIFIC RATES *AGE-STANDARDIZED RATES (Also called age-adjusted rates)
LIFE EXPECTANCY
Average number of years of life remaining at
specified ages if current mortality trends
continue
Standardization:
The process by which you derive a
summary figure to compare health
outcomes of groups
The process can be used for mortality,
natality, or morbidity data
Infectious disease
– diseases involving an agent
(bacterial, viral, parasitic) which is
transmitted from infected to non-
infected hosts
Infectious diseases
– bacterial
• salmonellosis, campylobacteriosis
– viral
• polio, HIV, influenza
– parasitic
• malaria, lymphatic filariasis
Non-infectious diseases
– cancers, heart disease, diabetes
mellitus
Unlike non-infectious diseases, the
occurrence of infectious disease events
in a given host depends on
– occurrence of the disease in other
members of the host population
– length of time that infected hosts
remain infectious
Infectious disease epidemiology
– centres around understanding the
relationship between the host and
infectious agent (or parasite) and the
transmission of the infectious agent
between hosts
Components of the infectious process
1. Agent
2. Reservoir
3. Portals of entry and exit
4. Transmission
5. Immunity
Agent
1. infectious = biologic
organism living and
replicating within a host
2. infectious disease =
infection + illness
3. contamination = agent
on exterior surface near
host
Reservoirs the whole species or other
environment where the agent
multiplies
without the reservoir, the agent
can’t perpetuate
types of reservoirs
symptomatic cases
carriers
animals
inanimate objects
Source of infection
Any living organism in which
infectious agent lives, multiplies &
from which the host acquires the
agent.
Transmission
– contact
– direct (host host)
– indirect (host exudate or
secretion host)
– droplet (airborne short distance)
– nuclei (airborne suspended)
– intermediary
– vector (animate)
– vehicles (inanimate)
Terminology used to describe the various
phases an individual goes through during
the time course of an infectious disease
process …
• Latent period
– the time interval from infection to
development of infectiousness
• Incubation period
– the time interval from infection to
development of signs of disease
– not always a fixed period of time,
shows some variation due to
• route of incubation
• virulence of agent
• host factors (age, immune status)
– follows a log normal distribution
• Infectivity
– the ability of an agent to cause
infection in a susceptible host
– in theory, depends on the minimum
number of infectious particles
required to establish infection
– in diseases spread from person to
person, the proportion of susceptible
individuals who develop infection
after exposure
– the secondary attack rate is a
measure of infectivity
• Pathogenicity
the ability of the infectious agent to
induce disease
agents with high pathogenicity
• viruses causing rabies, smallpox,
measles, chicken pox
agents with low pathogenicity
• polio virus, arboviruses
(mosquito borne)
•
• Virulence
describes the severity of disease,
after infection has occurred
measured using the case fatality rate
• Carrier = contagious person without
discernable signs of disease
types of carriers
inapparent throughout (e.g.
polio)
incubatory (e.g. Hepatitis B,
HIV)
convalescent (e.g.
Salmonella typhi)
The carrier state
– asymptomatic carriers of infection
– play an important role in
spreading disease
– ‘Typhoid Mary’
• Irish cook in New York in the
early 1900s
• worked as a cook in many
homes where residents
developed typhoid fever after
she was hired
• 53 cases of typhoid fever traced
to her
Infectious period
– the time during which time the
host can infect another susceptible
host
–
Non-infectious period
– the time interval between infection
to development of clinical disease
• Terms used to describe the temporal
pattern of disease in a population:
– endemic
disease occurs at expected frequency
– epidemic
disease occurs at greater than expected
frequency
– pandemic
huge epidemic (international)
– sporadic
single case or a cluster of cases
• Endemic
– disease occurs at expected
frequency
– disease present in population or
region at all times
– level of disease usually low and
predictable
• Epidemic
– disease occurs at greater than
expected frequency
– incidence exceeds expected
– usually infectious disease or
poisoning
– may be point source or
propagated
• Sporadic
– single case or cluster of cases
– infrequent disease occurrence
– irregular and unpredictable
– examples:
• Legionnaire’s disease
• food poisoning
• Seasonal variation
– vector-transmitted diseases
(malaria, dengue fever, St Louis
encephalitis) depend on exposure
to infected mosquito vectors
– disease transmission only occurs
during the warmer months of the
year
• Annual variation
– many infectious diseases exhibit
marked and repetitive cyclical
trends
– due to infection exhausting the
susceptible population and then
infecting the birth cohort
replenishing it
– examples
• measles, pertussis, polio
• Temporal patterns of onset can
provide insight into the nature of the
disease we’re dealing with (even
when we don’t know the cause)
Type of epidemics
1.Common source
2.Propagated
1. Common source epidemics
– subjects are exposed to a common
noxious influence
– common point source epidemics
• group is exposed over a relatively
short period then disease cases
will emerge over one incubation
period
• curve rises rapidly and contains a
definite peak at the top, followed
by a gradual decline
– common continuous source epidemics
• group is exposed continuously
and cases emerge over more than
one incubation period curve rises
rapidly, no definite peak
2. Propagated epidemics
– occur when the agent is transmitted
through the population from host to
host (typically infectious conditions)
– propagated epidemics, in theory,
show a series of progressively
taller peaks one incubation period
apart
Nature of epidemic depends on
• characteristics of an agent
(virulence) and host
(susceptibility)
• contact rate
• population density
CHAIN OF INFECTION
AGENT
↓
TRANSMISSION
↓
HOST
FACTORS REQUIRED TO PRODUCE INFECTIOUS
DISEASE
ETIOLOGIC AGENT
RESERVOIR
PORTAL OF EXIT
TRANSMISSION
PORTAL OF ENTRY
SUCCEPTIBLE HOST
Methods of transmission of an infectious agent
Direct transmission
Touching
Kissing
Sexual intercourse
Other contact (e.g. childbirth, medical
procedures, injection of drugs, breast-feeding)
Airborne, short-distance (via droplets,
coughing, sneezing) Transfusion (blood)
Transplacental
Indirect transmission
Vehicle-borne (contaminated food, water,
towels, farm tools, etc.)
Vector-borne (insects, animals)
Airborne, long-distance (dust, droplets)
Parenteral
(injections with contaminated syringes)
The routes of infection vary for different viruses and bacteria. Generally, there are one or
two main methods of transmission; no infectious agent can transmit via all the methods in
the table. A detailed knowledge of these transmission routes is required for effective
primary prevention.
Investigation of an epidemic
1. Verification of diagnosis
2. Confirmation of the existence of an epidemic
3. Identification of cases and their characteristics
4. Definition and investigation of the population at risk
5. Formulation of a hypothesis as to the source and spread of the epidemic
•
Types of epidemiological study
Type of study Alternative name Unit of study
Observational studies
Descriptive studies
Analytical studies
Ecological Correlational Populations Cross-sectional Prevalence Individuals Case-control Case-reference Individuals Cohort Follow-up Individuals
Experimental studies Intervention studies Randomized controlled trials Clinical trials Patients Field trials Healthy people Community trials Community intervention Communities
studies
This classification of study types includes the most commonly used terms for different study
designs. Unfortunately, not all epidemiologicaj study reports use the terms in the same way,
and in addition, terms other than those listed here may occasionally appear in the literature.
The terms in the left column are those recommended for use in WHO-sponsored research
and teaching programmes.
Design of a case-control study
Start with: Exposed ‹——————— ,
___ cases ________(people with
Not exposed‹—————— disease) POPULATION Exposed‹———————
controls ———————— (people without
Not exposed‹—————— disease)
TIME
————————›
direction of inquiry
<—————————
In a case-control study the starting point is cases with the disease in a population. The previous
exposure of the cases is analysed and a representative sample of controls (without the disease
being studied) selected from the same population. The direction of enquiry is always backwards in
time, but the actual data collection can be carried out in either a retrospective manner (backwards
in time from the time the study started) or a prospective manner (forwards in time from the start of
the study).
Design of a cohort study
direction of inquiry
TIME ———————————› ———————————›
———→Exposed —→ disease —→ no disease
People without
the disease ————→Not —→ disease
exposed —→ no disease
Population———→
In a cohort study a group of people free of the disease which is being investigated is selected.
Exposure to the factor which is being studied is measured during "follow up", as is the subsequent
development of the disease. The direction of enquiry is always in the same direction as time, but data
can be recorded both retrospectively and prospectively, as for case-control studies.
———————↓
Participants ↓ Randomization ————————↓ Control ↓ Treatment
Design of a randomized controlled trial
Study population
↓
↓——————Selection by defined criteria ——————↓
Non
participants
Nonparticiparits (do
not meet selection
criteria)
Potential
participants
↓
Invitation to
participate
↓
This scheme does not indicate that examination of the impacts of interventions on the health of
the treatment and control groups will be similar to the study of disease or absence of disease in
the cohort design. The random allocation of participants to either a group receiving some
treatment or subject to an intervention, or to a group not subject to any intervention (the control),
provides a very powerful scientific basis for drawing conclusions about causation of the health
impact of the interventions. In the ideal cohort study the occurrence of exposure is a random
event, as in the randomized controlled trial.
40
Applications of different observational study designs
Ecological Cross- Case- Cohort sectional control Investigation of rare + + + + — + + + + + — Disease Investigation of rare + + — — + + + + + cause Testing multiple effects + + + — + + + + + of cause Study of multiple + + + + + + + + + + + exposures and determinants Measurements of time + + — +b +++++ Relationship Direct measurement of — — +c +++++ Incidence Investigation of long — — + + + — latent periods
This table makes it possible to assess the most appropriate study
design for different situations. For example, if a rare disease is being
studied, an ecological or case-control design would be the most
suitable. If the cause is rare, a cohort design would be more
appropriate. The indications of applicability of different designs
should be used only for initial guidance. The final choice of design will
depend on availability of crucial data and other operational issues,
and deviation from the table's proposals may be required.
a Key: + ... + + + + + indicates the degree of suitability - not suitable
b If prospective. c If population-based.