Introduction to Epidemiology (Year 2 A &B).pptx

SCOPE OF EPIDEMIOLOGY
What is epidemiology?
The study of the distribution and determinants of health
related states and events in populations and the
application of this study to control of health problems
Last JM: A Dictionary of Epidemiology
The study of the distribution of a disease or a
physiological condition in human populations and of the
factors that influence this distribution
Lilienfeld A: in Foundations of Epidemiology

HEALTH RELATED STATES AND EVENTS
Epidemics of communicable diseases – original focus
Current scope:
- endemic communicable diseases
- non-communicable infectious diseases
- chronic diseases, injuries, birth defects, maternal-child health,
occupational health, and environmental health
- health-related behaviors: exercise, seat belt use,
- …..

DISTRIBUTION
Includes frequency and pattern
Frequency: the number of health events (e.g. number of cases of diabetes in a
population), also the relationship of that number to the size of the population
Pattern: the occurrence of health-related events by time, place, and person
Time patterns : annual, seasonal, weekly, daily, hourly, weekday
versus weekend,
Place patterns: geographic variation, urban/rural differences, and
location of work sites or schools
Personal characteristics: demographic factors (age, sex, marital
status, and socioeconomic status), as well as behaviors and environmental
exposures

DETERMINANTS
Causes and other factors that influence the
occurrence of disease and other health-related
events
Illness does not occur randomly in a population, but
happens only when the right accumulation of risk
factors or determinants exists in an individual

EPIDEMIOLOGY AS POPULATION SCIENCE
Epidemiological concept of population important
Populations are made up of distinct individuals
Individuals are generally organized into some kind
of societal grouping
 family
 other larger grouping / sub-cultures
Thinking about population and specifically about
health (and disease) in populations
What extent do individuals and their societies have
effects?

IMPORTANCE OF POPULATION
Define population (of interest) with view to
drawing sample
Examples –men aged 45-64 for CHD events study
 why leave women out? –implies belief in
different risk for women
 studying pregnancy and childbirth –restrict
population to females (of child bearing age)
Looking to generate study research findings
 generalize these to wider population

EPIDEMIOLOGY AND DEMOGRAPHY-
INTERDEPENDENT POPULATION SCIENCE
Taking a sample –where do we start?
Need information about population
Census data –every 5 years in Sierra Leone
 Varies between countries
Planning future health care needs –reliant on
some demographic data
 need to know the size of a problem now,
plus some indication of future trends

EPIDEMIOLOGY AS DISEASE DISTRIBUTION SCIENCE
Disease Distribution refers to
analysis of disease patterns
according to Person, Place
and Time
Triad of epidemiological questions:
Time, Place, Person -exposure
variables
Person:
 Age
 Sex
 Race
 Ethnic Group
 Socioeconomic Status
 Occupation
 Religion and
 Marital Status

EPIDEMIOLOGY AS DISEASE DETERMINANTS
Disease determinants are factors that
bring about a change in a
person’s health.
These are factors that either cause a
healthy individual to become sick
or cause a sick person to recover
It include both causal and preventive
factors
Determinants include: Individual,
Environmental and Societal
IndividualDeterminants
 Genetic make-up
 Gender
 Age
 Immunity
 Diets
 Behaviors
 Existing Diseases

ENVIRONMENTAL/SOCIETAL DETERMINANTS
Presence of infectious agents
Reservoirs in which the organism multiplies
Vectors that transport the agents
Poor and crowded housing conditions
Political instability

Dahlgren & Whitehead (1991) Influences on Health
(Cited Speller, 2007: no pagination)

TWO BROAD TYPES OF EPIDEMIOLOGY
Examining the distribution of a
disease in a population, and
observing the basic features of its
distribution in terms of time, place,
and person
Typical study design:
community health survey
(approximate synonyms - cross-
sectional study, descriptive study)
Testing a specific hypothesis about
the relationship of a disease to a
putative cause, by conducting an
epidemiologic study that relates the
exposure of interest to the disease
of interest
Typical study designs: cohort, case-
control
DESCRIPTIVE EPIDEMIOLOGY ANALYTIC EPIDEMIOLOGY

THE 5W'S OF DESCRIPTIVE EPIDEMIOLOGY
What = health issue of concern
Who = person
Where = place
When = time
Why/how = causes, risk factors, modes of transmission
Analytic epidemiology
Tests hypotheses about:
Why
How
Comparing groups with different rates of disease occurrence and with
differences in demographic characteristics, genetic or immunologic make-up,
behaviors, environmental exposures, and other potential risk factors

AN EPIDEMIOLOGIST
An epidemiologist:
Counts
Divides
Compares
Counting based on case definition i.e. a set of standard
criteria for classifying whether a person has a particular
disease, syndrome, or other health condition
Divide by the number of cases divided by the size of the
population or by the size of the population per unit of
time

RATE OF A DISEASE IN A POPULATION
Attack rate
Morbidity rate
Mortality rate
Incidence
Prevalence
Epidemic
Pandemic
Typesof Disease Frequency
The two basic types are
 Incidence
 Prevalence

INCIDENCE AS DISEASE FREQUENCY
Incidence is the occurrence of new
cases of disease that develop in a
candidate population over a
specific time period.
 New disease events
 Candidate population
 Specific amount of time
• Incidence cases:
Typesof Incidence
Cumulative incidence-is defined as
the proportion of a candidate
population that becomes
diseased over a specific period of
time.
# of new cases of disease/#of
candidate pop over specific time
period
Incidence Rate-is the occurrence of
new cases of disease during a
person time of observation.
# of new cases of disease/Person-
time of observation in candidate
population.

PREVALENCE AS DISEASE FREQUENCY
Prevalence is defined as the proportion of the total population that is
diseased.
 Point Prevalence
 Period Prevalence
• Prevalence case:
Example: Thus a population with a heart disease prevalence of 0.25
indicates What?

RISK AS DISEASE FREQUENCY
Risk, also known as incidence,
cumulative incidence, incidence
proportion, or attack rate (although
not really a rate at all) is a measure of
the probability of an unaffected
individual developing a specified
health outcome over a given period
of time. For a given period of time
(i.e.: 1 month, 5 years, lifetime):
Example: A 5-year risk of 0.10
indicates that an individual at risk
has a 10% chance of developing
the given health outcome over a
5-year period of time
Risk= #of new cases/Total number of
individual at risk
Another way to look at risk: Risk is
the proportion of an at-risk
population that develops a
specific health outcome within a
specified amount of time. The
numerator for risk is incident
cases, and the denominator
includes only those at-risk of
developing the outcome of
interest at the beginning of study
follow-up.

USE OF INCIDENCE AND PREVALENCE
Incidence is most helpful for evaluating the effectiveness of a program that
try to prevent disease from occurring in the first place
Prevalence are useful for estimating the needs of medical facilities for
allocating resources for treating people who already have a disease.

CHARACTERISTICS OF INCIDENCE AND PREVALENCE
Measure Type of # Units Range Types of
cases
Major use
Cumulativ
e
incidence
Proportion None 0 to 1 New Prevention
/Tx of
diseases
Incidence
Rate
Time Rate 1/time 0 to infinity New Prevention
/Tx of
diseases
Prevalenc
e
Proportion None 0 to 1 Existing Resources
Planning

COMPARING MEASURES OF OCCURRENCES

OTHER MEASURES OF DISEASE FREQUENCY
Crude Mortality rate ( or death): Total number of deaths from all causes per
100,000 population per year. Crude means the rate is based on raw
numbers
Cause –specific mortality ( or death) rate: Number of death from specific
causes per 100,000 per year.
Age-Specific mortality (or death): Total umber of deaths from all causes
among individuals in a specific age category per 100,000 population per
year in the age category.
Years of potential life lost: The number of years that an individual was
expected to live beyond his or her death.

Livebirth rate: Total number of Livebirth per 1000 population per year. A
Livebirth is a pregnancy that results in a child who after separation,
breathes or shows any other evidence of life.
Infant mortality rate: Number of deaths of infants less than one year of age
per 1000 Livebirth per year. It can be neonatal deaths occurring during
the first 27 days following births or it post-neonatal deaths occurring
from 28 days through 12 months.
Birth defects (congenital anomaly or malformation) rate: Number of
children born with defects per 10,000 births. The denominator and
numerator often include live and stillbirths

Morbidity rate: Number of existing or new cases of a particular disease or
condition per 100 population. It is a general word that can apply to a
disease, condition or event.
Case fatality rate: Number of deaths per number of cases of a disease.
Survival rate: Number of living cases per number of disease.

MEASURING FREQUENCY
To measure frequency of a disease or event, pay
attention to the numerator (cases) and the
denominator (population at risk)
Key point in making sense of the numbers

MEASURES OF DISEASE FREQUENCY
ratios
proportions
prevalence, incidence
risks, rates, odds
all functions of numerators (cases) and denominators (population at
risk or those at risk but disease free)
• Ratios: the relative magnitudes of two quantities (usually
expressed as a quotient) (A/B)
• Proportions: a ratio that relates the part (the numerator) to the
whole (the denominator) — numerator always part of the
denominator (A/A+B)

PREVALENCE
The prevalence of a disease or
condition in a population is defined as:
The total number of cases (existing
cases) of the disease in the population
at a given time
or
The total number of cases in the
population, divided by the number of
individuals in the population
It is a proportion and is usually
expressed as a percentage
Incidence
The incidence of a disease in a
population is defined as:
The total number of NEW cases of the
disease in a population at risk of the
disease in a defined time period
or
The total number of NEW cases in the
population, divided by the total
number of individuals at risk of the
disease in the population
Again, it is a proportion (RISK) and can
be expressed as a percentage

ODDS OF DISEASE
Provides an alternative way to express a probability (likelihood of an event)
Risk = A / N
Odds = A / (N-A)
Odds = probability / (1 + odds)
Probability = odds / (1 - odds)
Riskand odds
Risk is number of events over number of possible events
Odds is defined as the number of events to the number of non-events
Example: number of cases in exposed group 60, number of cases in
unexposed group 10, odds are six to one (60/10) and risk is 86% (60/70)
The odds has properties that make it very useful in epidemiology

RATE
Rate or velocity at which new cases of a particular disease (or outcome of interest)
occur in a population at risk for the disease
Calculated as: Number of individuals developing disease over specified time period
----------------------------------------
Sum of the “disease-free” time experienced by study participants at risk of disease
Measures of association
Measure the strength of association between the exposure and outcome, e.g. How
likely are cigarette smokers likely to develop lung cancer?
Could be relative (ratios) or absolute (differences)
Risk ratio
Odds ratio

RISK RATIO
Risk ratio = Re/ Ru
= (120/5000)/(50/5000)
= 2.4
Case Control
Exposed a b
Unexposed c d
Risk in exposed (Re) = a/(a+b)
Risk in exposed (Ru)= c/(c+d)
Risk ratio = Re/ Ru
5000
4950
50
No family history
(unexposed)
170
120
Number
developed
disease
1000
9830
Total
5000
4880
Family history
(exposed)
Total
Number
disease-free
5000
4950
50
No family history
(unexposed)
170
120
Number
developed
disease
1000
9830
Total
5000
4880
Family history
(exposed)
Total
Number
disease-free

ODDS RATIO
Odds ratio = Re/ Ru
= (120/4880)/(50/4950)
= 2.4
Case Control
Exposed a b
Unexposed c d
Odds of a case being exposed (Re) = a/b
Odds of a control being exposed (Ru)= c/d
Odds ratio = Re/ Ru = (a/b)/(c/d) = ad/bc
5000
4950
50
No family history
(unexposed)
170
120
Number
developed
disease
1000
9830
Total
5000
4880
Family history
(exposed)
Total
Number
disease-free
5000
4950
50
No family history
(unexposed)
170
120
Number
developed
disease
1000
9830
Total
5000
4880
Family history
(exposed)
Total
Number
disease-free

FEATURES OF ODDS RATIOS
• Often the only measure calculable for case-control studies
• Approximates the risk ratio when the disease is rare
• Based on artificially sampled case and control populations, which may
not reflect the population rate or risk of disease
• If the prevalence of disease is high (high initial risk), the odds ratio can
under- or overestimate the risk ratio
• Often used in genomic epidemiology because the largest set of studies
are case-control designs based on disease definitions and often
sampled from patient populations

INFECTIOUS DISEASE MODEL
Model
Host
Pathogen
Environment
disease

INCIDENCE AND PREVALENCE OF INFECTIOUS DISEASES
Incidence of an infectious disease: number of new cases in a given
time period expressed as percent infected per year (cumulative
incidence) or number per person time of observation (incidence
density).
Prevalence of an infectious disease: number of cases at a given
time expressed as a percent at a given time. Prevalence is a
product of incidence x duration of disease, and is of little
interest if an infectious disease is of short duration (i.e.
measles), but may be of interest if an infectious disease is of
long duration (i.e. chronic hepatitis B).

EPIDEMIC
“The unusual occurrence in a
community of disease, specific
health related behavior, or other
health related events clearly in
excess of expected occurrence”
(epi= upon; demos= people)
Epidemics can occur upon endemic
states too.
Endemic
It refers to the constant presence of a
disease or infectious agent within
a given geographic area or
population group. It is the usual
or expected frequency of disease
within a population.
(En = in; demos = people)

HYPERENDEMIC AND HOLOENDEMIC
The term “hyperendemic” expresses that the
disease is constantly present at high
incidence and/or prevalence rate and affects
all age groups equally.
The term “holoendemic” expresses a high
level of infection beginning early in life and
affecting most of the child population,
leading to a state of equilibrium such that
the adult population shows evidence of the
disease much less commonly than do the
children (e.g. malaria)

PANDEMIC AND EXOTIC
An epidemic usually affecting a large
proportion of the population, occuring
over a wide geographic area such as a
section of a nation, the entire nation, a
continent or the world, e.g. Influenza
pandemics.
Exotic diseases are those which are
imported into a country in which they do
not otherwise occur, as for example,
rabies in the UK.

SPORADIC
The word sporadic means “scattered
about”. The cases occur irregularly,
haphazardly from time to time, and
generally infrequently. The cases are few
and separated widely in time and place
that they show no or little connection
with each other, nor a recognizable
common source of infection e.g. polio,
meningococcal meningitis, tetanus….
However, a sporadic disease could be the
starting point of an epidemic when the
conditions are favorable for its spread.

COMPLIMENTARY TEACHING MATERIALS
PARAMETERS OF AN INFECTION

THE EPIDEMIC CURVE
OF AN EXTENDED
SOURCE EPIDEMIC IN A
LIMITED POPULATION
(SIMPLIFIED).

FIG. 2.3. EPIDEMIC TYPES

INVESTIGATION OF A POINT-SOURCE
EPIDEMIC.

INVESTIGATION OF A PROPAGATED-SOURCE
EPIDEMIC.

BASIC REPRODUCTIVE RATE INCREASING,
I.E. >1. MAXIMAL TRANSMISSION: EVERY
INFECTION PRODUCES A NEW CASE.

FIG. 2E DECREASING, I.E. <1. UNSUSTAINED
TRANSMISSION: EACH TRANSMISSION GIVES RISE
TO LESS THAN ONE NEW CASE AND THE INFECTION
DIES OUT.

FIG. 2.8. THE FOCALITY OF ENDEMICDISEASE. (A) A
UNIVERSALLY HOMOGENEOUS PREVALENCE RATE IS
MEASURED IN AN AREA. (B) ONCE CONTROL
MEASURES HAVE BEEN IMPLEMENTED, FOCI OF
PERSISTENT TRANSMISSION ARE REVEALED.

FIG. 2.9. THEORETICAL ENVIRONMENTAL
CONTROL OF SCHISTOSOMIASIS.

48
Prevalence of disease is a measure of burden – how many people have the
illness.
 Population of a place – often a particular geographic area (city, state)
 Defined period of time. Could be a particular point in time (point prevalence) or
during a defined time period, e.g., over a year (period prevalence)
 May be expressed as a percentage or other ratio (per thousand)
Basic formula:
Number of persons with the disease
Number of persons in the population
PREVALENCE: DEFINITION AND CALCULATION

49
Prevalence data are typically available from routine surveillance systems and
surveys collected by health agencies at the local, state or national levels.
 For example the US CDC collects and provides asthma prevalence data
 https://www.cdc.gov/asthma/asthmadata.htm
Example from Shikowski et al. 2014
In the European Community Respiratory Health Survey cohort COPD
prevalence was 3.4%
PREVALENCE: DATA AND EXAMPLES

50
Incidence of disease is the number of new cases of disease
 Defined place – often a particular geographic area (city, state)
 Population at risk
 Defined period of time
 May be expressed as a percentage or other ratio (per thousand)
Basic formula:
Number of new cases of disease in a population over defined time
Number of persons at risk in the population during that time
INCIDENCE: DEFINITION AND CALCULATION

51
Incidence data are typically available
from routine surveillance systems
and surveys collected by health
agencies at the local, state or
national levels.
Incidence data for cancer are gathered
and disseminated by cancer
registries
Example from Shikowski et al. 2014
In the European Community
Respiratory Health Survey COPD
incidence was 3.4%
Measuresof Associationand Risk
 Relative risk (RR)
 Odds ratio (OR)
 Attributable risk (AR)
INCIDENCE: DATA AND EXAMPLES

52
Relative risk is a ratio of risk
comparing two groups on the basis
of their exposure status
Used to determine if a particular
exposure increases or decreases
risk or probability of developing a
disease
 Exposures could be to chemical,
microbial, physical or psychosocial
stressors
Basic formula: Incidence in exposed
group
Incidence in un-exposed
group
Relativerisk:Dataand Examples
Relative risk can be calculated from
cohort study data
 Cohort studies follow groups of people
defined by exposure status over time to
see whether disease develops (or not)
Example from Rajagopalan et al 2018
Short-term increases in exposure to
PM2.5 increase the relative risk of
cardiovascular mortality by 1% to
3%
RELATIVE RISK: DEFINITION AND CALCULATION

53
Odds ratio (or relative odds) is defined as a ratio of the odds of developing
disease in exposed persons to the odds of developing disease in un-
exposed persons
Odds ratios are similar to relative risk
Basic formula=
A/B ÷ C/D
ODDS RATIO: DEFINITION AND CALCULATION
Exposed Developed
Disease
Did not
develop
disease
Yes A B
No C D

54
Odds ratios are calculated from
case-control studies
 A case-control study starts by
identifying those with and
without disease absent other
knowledge of the incidence of
disease in the underlying
population. Then exposures are
assessed in both cases and
controls to examine potential
association with disease.
Example from Lee et al. 2014
All studies in this review that
assessed the association of
carbon monoxide (CO)
exposure and allergic diseases
reported odds ratios greater
than 1, indicating increased risk
with exposure
Attributable risk: Definition
Attributable risk is a measure of
how much of the disease risk is
due to a certain exposure, after
accounting for the background
risk of disease (in unexposed
people).
Attributable risk is determined by
subtracting the risk of disease
in the unexposed group from
risk in the exposed group
ODDS RATIO: DATA AND EXAMPLES

55
ATTRIBUTABLE RISK: CALCULATION
Attributable
risk
measures
In Exposed Group In Total Population
Incidence
attributable
to exposure
[Incidence in
exposed group] -
[Incidence in
unexposed group]
[Incidence in total
population] - [Incidence
in unexposed group]
Proportion of
incidence
attributable
to exposure
[Incidence in
exposed group] -
[Incidence in
unexposed group]
÷
[Incidence in
[Incidence in total
population] - [Incidence
in unexposed group]
÷
[Incidence in total
population]

56
Attributable risk data are developed from a combination of health risk research
and population health statistics.
Example:
“Air pollution, both outdoor and household burning of solid fuels, was the ﬁfth
leading risk factor for mortality following diet, high blood pressure, tobacco
and high fasting blood glucose, contributing to 4.9 million deaths
worldwide, or 8.7% of global mortality in 2017.” (Boogaard et al. 2019, pg.
418)
ATTRIBUTABLE RISK: DATA AND EXAMPLES

BREAK THE CYCLE THEN CONTROL THE INFECTION
CHAIN OF INFECTION
This refers to a logical sequence of factors or
links of a chain that are essential to the
development of the infectious agent and
propagation of disease. The six factors to
consider in disease transmission
 Infectious agent (etiology or causative
agent)
 Reservoir
 Portal of exit
 Mode of transmission
 Portal of entry
 Susceptible host
Infectious Agents
 An organism that is capable of producing
infection or infectious disease.
 On the basis of their size, etiological agents
are generally classified into:
 Metazoa (multicellular organisms e.g.
Helminths).
 Protozoa (Unicellular organisms e.g.
Ameobae)
 Bacteria (e.g. Treponema pallidum,
Mycobacterium tuberculosis)
 Fungus (e.g. Candida albicans)
 Virus (e.g. Chickenpox, polio)

RESERVOIR OF INFECTION
Any person, animal, arthropod, plant,
soil or substance (or combination of
these) in which an infectious agent
normally lives and multiplies.
on which it depends primarily for survival
and where it reproduces itself in such a
manner that it can be transmitted to a
susceptible host.
Typesof Reservoir
 Man: There are a number of important
pathogens that are specifically adapted
to man, such as: measles, smallpox,
typhoid, meningococcal meningitis,
gonorrhea and syphilis.
 The cycle of transmission is from
human to human
 Animals: Some infective agents that
affect man have their reservoir in
animals.
 The term “zoonosis” is applied to
disease transmission from animals to
man under natural conditions
 Bovine tuberculosis - cow to man
 ƒ
Brucellosis - Cows, pigs and goats to
man
 Non-living things: Many of the agents
are living in soil and fully adapted to
live freely in nature.
 Tetani etiologic agent of Tetanus
 Clostridium welchi etiologic agent of
gas gangrene

PORTAL OF EXIT
This is the site through which the
agent escapes from the reservoir.
Examples include:
 GIT: typhoid fever, bacillary
dysentery, amoebic dysentery,
cholera etc.
 Respiratory: tuberculosis,
common cold, etc. ƒ
 Skin and mucus membranes:
Syphilis
Modeof transmission(Direct)
The mechanisms by which an
infectious agent is transferred from
one person to another or from a
reservoir to a new host.
Transmission may be direct or
indirect.
Direct Transmission-
Direct Vertical: transplacental
transmission of syphilis, HIV
Direct horizontal Direct touching,
biting, kissing, sexual intercourse,
sneezing, coughing, spitting or
talking; Usually limited to a distance
of about one meter or less.

MODE OF TRANSMISSION (INDIRECT)
Indirect Transmission
Vehicle-borne transmission: Indirect contact through contaminated
inanimate objects:
 Bedding, toys, handkerchiefs, soiled clothes, cooking, eating utensils,
surgical instruments. Contaminated food and water
 Biological products like blood, serum, plasma, IV-fluids
 Any substance serving as intermediate means by which an infectious
agent is transported and introduced into a susceptible host through a
suitable portal of entry.
 The agent may or may not multiply or develop in the vehicle before it is
introduced into man

MODE OF TRANSMISSION(INDIRECT)
 Vector-borne transmission: Occurs when the infectious agent is
conveyed by an arthropod (insect) to a susceptible host.
 Mechanical transmission: The arthropod transports the agent by soiling
its feet or proboscis, in which case multiplication of the agent in the
vector does not occur. (common house fly.)
 Biological transmission: This is when the agent multiplies in the
arthropod before it is transmitted, such as the transmission of malaria
by mosquito
 Air-borne transmission: Dissemination of microbial agent by air to a
suitable portal of entry, usually the respiratory tract
 Dust: small infectious particles of widely varying size that may arise
from soil and be re-suspended by air currents.
 Droplet nuclei : Small residues resulting from evaporation of fluid.They
usually remain suspended in the air for long periods of time

PORTAL OF ENTRY
 The site in which the infectious
agent enters to the susceptible
host.
 Mucus membrane
 Skin
 Respiratory tract
 GIT
 Blood
Susceptible Host
 Susceptible host (host factors): A
person or animal lacking
sufficient resistance to a
particular pathogenic agent to
prevent disease if or when
exposed.
 Occurrence of infection and its
outcome are in part determined
by host factors.
 The term “immunity” is used to
describe the ability of the host to
resist infection.

INFECTION RESISTANCE
Resistance to infection is determined by non-specific and specific factors:
Non-specific factors
 Skin and mucus membrane
 Mucus, tears, gastric secretion
 Reflex responses such as coughing and sneezing.
Specific factors
 Genetic-hemoglobin resistant to Plasmodium
 Naturally acquired or artificially induced immunity.
 Active immunity- acquired following actual infection or immunization.
 Passive immunity- pre-formed antibodies given to the host

TYPES OF CARRIER
A carrier is an infected person or animal who does not have apparent
clinical disease but is a potential source of infection to others.
Healthy or asymptomatic carriers: These are persons whose infection
remains unapparent. For example, in poliovirus and hepatitis virus
infections, there is a high carrier rate.
Incubatory carriers: These are individuals or persons who excrete the
pathogen during the incubation period- before the onset of symptoms
examples measles, mumps, chickenpox and hepatitis.
Convalescent Carriers: These are those who continue to harbor the
infective agent after recovering from the illness. E.g. Diphtheria, Hepatitis
B virus.
Chronic Carriers: The carrier state persists for a long period of time. E.g.
Typhoid fever, Hepatitis B virus infection

TIME COURSE OF INFECTIOUS DISEASES
Incubation period: It is the interval of time between infection of the host
and the first appearance of symptoms and signs of the disease.
Prodormal period: It is the interval between the onset of symptoms of an
infectious disease and the appearance of characteristic manifestations (In
a measles patient, fever and coryza occur in the first three days and
Koplick spots in the buccal mucosa)
Period of communicability: The period during which that particular
communicable disease (infectious agent) is transmitted from the infected
person to the susceptible host

WHAT DESIGNS EXIST TO
IDENTIFY AND INVESTIGATE
FACTORS IN DISEASE?

Case report
Case series
Descriptive
Epidemiology
Descriptive
RCT
Before-After
study
Cross-sectional
study
Case-Crossover
study
Case-Control
study
Cohort study
Analytic
Ecologic study

TIMEFRAME OF STUDIES
Prospective Study - looks forward,
looks to the future, examines future
events, follows a condition,
concern or disease into the future
time
Study begins here

TIMEFRAME OF STUDIES
Retrospective Study - “to look
back”, looks back in time to study
events that have already occurred
time
Study begins here

STUDY DESIGN SEQUENCE
Case reports Case series
Descriptive
epidemiology
Analytic
epidemiology
Clinical
trials
Animal
study
Lab
study
Cohort Case-
control
Cross-
sectional
Hypothesis formation
Hypothesis testing

Descriptive Studies
Case-control Studies
Cohort Studies
Develop
hypothesis
Investigate it’s
relationship to
outcomes
Define it’s meaning
with exposures
Clinical trials
Test link
experimentally
Increasing
Knowledge
of
Disease/Exposure

DESCRIPTIVE STUDIES
CASE REPORTS
Detailed presentation of a single case
or handful of cases
Generally report a new or unique
finding
e.g. previous undescribed disease
e.g. unexpected link between diseases
e.g. unexpected new therapeutic effect
e.g. adverse events

CASE SERIES
Experience of a group of patients with a
similar diagnosis
Assesses prevalent disease
Cases may be identified from a single or
multiple sources
Generally report on new/unique
condition
May be only realistic design for rare
disorders

CASE SERIES
Advantages
Useful for hypothesis generation
Informative for very rare disease with few
established risk factors
Characterizes averages for disorder
Disadvantages
Cannot study cause and effect relationships
Cannot assess disease frequency

Case Report
Case Series
Descriptive
Epidemiology Study
One case of unusual
findings
Multiple cases of
findings
Population-based
cases with denominator

ANALYTICAL STUDIES
STUDY DESIGNS -
ANALYTIC EPIDEMIOLOGY
Experimental Studies
 Randomized controlled clinical trials
 Community trials
Observational Studies
 Group data
 Ecologic
 Individual data
 Cross-sectional
 Cohort
 Case-control
 Case-crossover

EXPERIMENTAL STUDIES
treatment and exposures occur in a
“controlled” environment
planned research designs
clinical trials are the most well known
experimental design. Clinical
trials use randomly assigned data.
Community trials use nonrandom
data
ObservationalStudies
non-experimental
observational because there is no
individual intervention
treatment and exposures occur in a
“non-controlled” environment
individuals can be observed
prospectively, retrospectively, or
currently

CROSS-SECTIONAL STUDIES
An “observational” design that surveys exposures and disease status at a single
point in time (a cross-section of the population)
time
Study only exists at this point in time

CROSS-SECTIONAL DESIGN
time
Study only exists at this point in time
Study
population
No Disease
Disease
factor present
factor absent
factor present
factor absent

Often used to study conditions that are relatively frequent with long duration of
expression (nonfatal, chronic conditions)
It measures prevalence, not incidence of disease
Example: community surveys
Not suitable for studying rare or highly fatal diseases or a disease with short
duration of expression

Disadvantages
Weakest observational design,
(it measures prevalence, not incidence of
disease). Prevalent cases are survivors
The temporal sequence of exposure and
effect may be difficult or impossible to
determine
Usually don’t know when disease occurred
Rare events a problem. Quickly emerging
diseases a problem

EPIDEMIOLOGIC STUDY DESIGNS
Case-Control Studies
an “observational” design comparing
exposures in disease cases vs. healthy
controls from same population
exposure data collected retrospectively
most feasible design where disease
outcomes are rare

CASE-CONTROL STUDIES
Cases: Disease
Controls: No disease

Study
population
Cases
(disease)
Controls
(no disease)
factor present
factor absent
factor present
factor absent
present
past
time
Study begins here

CASE-CONTROL STUDY
Strengths
Less expensive and time consuming
Efficient for studying rare diseases
Limitations
Inappropriate when disease outcome for a specific
exposure is not known at start of study
Exposure measurements taken after disease
occurrence
Disease status can influence selection of subjects

HYPOTHESIS TESTING: CASE-CROSSOVER
STUDIES
Study of “triggers” within an individual
”Case" and "control" component, but information of both components will come from
the same individual
”Case component" = hazard period which is the time period right before the disease
or event onset
”Control component" = control period which is a specified time interval other than the
hazard period

EPIDEMIOLOGIC STUDY
DESIGNS
Cohort Studies
an “observational” design comparing
individuals with a known risk factor or
exposure with others without the risk factor
or exposure
looking for a difference in the risk
(incidence) of a disease over time
best observational design
data usually collected prospectively (some
retrospective)

time
Study begins here
Study
population
free of
disease
Factor
present
Factor
absent
disease
no disease
disease
no disease
present
future

PROSPECTIVE COHORT STUDY
Measure exposure
and confounder
variables
Exposed
Non-exposed
Outcome
Outcome
Baseline
time
Study begins here

RETROSPECTIVE COHORT STUDY
Measure exposure
and confounder
variables
Exposed
Non-exposed
Outcome
Outcome
Baseline
time
Study begins here

COHORT STUDY
Strengths
Exposure status determined before disease
detection
Subjects selected before disease detection
Can study several outcomes for each exposure
Limitations
Expensive and time-consuming
Inefficient for rare diseases or diseases with
long latency
Loss to follow-up

investigator can “control” the exposure
akin to laboratory experiments except
living populations are the subjects
generally involves random assignment to
groups
clinical trials are the most well known
experimental design
the ultimate step in testing causal
hypotheses

In an experiment, we are interested in the consequences of some treatment on
some outcome.
The subjects in the study who actually receive the treatment of interest are
called the treatment group.
The subjects in the study who receive no treatment or a different treatment
are called the comparison group.

EPIDEMIOLOGIC STUDY DESIGNS
Randomized Controlled Trials (RCTs)
a design with subjects randomly assigned to
“treatment” and “comparison” groups
provides most convincing evidence of
relationship between exposure and effect
not possible to use RCTs to test effects of
exposures that are expected to be harmful, for
ethical reasons

time
Study begins here (baseline point)
Study
population
Intervention
Control
outcome
no outcome
outcome
no outcome
baseline
future
RANDOMIZATION

RANDOMIZED CONTROLLED TRIALS
Disadvantages
Very expensive
Not appropriate to answer certain
types of questions
it may be unethical, for example,
to assign persons to certain
treatment or comparison groups
ReviewQuestions(Developedby the Supercourse team)
Describe the link between exposure
and disease
Describe study design sequence
Describe strengths and weaknesses of
each design
EpidemiologicStudy Designs
Randomized Controlled Trials (RCTs)
 the “gold standard” of research
designs
 provides most convincing evidence
of relationship between exposure
and effect
 trials of hormone replacement
therapy in menopausal women
found no protection for heart
disease, contradicting findings of
prior observational studies

SURVEILLANCE FOR PRIORITY DISEASES, CONDITIONS AND EVENTS IN HUMAN HEALTH,
ANIMAL HEALTH AND ENVIRONMENT
PUBLICHEALTHSURVEILLANCE
Ongoing, systematic collection, analysis, and interpretation of health-
related data essential to the planning, implementation, and evaluation of
public health practice, closely integrated with the timely dissemination of
these data to those responsible for preventing and controlling disease and
injury (Stephen Thacker, CDC)
“The reason for collecting, analyzing, and disseminating information on a disease
is to control that disease. Collection and analysis should not be allowed to
consume resources if action does not follow” (William Foege et al)
DATA FOR ACTION

USES OF SURVEILLANCE DATA
Utility Examples
1. Immediate
detection of
Epidemics
Newly emerging health
problems
Changes in distribution of at risk
populations
2. Periodic
dissemination for
Estimating the magnitude of a
health problem, including cost
Assessing control activities
Determining risk factors for
disease
Monitoring changes in health
practices

REPORTING 1/4
 Every level of the one health system has
a role in carrying out ongoing
surveillance for priority diseases,
conditions and events.
 If a disease is identified at a local level,
for example, but the information is not
reported to the next level, an
opportunity for timely response is lost.
Reporting 2/4
Gathering data about diseases, conditions
and events in a health facility, district
or other administrative areas helps the
one health teams to use the data for
action and to:
Identify emerging problems and plan
appropriate responses
– Take action in a timely way
– Monitor disease trends in the area
– Evaluate the effectiveness of the
response

REPORTING 3/4
What is reported to each level and how
often is usually guided by national
policy. The policy will specify whether
the data are reported immediately,
weekly, monthly, or quarterly.
How the information is reported depends
on the capacity in your area.
 For example, reporting may be
done by electronic methods such
as email or other electronic
transmission, by regular mail, or
by radiophone or cell phone SMS
text reporting
Reporting 4/4 .
The decision about what, when and where
to report disease information will
depend on specific disease control
priorities and activities in your country
or district.
In addition to priority diseases that are
targets of national policy, districts
should also report any unusual event
that has the potential to affect human
health.

IMMEDIATELY REPORT INFORMATION ABOUT ACUTE
EPIDEMIC-PRONE DISEASES OR EVENTS
Immediate reporting means that information about a disease, condition, or event
is reported to the next level as soon as an epidemic-prone disease is
suspected.
The information that is reported immediately is often referred to as case-based
reporting. This means that specific information about each case is included
in the report.

DISEASES REQUIRING
IMMEDIATE REPORTING
1.Acute haemorrhagic fever
syndrome(Ebola, Lassa,
Marburg, CCHF etc)
2.Acute Jaundice Syndrome
3.Adverse effects following
immunization (AEFI)
4.Animal Bites (Dog and
Snake bite
5. Anthrax
6. Bacterial Meningitis
7. Chikungunya
8. Cholera
9. Dengue fever
10.Diarrhoea with blood
(Shigella)
11.Human Rabies
12.Cholera
12. Maternal deaths
13.Monkey pox
14.Measles
15.Plague
16.SARS
17.Yellow fever
18.Dracunculiasis
19. Plague
20. SARIs**
21. Typhoid fever
22. Yellow fever
23. Zika virus disease
24. Also: A cluster of deaths in the
community (animal or human
deaths)
A cluster of unwell people or
animals with similar symptoms

DISEASE, CONDITION AND EVENTS FOR IDSR
WEEKLY REPORTING (NEW LIST)
1. Acute haemorrhagic fever
syndrome (Ebola Virus disease,
Marburg, Lassa Fever, RVF, Crimean-
Congo)
15. Measles
2. Acute Jaundice Syndrome 16. Monkey Pox
3. Adverse effects following
immunization (AEFI)
17. Neonatal Tetanus
4a. Animal Bites (Dog) 18. Non-Neonatal tetanus
4b. Animal Bite (Snake)
19. Perinatal and Neonatal deaths
5. Anthrax 20. Plague
6. Bacterial Meningitis
21. Poliomyelitis (Acute Flaccid Paralysis)
(AFP)
7. Buruli Ulcer 22. Rabies (Human)
8. Cholera 23. Yellow fever
9. Diarrhoea with blood (Shigella)
24. MDR/XDR Tuberculosis – Lab.
Surveillance
10. Diarrhoea with severe dehydration
less than 5 years of age
25. Severe malnutrition in children under 5
years of age
11. Dracunculiasis (Guinea Worm
disease
26. Severe pneumonia less than 5 years of
age

DISEASE, CONDITION AND
EVENTS FOR IDSR MONTHLY
REPORTING OR QUARTERLY
(NEW LIST)
1. Lymphatic filariasis
10. Injuries (Road traffic
Accidents)
2. Noma 11. Sickle Cell Disease (SCD)
3. Onchocerciasis
12. Soil Transmitted
Helminthiasis (STH)
4. Yaws 13. STIs
5. Asthma 14. Trachoma
6. Diabetes Mellitus 15. Trypanosomiasis
7. Epilepsy 16. Tuberculosis –(quarterly)
8. HIV/AIDS (new cases)
17. Leprosy –(quarterly)
9.Hypertension
Laboratory : Newly diagnosed HIV infection, TB (MDR/XDR)

SURVEILLANCE CAPACITY E.G. PANDEMIC
INFLUENZA
A system that can detect, analyze and report
events (including rumors and other ad hoc
reports) that are a potential risk to public
health
Laboratory capacity to identify influenza
viruses with pandemic potential, or rapid
access to outside laboratory facilities with
this capacity and the ability to ship samples;
Ability to collect and share samples under the
pandemic influenza preparedness (PIP)

SURVEILLANCE CAPACITY E.G. PANDEMIC INFLUENZA
Epidemiological expertise to carry out the
initial investigation of signal events
(including clusters), data analysis and
reporting
National IHR focal point to report PHEIC to
WHO
Outbreak response and pandemic
preparedness plans
Materials and personnel for rapid containment

ENVIRONMENTAL PUBLIC HEALTH
SURVEILLANCE
Surveillance on hazards,
exposures, and health
outcomes.
Challenging due to difficulty
in linking specific
environmental causes to
adverse health outcomes.
Exposure data (e.g., air
sampling) may be difficult
to obtain in developing
countries.
Biologic markers (e.g., blood
lead levels) are useful and
feasible.

SURVEILLANCE LIMITATIONS
Completeness
Unreported
cases
Incomplete
reports
Consistency of
reporting

QUALITY OF THE DATA
Completeness of
case
ascertainment
Assessment of
completeness,
accuracy, and
timeliness of
reports
‘Tip of the iceberg’
Reported
Diagnosis is
made
Individual cases
are seen by
human/animal
healthcare
provider
Sick human or animal

POINTS TO REMEMBER
1. Report priority diseases to the next level
2. Know which diseases and events require immediate
reporting and monthly reporting
3. Be sure to know who to send the report to and the reporting
format
4. Involve the labs and community to foster communication
and develop a clear profile for the disease and target
populations

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