.
Debre Markos University
College of Medicine and Health Science
Department of Public Health
Epidemiology for BSc Environmental
Health 2nd year students
By
Mengistie K (BSc, MPH Field Epidemiology)
March, 2023
Debre Markos, Ethiopia

Course syllabus
 Year: II
 Semester: II
 Course Tile: Epidemiology
 Module category: Core
 Module code: Enph-M2091
 Module number:9
 Course Code: PuHe2093
 Course Credit: 5ECTS(3 cr hr)=6o hrs
 Course Delivery Duration: 14/07/2015 -01/08/2015

Course objectives
• At the end of the course the student will be able to:
 Define epidemiology and discuss its importance to health science student
s in general and to environmental health students in particular

 Understand and make use of the principles of Epidemiology

 Describe concepts of disease causation

 Calculate the measures of disease and death

 Differentiate types of study design

 Investigate and control outbreaks and epidemics

 Describe the purpose and types of surveillance

 Identify the factors that affect validity of studies

Introduction to Epidemiology
Learning Objectives:
At the end of this unit the student is expected to:
• Define Epidemiology
• Identify the main issues in the definition
• Discuss the uses of Epidemiology

Definition
• Epidemiology is the study of the frequency,
distribution and determinants of diseases and
other health related conditions in human
populations, and the application of this study to the
promotion of health, and to the prevention and
control of health problems.

Major components of the definition
1. Population. The main focus of epidemiology is on
the effect of disease on the population rather
than individuals. For example malaria affects
many people in Ethiopia but lung cancer is rare. If
an individual develops lung cancer, it is more
likely that he/she will die. Even though lung
cancer is more killer, epidemiology gives more
emphasis to malaria since it affects many people.
2. Frequency. This shows that epidemiology is
mainly a quantitative science. Epidemiology is
concerned with the frequency (occurrence) of
diseases and other health related conditions.
Frequency of diseases is measured by morbidity
and mortality rates.

Major components of the definition…
3. Health related conditions. Epidemiology is
concerned not only with disease but also with other
health related conditions because every thing around
us and what we do also affects our health. Health
related conditions are conditions which directly or
indirectly affect or influence health. These may be
injuries, births, health related behaviors like smoking,
unemployment, poverty etc.
4. Distribution. refers to the geographical
distribution of diseases, the distribution in time, and
distribution by type of persons affected

Major components of the definition…
5. Determinants. are factors which determine
whether or not a person will get a disease.
6. Application of the studies to the promotion of
health and to the prevention and control of health
problems.
The whole aim in studying the frequency,
distribution, and determinants of disease is to
identify effective disease prevention and control
strategies

History of Epidemiology
Although epidemiological thinking has been traced to
the time of Hippocrates, the discipline did not flourish as
an independent discipline until the 20th century.
Some key dates and contributions to the
development of epidemiologic thinking and methods
include:
460 B.C – Hippocrates, the father of modern medicine.
For the first time in the fifth century B.C.
he suggested that the development of
human disease might be related to the external as well
as personal environmental of an individual
9
introduction to Epidemiology

1854 - John Snow demonstrated that the risk of mortality
due to cholera was related to the drinking water provided
by a particular supplier in London.
 He used a "natural experiment" to test his
hypothesis.
In another study conducted by Snow in 1854, he linked an
epidemic of cholera to a specific pump, the "Broad Street
Pump".
History of Epidemiology
10
introduction to Epidemiology

Basic Epidemiologic Assumptions
Human disease does not occur at random: there are
patterns of occurrence in which some behavioral and
environmental factors (exposures) increase the risk of
acquiring/developing a particular disease among group
of individuals.
Human disease has causal and preventive factors that
can be identified through systematic investigation of
populations or group of individuals within a population
in different places or at different times.
Thus, identifying these factors creates
opportunity for prevention and control of diseases in
human population either by eliminating the cause or
introducing appropriate treatment.
11
introduction to Epidemiology

Scope of epidemiology
Its scope at the beginning was limited to understanding
epidemics.
 Now it is the basis of advancing our understanding of all kinds
of diseases whether they belong to communicable, non
communicable or injury category.
It is used in laboratory sciences, clinical medicine and public
health.
Its scope in public health ranges from routine surveillance to
research strategies for the testing of hypotheses about causes,
measurement of health and disease risks and evaluations of
preventive, diagnostic and therapeutic programme and
technologies
12
introduction to Epidemiology

Scope of epidemiology…
Epidemiology is also a collection of applied disciplines, i.e.,
every disease entity has its own epidemiology (infectious,
cardio-vascular,
cancer, etc.).
Other studies focus on health risks (occupation, smoking,
diet, social conditions, etc.).
More recently, epidemiologic methods have been applied
to chronic diseases, injuries, birth defects, maternal and
child health, occupational health, and environmental
health.
Now, even health behaviors, such as care-seeking, safety
practices, violence, and hygienic practices are valid
subjects for epidemiologic investigation.
13
introduction to Epidemiology

Uses of Epidemiology
 to determine, describe, and report on the natural
course of disease, disability, injury, and death
 to aid in the planning and development of health
services and programs
 to provide administrative and planning data
 to study the cause (or etiology) of disease(s), or
conditions, disorders, disabilities, etc.

Uses of Epidemiology…
 to determine the primary agent responsible or
ascertain causative factors
 to determine the characteristics of the agent or
causative factors to determine the mode of
transmission
 to determine contributing factors
 to identify and determine geographic patterns

Two Broad Types of Epidemiology:
1. Descriptive Epidemiology - Defines the amount and
distribution of health problems in relation to person,
place and time.
It answers the questions who, where and when.
2. Analytic Epidemiology – involves explicit
comparison of groups of individuals to identify
determinants of health and diseases.
 It answers the questions why and how
Descriptive Epidemiology is antecedent to analytical
epidemiology

Con…
Three essentials characteristics (Variables) of
disease that we look for in descriptive studies are
 Person
Place
Time

Person
age, gender, ethnic group
genetic predisposition
concurrent disease
diet, physical activity, smoking
risk taking behavior
SES, education, occupation

Geographic Place
 presence of agents or vectors
Climate
Geology
population density
economic development
 nutritional practices
medical practices

Time
calendar time
 time since an event
physiologic cycles
age (time since birth)
Seasonality
temporal trends

Communicable disease epidemiology
Definition
It is the study of circumstances under which infection
disease occur in a population; including factors that
influence their frequency, spread and distribution.
21

Cont…
• Occur in epidemic forms, and can be sudden public
health problem (Plague, SARS, bird flue, etc)
• The problem is aggravate by:
• Poor socio-economic status
• Poor personal and environmental hygiene
• Inadequate health service coverage, etc.
• Epidemiological transition
• Change in demographic characteristics of
people
• Emergence of antibiotic resistant strains of
microbes
22

Natural history of disease
definition : is the course of a disease in an individual
over time without intervention.
• The process begins with exposure to the causative
agent capable of causing disease. Without medical
intervention, the process ends with recovery,
disability, or death

Stage natural history of a disease
There are four stages in the natural history of a disease.
These are
1. Stage of susceptibility-period of exposure:
• Disease has not yet developed ,but there are factors
that favor occurrence.
Examples:
• A person practicing casual and unprotected sex has a high risk
of getting HIV infection.
• An unvaccinated child is susceptible to measles.
• High cholesterol level increases the risk of coronary heart
disease.
2. Stage of sub clinical disease (pre-symptomatic stage) :
• the disease process has already begun but , the
disease is not manifested .
• The disease can only be detected through special
tests.

3. Stage of clinical disease :
• signs and symptoms of the disease are
manifested in this stage .
4.Stage of disability or death :
• The disease has occurred and left over damage
to the body that limits the activity of the
victim(disability) or has ended with the death of
the victim.
Examples:
• Trachoma may cause blindness.
• Meningitis may also result in death.
N.B. recovery can takes place at any stage in the
course of the disease.

The stages in the natural history of disease
cont..

Course of an infectious disease over time
The different periods that are encountered in the
course of infectious disease :
• Prepatent period
• Incubation period
• Communicable period
• Generation time
• Latent period

Course of an infectious disease cont…
Prepatent period
• This is the time interval between infection (or
biological onset), and
• The point at which the infection can first be
detected
• It measured by the time of first shedding of the
agent by the host
• In some conditions, like the AIDS, it is the so called
"window period"

Course of an infectious disease cont..
Incubation period
• This is the time interval between infection and the
first clinical manifestations of the disease
• Between the biological onset and clinical onset
Communicable period
• The period during which an infected host can
transmit the infection to others which can be
measured by the time interval during which the
agent is shed by the host.

Generation time/period
• The period b/n the onset of infection in a host and
the maximal communicability of that host
• The maximal communicability may be during or after
the incubation period
Latent period
• The time interval between recovery and the
occurrence of a relapse in clinical disease
• E.g. in cases of malaria and epidemic typhus.

Time Course of a Disease in Relation to Its Clinical
Expression and Communicability
31 Generation period

Components of Infectious Disease Process
• Infectious diseases result from the interaction between
the infectious agent, host/reservoir and environment.
Fig 2. Components of infectious diseases
Host
Environment
Agent
32

Models in infectious disease
1. Epidemiologic triangle and triad (balance beam).
Traditional model of infectious disease causation
Agent
Host Environment
Epidemiologic triangle
Host
Agent
Environment
Balance beam
33
N

Sufficient and component causes model
• It is also called multiple causality of diseases
• In this traditional model, each component cause is seen as
necessary and sufficient cause in itself to produce the
effect.
• Necessary cause: A causal factor whose presence is
required for the occurrence of the disease.
• Sufficient cause. A causal factor or collection of factors
whose presence is always followed by the occurrence of
the disease.
34

Levels of Disease Occurrence
Diseases in a community occur
1. At difference levels of existence and
2. Excess of predictable levels (expected)
1. Level of occurrence of disease
Endemic: the usual presence of disease from low to moderate
level
Hypo/Hyper-endemic: a persistently lower or high level of
disease
Sporadic: Normally does not occur, but occasional cases occur
at irregular intervals
35

Cont….
Excess of expected levels
Epidemic/ Outbreak : An excess occurrence of disease
over expected level at certain time.
Pandemic: An epidemic that affects several countries or
continents. (eg HIV/AIDS, Swan flue, etc)
36

Chain of disease transmission
• IT is a series of events , which must occur in order for
disease causing organisms to cause infection .
• There are six successive events implicated in the chain
of disease transmission.
1. The agent
2. Its reservoir(s)
3. Its portal(s) of exit
4. Its mode(s) of transmission
5. Its portal(s) of entry
6. The human host

1.Infection agent
This is an organism capable causing infection or
infectious disease .
metazoan ,protozoa , bacteria , fungus ,Virus.
• A. Infectivity :- ability of an agent to cause infection
to susceptible host .
• Can be measured by infection rate
• The proportion of exposed persons who become infected.
infection rate = total no of infected person x100
total no susceptible people exposed

B. Pathogenicity
• Ability of micro organism to induce disease
• The proportion of infected persons who develop
clinical disease.
Pathogenicity =total no of clinical cases
total no of subclinical cases

C. From disease to disease outcome
 Virulence: the proportion of persons with clinical
disease who become severely ill or die
It is measured by Case-fatality-rate or hospitalization
rate
Case-fatality-rate =
No of death of a specific disease
Total No of cases of that specific disease
X 100
Hospitalization rate =
No of hospitalized persons of a specific disease
Total No of cases of that specific disease
X 100
40

D. Resistance
• ability of the agent to survive adverse
environmental conditions during transmission from
one host to another

Outcomes at Each Stage of Infection
Exposure Infection Disease Disease Outcome
Infectiousness
(Infection rate)
Pathogenesis
(Clinical to sub-clinical
ratio)
Virulence
(Case-fatality rate,
Hospitalization rate)
42

2. Reservoir of infection
• It is an organism or habitat, in which an infectious
agent normally lives, growth and multiplies.
Types of reservoirs:
1. human reservoir
• Agents with a human reservoir include
• measles
• mumps, and
• most respiratory pathogens.
• Human reservoirs may be persons with symptomatic
illness, or carriers.

Reservoir of infection cont…
• A carrier is a person without apparent disease who is
nevertheless capable of transmitting the agent to others.
Carriers may be:
• Asymptomatic carriers (transmitting infection without ever showing
signs of the disease),
• Examples: - measles, chicken pox, mumps, viral hepatitis, AIDS, rabies.
• Incubatory carriers (transmitting infection by shedding the agent
before the onset of clinical manifestations),
• Examples: - typhoid fever, Diphtheria, HBV
• Convalescent carriers (transmitting infection after the time of
recovery from the disease).
• Examples:- polio , amebiasis , VHA, meningococcal
• Chronic carriers shed the agent for a long period of time, or even
indefinitely.
• Examples: HBV and typhoid fever.

Reservoir of infection cont…
2. Animals reservoir :-cause zoonotic diseases
-bovine TB-cow to man
- brucellosis –cows, pigs and goats to man
- anthrax -cattle, sheep, goats, horses to man
- rabies -dogs, foxes and other wild animals to
man
3. Non-living things:- soil , food , water . E.g.
clostridium tetani ,clostridium botulinum etc

3. Portal of exit
This is the site through which the agent escapes from
the reservoir.
• All body secretions and discharges.
• GIT: TF, dysentery, cholera, ascariasis
• Respiratory: TB, Common cold,
• Skin and mucus membranes: syphilis
• Blood and tissues (placenta).

4.Mode of transmission
• This is the mechanism by which an infectious agent is
transferred from a reservoir of infection to anew host .
• There are two major modes:
1. Direct Transmission- immediate transfer of the agent
from a reservoir to a susceptible host by direct
contact or droplet spread.
• Example:
⎯ Touching
⎯ Kissing
-direct projection
⎯ Sexual intercourse
⎯ Blood transfusion
⎯ Trans-placental

Cont…
2. Indirect Transmission- an agent is carried from
reservoir to a susceptible host by suspended air
particles or by animate (vector-mosquitoes, fleas,
ticks...) or inanimate (vehicle-food, water, biologic
products, fomites) intermediaries.
• Example:
• ⎯ Vehicle-born: food, water, towels, ...
• ⎯ Vector-borne: insect animals, ...
• ⎯ Airborne: dust, droplets
• ⎯ Parenteral injections

5.Rout of entry /portal of entry
• This is the site on the susceptible host through
which an infectious agent gets in to the susceptible
host.
GIT:-TF
Respiratory:- TB
skin :- STD, scabies
The manner of entry is one of the factors which
determine whether or not the agent will establish
infection .

6. Susceptible host
Definition:- A person lacking sufficient resistance to
a particular pathogenic agent to prevent disease if
exposed.
Level of susceptibility depends up on
• Age : extreme of age
• Nutritional status
• Stress
• pre- existing medical conditions
• Immune status

Principles of communicable disease control
• Definition :- reduction of incidence and prevalence
of communicable diseases to a level where it can
not be a major public health problem.
There are three main methods of controlling
communicable diseases:
1. Elimination of the Reservoir
a. Man as reservoir: When man is the reservoir,
eradication of an infected host is not a viable
option. Instead, the following options are
considered:

Cont…
Detection and adequate treatment of cases:
arrests the communicability of the disease
Isolation: separation of infected persons for a period
of communicability of the disease.
Quarantine: limitation of the movement of
apparently well person or animal who has been
exposed to the infectious disease for a duration of
the maximum incubation period of the disease.

Cont…
b. Animals as reservoir: Action will be determined by
the usefulness of the animals, how intimately they
are associated to man and the feasibility of
protecting susceptible animals.
For example:
• Plague: The rat is regarded as a pest and the objective
would be to destroy the rat and exclude it from human
habitation.
• Rabies: Pet dogs can be protected by vaccination but
stray dogs are destroyed.

Cont…
• c. Reservoir in non-living things: Possible to limit
man’s exposure to the affected area (e.g. Soil,
water, forest, etc.).

2. Interruption of transmission
• This involves the control of the modes of
transmission from the reservoir to the potential
new host through:
• Improvement of environmental sanitation and
personal hygiene
• Control of vectors
• Disinfections and sterilization

3. Protection of susceptible host:
• The chain of infection may be interrupted if the
agent does not find a susceptible host.
This can be achieved through:
• Immunization: Active or Passive
• Chemo-prophylaxis- (e.g. Malaria, meningococcal
meningitis, etc.)
• Better nutrition
• Personal protection. (e.g. wearing of shoes, use of
mosquito bed net, insect repellents, etc.)

Levels of Disease Prevention
• Disease prevention means to interrupt or slow the
progression of disease
• Epidemiology plays a central role in disease
prevention by identifying those modifiable causes.
• There are three levels of prevention

1. Primary prevention
• The main objectives of primary prevention are promoting
health, preventing exposure and preventing disease
• keeps the disease process from becoming established by
eliminating causes of disease or increasing resistance to
disease
• Has 3 components.
A. Health promotion:- consists of general non-specific
interventions that enhance health and the body's ability to
resist disease
• Improvement of socioeconomic status, provision of
adequate food, housing, clothing, and education are
examples of health promotion

Primary prevention…
B. Prevention of exposure:- is the avoidance of
factors which may cause disease if an individual is
exposed to them.
• Examples can be provision of safe and adequate
water, proper excreta disposal, and vector control

Primary prevention…
C. Prevention of disease:- is the prevention of disease
development after the individual has become exposed to
the disease causing factors
• Immunization is an example of prevention of disease.
• Immunization acts after exposure has taken place
• Immunization does not prevent an infectious organism
from invading the immunized host, but does prevent it
from establishing an infection.
• If we take measles vaccine, it will not prevent the virus
from entering to the body but it prevents the
development of infection/disease

2. Secondary prevention
• its objective is to stop or slow the progression of
disease so as to prevent or limit permanent damage.
• It can be achieved through detecting people who
already have the disease as early as possible and treat
them.
• It is carried out before the person is permanently
damaged.
• Examples:
• Prevention of blindness from Trachoma
• Early detection and treatment of breast cancer to
prevent its progression to the invasive stage, which is the
severe form of the disease

3. Tertiary prevention
• is targeted towards people with permanent
damage or disability.
• It is needed in some diseases because primary and
secondary preventions have failed, and in others
because primary and secondary prevention are not
effective.
• It has two objectives:
1. Treatment to prevent further disability or death
2. To limit the physical, psychological, social, and
financial impact of disability
It can be done through rehabilitation
Retraining of the remaining functions for maximal
effectiveness.

Tertiary prevention…
• Example: When a person becomes blind due to
vitamin A deficiency, tertiary prevention
(rehabilitation) can help the blind or partly blind
person learn to do gainful work and be
economically self supporting.

.
Basic measurement in epidemiology

Epidemiologic Questions (Measurement)
Epidemiology ≈ Measuring ≈ Quantitative Method
3/26/2023 65
Frequency Association/Effect Impact
What proportion of the
population is affected by
the
health related event? (who,
when, where?)
How fast is the exposure or
outcome spreading?
Are there differences in the
outcome between exposed
and not exposed
Are there differences in
exposure between people
with the outcome and those
without the outcome of
interest?
Is the health event a problem
any more?
What is the
impact of
introducing
a preventive
exposure?
What is the
impact of
removing a
harmful
exposure?

Epidemiologic Measures
Epidemiology ≈ Measuring ≈ Quantitative Method
3/26/2023 66
Frequency Association/Effect Impact
Prevalence
( Point, Period)
Incidence
(Cumulative,
Density)
Relative Risk
Odds Ratio
Crude Vs
specific
Crude Vs
Adjusted
Attributable Risk
among exposed
Crude Vs
Adjusted
Crude Vs Adjusted
Relative Risk
Reduction
Attributable
Risk among
the population
Prevented
Fraction

Measures of Frequency
• Counts
• Ratios
• Proportions
• Rates
3/26/2023 67

Count
Common descriptive measure
First step in calculating rates
 Essential for service delivery, planning
3/26/2023 68

Targets and Number of < 5 years children who have been
measured for their weight in Oromia Zone, Amhara
Region, Ethiopia from 2013 to 2016
Years
Targets for weight
measurement
Number of weights
measured
2013 39905 41449
2014 70402 98216
2015 71650 157865
2016 74256 181619
3/26/2023 69

Ratio
 One number divided by another number
 No specific relationship necessary between the
numerator and denominator(numerator NOT
necessarily included in the denominator
 Common descriptive measure
 Numerator and denominator can be unrelated
 Either numerator or denominator usually set to 1
3/26/2023 70

Ratio…
• Numerator not included in Denominator
3/26/2023 71

Ratio….
• A city of 4 million people has 400 clinics.
• Calculate the ratio of clinics per person.
• Ratio=400/4000,000
=1 clinic per 10,000
3/26/2023 72

Proportion
• Definition: comparison of a part to a whole
• Numerator must be included in the denominator
• Ranges between 0 and 1 (0–100%)
• Percentage = proportion x 100
3/26/2023 73

Proportion…
• Numerator included in Denominator
3/26/2023 74

Rate
• Numerator – number of EVENTS observed for a
given time Denominator – population in which the
events occur – includes time interval Example: rate
of new acute malnutrition cases in City A
• 200 cases/100,000 population/year
3/26/2023 75

Rate
• Numerator – number of EVENTS observed for a
given time Denominator – population in which the
events occur – includes time interval Example: rate
of new acute malnutrition cases in City A
• 200 cases/100,000 population/year
3/26/2023 76

Risk
Definition: proportion of an initially disease-free
population that develops disease during a specified
(usually limited) period of time
new cases during period
Size of population at start of period
Synonyms:
- "Attack rate"
- Probability of getting disease
- Cumulative incidence
- Incidence proportion
3/26/2023 77

Summary
Ratio
Division of two numbers,
related or unrelated
Proportion
Division of two related numbers;
numerator is a subset of
denominator
Rate
Division of two related numbers;
denominator is the population;
Denominator includes time

Summary…
• All rates are proportions
• All rates are ratios too
• All proportions are ratios
• But all proportions are not rates
• All ratios are not proportions
3/26/2023 79

Common measures of Disease Frequency
(Morbidity)
• Incidence (risk): Measures new cases of a disease
that develop over a period of time
• Prevalence (Burden): Measures existing cases of a
disease at a particular point in time or over a
period of time
3/26/2023 80

Incidence
Incidence
3/26/2023 81
Cumulative Incidence (CI)
or Risk of Disease
Incidence Rate (IR)

Incidence…
Risk (Cumulative Incidence)
• Measures the risk (the likelihood, probability) that
an individual will contract the disease during a
certain time period
• Cumulative incidence relates occurrences of new
cases to the population at the beginning of the
study period
3/26/2023 82

Incidence…
• Incidence rate must take into account
• Number of individuals who become ill in a population
• Time periods experienced by member of the
population during which the events occur
3/26/2023 83

Attack rate
A type of incidence used during disease outbreak
in a narrowly defined population over a short
period of time
Measured during an outbreak
The denominator of the attack rate is the
number of people who are at risk of becoming ill
Example: x = 30 people got sick, out of y = 100
people who attended dinner party
Attack Rate = 30/100 = 0.30 = 30%
3/26/2023 84

Prevalence
The size of cases present in a population
(estimates disease burden)
 Commonly used with chronic disease or those
with long duration
Two types of Prevalence
•Point prevalence
•Period prevalence
3/26/2023 85

3/26/2023 86

Comparing Incidence and Prevalence
Incidence
New cases or events
over period of time
Useful to study factors
causing disease,
Useful in “risk”
estimation
Prevalence
All cases at
point/period of time
Useful for measuring
size of problem
For planning
3/26/2023 87

Increased By
By longer duration of the disease
Prolongation of life of patients
without cure
Increase in new cases (increase
in incidence)
In-migration of cases
Out-migration of healthy people
In-migration of susceptible
people
Improved diagnostic facilities
(better reporting)
Decreased By
Shorter duration of the disease
High case fatality
Decrease in new cases
(decrease in incidence)
In-migration of health people
Out-migration of cases
Out-migration of susceptible
people
Improved cure rate of cases)
Factors influencing Prevalence
3/26/2023 88

Measures of disease frequency Example
Suppose we followed a population of 150
persons for one year, and 25 had a disease of
interest at the start of follow-up and another
15 new cases developed during the year
What is the point prevalence at the start of the
period?
25/150 = 0.17 = 17%
What is the period prevalence for the year?
(25 + 15) / 150 = 0.27 or 27%
What is the cumulative incidence for the one
year period?
15/125 = 0.12 = 12%
3/26/2023 89

Measurements of Mortality
Mortality rates and ratios measure the occurrence
of deaths in a population using different ways.
Rates whose denominators are the total
population are commonly calculated using either
the mid - interval population or the average
population.
This is done because population size fluctuates
over time due to births, deaths and migration.
3/26/2023 90

Crude Death rate (CDR)
= Total no. of deaths reported during a given time interval X 1000
Estimated mid interval population
The Crude Death Rate measures the proportion of the population
dying every year, or the number of deaths in the community, per
1000 population
It reflects the risk of death in that community or country
3/26/2023 91

Age- specific mortality rate
= No. of deaths in a specific age group during a given time X 1000
Estimated mid interval population of specific age group
• One example of age specific mortality rate is Infant Mortality
Rate.
Sex- specific mortality rate
= No. of deaths in a specific sex during a given time X 1000
Estimated mid interval population of same sex
3/26/2023 92

Proportionate mortality ratio
= No. of deaths from a specific cause during a given time x 100
Total no. of deaths from all causes in the same time
The proportionate mortality ratio asks the question:
What proportion of deaths are due to a certain cause?
For example when we say the proportionate mortality ratio for
HIV/AIDS is 30 %, this means out of 100 total (of all) deaths 30
of them died from HIV/AIDS.
3/26/2023 93

Case Fatality Rate (CFR)
• = No. of deaths from a specific disease during a given time x 100
No. of cases of that disease during the same time
• Case fatality rate represents the probability of death among
diagnosed cases or the killing power of a disease.
• Example: In 1996 there were 1000 tuberculosis patients in one
region. Out of the 1000 patients 100 died in the same year.
• Calculate the case fatality rate of tuberculosis.
• CFR = 100 x 100 = 10 % 1000 That means 10% of tuberculosis
patients will die once they develop the disease
3/26/2023 94

Neonatal Mortality Rate
= No. of deaths < 28 days of age reported during a given time X 1000
No. of live births reported during the same time
Example: In 1996 there were a total of 5000 live births in “Zone B”.
Two hundred of them died before 28 days after birth.
Calculate the Neonatal Mortality Rate (NMR). NMR = 200 X 1000 =
40 per 1000 live births 5000
That means out of 1000 live births in 1996, 40 of them died before 28
days after birth.
3/26/2023 95

Infant Mortality Rate (IMR)
= No. of deaths under 1 year of age during a given time X 1000
No. of live births reported during the same time interval
Infant mortality rate reflects the health of the community in
which the child is being brought up.
• Thus, it is high among people who have little health care,
chiefly because infections, such as pneumonia, diarrhea and
malaria, are common among their infants.
• Malnutrition is also one of the killer of infants in developing
countries.
• The infant mortality rate in Ethiopia is one of the highest in
the world (96.8 per 1000 live births).
• That means out of 1000 live births about 97 die before they
celebrate their first birth day.
3/26/2023 96

.
Public Health Surveillance

Learning Objectives
• Define Surveillance
• Describe the types of surveillance
• Discuss the activities of surveillance
• Identify public health important diseases that are
under surveillance in Ethiopia
• Identify Features of a good surveillance system

What is Public Health Surveillance?
• “Ongoing systematic collection, analysis,
interpretation, and dissemination of data regarding
a health related event for use in public health
action to reduce morbidity and mortality and to
improve health.”
• “Watch over careful” observation and timely
intervention.

Problem Response
Surveillance:
What
is the
problem?
Risk Factor
Identification:
What is the
cause?
Intervention
Evaluation:
What
works?
Implementation:
How do you
do it?
Public Health Approach

Types of Surveillance
1. Indicator based surveillance
• Structured data collection through routine
Surveillance
• It includes:
• Integrated diseases surveillance
• Laboratory surveillance
• Nutritional surveillance
2. Event based surveillances
• Media scanning (website and news paper}
• Rumor (patient report, community concern,
clinician concern)
• Sectoral information,

Types of Surveillance…
Population-based
Active
Surveillance
Passive
Sentinel
Active Passive

Types of Surveillance…
Population based
Surveillance
Targets the entire
population
Representative
Based on existing public
health structure
Increase potential for
detection of rare diseases
Relatively poor data
Sentinel Surveillance
Target the selected
population
Lack of representativeness
Selected physicians or
facilities involved
Specified diseases
reported
High quality data collected

Types of Surveillance…
Passive Surveillance
Provider initiated reporting
May not be representative
Less work for health
department, simple
Limited consistency of
reporter, reporting
May not be timely
May fail to identify
outbreaks
Voluntary
Active Surveillance
public health system
initiated reporting
Validate
representativeness
Requires more health
department resources
Assure more complete
reporting
Can be used with specific
investigations
Can be used for brief
periods

Purposes of Public Health Surveillance
Assess public health status
Trigger public health action
 Define public health priorities
Evaluate programs

Uses of Public Health Surveillance
♦ Estimate magnitude of a health condition
♦ Determine geographic distribution of illness
♦ Portray temporal trends (seasonality) of a disease
♦ Detect outbreaks/define a problem
♦ Generate hypotheses, stimulate research
♦ Evaluate prevention and control measures
♦ Monitor changes in infectious agents
♦ Detect changes in health practices
♦ Facilitate planning

Health Related Events Under Surveillance
Diseases, conditions, injuries, deaths
 Risk factors
Host - behavioral risk factors (e.g. smoking, lack of
exercise, poor diet, risky sexual behavior)
 Agent - Antibiotic resistance, antigenic patterns,
virulence
Environment - pollution, insect density
Health care practices
 Blood products, adverse drug effects

Disease under Surveillance in Ethiopia…
Immediately Reportable
1. Acute Flaccid Paralysis
2. Anthrax
3. Avian Human Influenza
4. Cholera
5. Dracunculiasis/Guinea warm
6. Measles
7. Neonatal tetanus
8. Pandemic Influenza A(H1N1)
9. Rabies
10. Small pox
11. SARS
12. Viral Hemorrhagic Fever(VHF)
13. Yellow Fever
14. Maternal death
15. Perinatal death
Weekly Reportable
1. Dysentery
2. Malaria
3. Meningitis
4. Relapsing
5. Typhoid Fever
6. Typhus
7. Severe Acute Malnutrition
8. New HIV infection
9. ARI
10. Viral load

What are the criteria
1. Diseases which have
high epidemic potential
2. Diseases of
international concern
3. Diseases targeted for
eradication or
elimination
4. Disease of public health
importance
5. Diseases which have
effective prevention and
control measures

Activities in Surveillance
• The different activities carried out under surveillance
are:
1. Data collection and recording
2. Data compilation, analysis and interpretation
3. Reporting and notification
4. Dissemination of information

Features of a good surveillance system
• Using a combination of both active and passive
surveillance techniques
• Timely notification
• Timely and comprehensive action taken in response to
notification
• Availability of a strong laboratory service for accurate
diagnoses of cases

Epidemiological study Design
112

Learning Objectives
• To overview different types of study designs
• To identify descriptive Epidemiological study
designs
• To identify Analytical Epidemiological study
designs
• To describe the advantage of observational and
experimental study designs
113

Epidemiological designs
Descriptive studies:
• Case Reports/Case series
• Correlational studies
• Cross sectional studies
Analytic study designs
• Case control, Cohort studies
• Experimental /Interventional studies
114

Descriptive study design
Descriptive epidemiology is a way of organizing data
related to health and health related events by person
(Who), place (Where) and time (When) in a population.
Information organized as such is easy to communicate
and provides information about:
1) the magnitude of the problem,
2) the populations at greatest risk of acquiring a
particular disease, and
3) the possible cause(s) of the disease.
115

Cross-sectional study design
♦Observation of a cross-section of a population at a
single point in time
– Unit of observation and analysis: The individual
♦Usually conducted to collect information about
prevalence
– Also known as “prevalence studies”
♦No independent reference groups
♦Non-directional in time = ‘snapshot
116

Uses of Cross-sectional
♦Estimate prevalence of disease or their risk factors
♦Estimate burden
♦Measure health status in a defined population
♦Plan health care services delivery
♦Set priorities for disease control
♦Generate hypotheses
♦Examine evolving trends
– Before / after surveys
– Iterative/repeated cross-sectional surveys
117

Data collected in cross-sectional study
♦ Disease
♦ Exposure to potential risk factors
♦ Practices
– Dietary intake
– Costs and utilization of health care services
– Healthy / unhealthy behaviors
– Physiologic measurements
118

Cross-sectional study design
♦Descriptive
– Estimate prevalence
♦Analytic
– Compare the prevalence of a disease in various
subgroups, exposed and unexposed
– Compare the prevalence of an exposure in various
subgroups, affected and unaffected
119

Limitations of Cross-sectional study design
• Limited capacity to document causality
• Exposure/outcome measured at the same time
• Prevalence-incidence bias
• Neyman
• Not suitable for the study of rare / short diseases
120

Analytic Epidemiological study designs
The subject of interest is the individual within the
population.
The objective is not to formulate but to test the hypothesis.
To evaluate an association between exposure and disease.
Focuses on the magnitude of the association between the
exposure and the health problem under the study.
121

Case control studies
Definition:
A case-control study is one in which persons with a
condition ("cases") and suitable comparison subjects
("controls") are identified, and then the two groups are
compared with respect to prior exposure.
 – subjects are sampled by their outcome status.
122

Case-Control Studies – Timing
Exposure
?
?
Exposed
Unexposed
Disease
Yes (case)
No (control)
Investigator

Case-Control Studies – Flow Chart
Source
Population
Cases
Controls
Exposed
Unexposed
Exposed
Unexposed

Con…
Information is obtained by simple observation of the
event.
Cases (subjects having a specific disease) and controls
(subjects not having the disease) are compared for their
exposure status.
Assess retrospectively on exposure status
Relatively cheaper, (Time and Cost)
Measure of association is using Odds ratio
125

Case-Control Studies – Steps
1. Identify cases of disease of concern
2. Identify appropriate non-diseased comparison
group (“controls”)
3. Document exposures among cases and
controls
4. Calculate odds ratios
5. Perform statistical tests or calculate confidence
intervals

Sources of Cases
• Hospitals.
• General population:
127

Where to Find Controls
• Population-based
• Hospital- or clinic-based
• Neighbors
• Friends
• Other, such as
• Co-workers
• Classmates
128

Why Have Controls?
• Provide estimate of prevalence of exposure in
population
• “Expected” prevalence of exposure among
cases if no association
129

Advantages of case control study
oUseful for studying several potential exposures.
oEfficient for rare diseases.
oEfficient in resources and time
oRelatively quick, disease and exposure
measurements can be made at the same point in
time.
Disadvantages of case control study
oProne to selection bias and recall bias.
oNot suitable for rare exposures.
oMay be difficult to establish that "cause ”preceded
"effect”
oCannot yield a population-level measure of disease
incidence or prevalence. 130

COHORT STUDIES
Definition: An epidemiologic design that identifies
comparison groups according to their exposure
status.
Or It is the analytical method of epidemiologic study
that compares exposed and non-exposed groups.
131

Definition
• Cohort study is a type of analytical study which is
undertaken to obtain additional evidence to refute or
support existence of association between suspected
cause and diseases.
• Other names of cohort study are Longitudinal study,
incidence study and forward looking study
132

Design of a Cohort Study
133
Individuals
“choose” their
exposure status

Types of Cohort study
1. Prospective - The outcome has not occurred at the
beginning of the study
2. Retrospective - Both exposure and outcome status
have occurred at the beginning of the study
134

Measure of Association for Cohort study
• Risk ratio if cumulative incidence study
• Rate ratio if person-time follow-up study
• Incidence of disease in exposed divided by
incidence of disease in unexposed
135

Advantage
valuable when the exposure is rare
 can examine multiple effects of a single exposure
 can reveal temporal relationship
 allows direct measurement of risk
minimize bias in ascertainment of exposure
136

Limitations
inefficient in evaluation of rare diseases
Expensive
time consuming
loss to follow-up create problem
137

Experimental/Intervention studies
Key Features of Experimental Design
1) Investigator manipulates the condition under study
2) Always prospective
138

Experimental Studies
• Investigator can “control” the exposure
• Similar to laboratory experiments except that 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
139

Cont…
• 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.
140

Classification of Intervention Studies:
Based on population
• Clinical trial - usually performed in clinical setting and
the subjects are patients.
• Field trial- used in testing medicine for preventive
purpose and the subjects are healthy people.
• Community trial - a field trial in which the unit of the
study is group of people/ community.
141

Classification of Intervention
Studies: Based on design
• Uncontrolled trial - no control group. control will be
past experience (history).
• Non-randomized controlled - there is control group
but allocation into either group is not randomized.
• Randomized controlled - there is control group and
allocation into either group is randomized.
142

Classification of Intervention Studies: Based on
Trial Objective
• Phase I - trial on small subjects to test a new drug with
small dosage to determine the toxic effect.
– Scale: 20-80 healthy individuals
• Phase II - trial on small group to determine the
therapeutic effect.
– Scale: 100-200 patients
• Phase III - study on large population to test
effectiveness.
– Randomized Controlled Trial (RCT)
• Phase IV
– Post marketing surveillance
– Long term prospective assessment of effects & side-
effects 143

Experimental studies: Advantages
• The major advantage of experimental studies lie in the
strength of causal inference that can be made.
• it is very difficult to make causal inferences based on
observational studies.
• Experimental studies offer the best design for
controlling confounding variables.
• Randomized Controlled Trials (RCTs)
• Gold standard for epidemiologic research
144

.
Outbreak/Epidemic Investigation

Learning Objectives
• Define common terms in outbreak investigation
• Identify Types of Epidemic
• Describe steps of Epidemic Investigation
• Discuss the purpose of Epidemic management
• Identify different epidemic management methods
146

Levels of disease occurrence
• Diseases occur in a community at different levels at a
particular point in time.
• Some diseases are usually present at a predictable level.
• This is called the expected level/endemic.
• The examples of expected level are endemic and hyper
endemic.
• But sometimes they occur in excess of what is expected.
• Outbreak
• Epidemic, and
• Pandemic
147

Definition of terms related to the level of
disease occurrence
1. Endemic: Presence of a disease at more or less stable
level.
Malaria is endemic in the lowland areas of Ethiopia.
2. Hyper endemic: Persistently high level of disease
occurrence.
3. Sporadic: Occasional or irregular occurrence of a
disease. When diseases occur sporadically they may
occur as epidemic.
4. Cluster: Aggregation of cases in a given area over a
particular period without regard to whether the
number of cases is more than expected
148

5. Epidemic: The occurrence of disease or other health
related condition in excess of the usual frequency in a
given area or among a specific group of people over a
particular period of time
6. Outbreak: Epidemics of shorter duration covering a
more limited area.
7. Pandemic: An epidemic involving several countries or
continents affecting a large number of people.
• For example the worldwide occurrence of HIV/AIDS is
a pandemic
149

Epidemics
Three points to be kept in mind
1. Epidemic refers to
 Acute and chronic infection
 Non infectious diseases
 Other related conditions
2. No minimum number
3. Knowledge of the usual frequency
150

Types of epidemics
• Epidemics (outbreaks) can be classified according to
• the method of spread or propagation,
• nature and length of exposure to the infectious agent, and
duration.
151

1. Common Source Epidemics
• Disease occurs as a result of
• exposure of a group of susceptible persons
to a common source of a pathogen,
• often at the same time or within a brief time
period.
• When the exposure is simultaneous, the resulting
cases develop within one incubation period of the
disease and this is called a point source epidemic.
• E.g. Food borne epidemic
152

153
Number of
cases
Time
Usual rate
Epidemic
Fig. The epidemic curve for point source
epidemic
Characteristics:
•Sharp rise and fall
•Unimodal peak
•Short duration

• If the exposure to a common source continues over
time, it will result in a continuous common source
epidemic.
• E.g. A waterborne outbreak that spreads through a
contaminated community water supply
• The epidemic curve may have a wide peak because of
the range of exposures and the range of incubation
periods.
154

155
Usual rate
Number of
cases
Time
Epidemic
Pattern of a continuous common source epidemic
Flat top

2. Propagated/ Progressive Epidemics
• The infectious agent is transferred from one host to
another.
• It can occur through direct person to person
transmission or it can involve more complex cycles in
which the agent must pass through a vector as in
malaria.
• Propagated spread usually results in an epidemic curve
with a relatively gentle upslope and somewhat steeper
tail.
• An outbreak of malaria is a good example of propagated
epidemic.
• When it is difficult to differentiate the two types of
epidemics by the epidemic curve, spot map (studying
the geographic distribution) can help
156

157
Time
Pattern of a propagated type epidemic
Usual rate
Number of
cases
Epidemic
Characteristics:
•Slow increase
•Several peaks
•Sharp fall

3. Mixed Epidemics
• The epidemic begins with a single, common source of
an infectious agent with subsequent propagated
spread.
• Many food borne pathogens result in mixed
epidemics.
158

Investigation of an Epidemic
• The purpose is
• to determine the specific cause or causes of the
outbreak at the earliest time and
• to take appropriate measure directed at
controlling the epidemic and preventing future
occurrence.
159

Steps in epidemic investigation
1. Verify (confirm) the existence of an epidemic
• This initial determination is often made on the basis
of available data.
• Compare the number of cases with the past levels to
identify whether the present occurrence is in excess
of its usual frequency.
• Instead of comparing absolute numbers it is
advisable to compare rates like incidence rate

Steps…
2. Prepare for fieldwork
 Before leaving for the field you should be well prepared to under take
the investigation.
 Preparations can include:
a. Investigation-
– appropriate scientific knowledge,
• supplies, and equipment to carry out the investigation
b. administration, and
c. consultation.
161

Steps in epidemic investigation….
3. Verify (confirm the diagnosis)
• Always consider whether initial reports are correct
• Carry out clinical and laboratory investigations on
the reported cases.
• For example the already collected blood film
slides can be seen by laboratory experts to
check whether the initial report was correct.
• It is important to investigate the index case (the
first case that comes to the attention of health
authorities) and other early cases.
• The sooner the index case and other early cases
are investigated, the greater the opportunity to
arrest the outbreak at earliest stage possible.
162

Steps in epidemic investigation….
4. Identify and count cases
Remember excess may be due to
• Changes in local reporting producers
• Changes in case definition
• Improvement of diagnostic
• A standard case definition is required to
differentiate cases and non cases
• Confirmed / definite – a case with clinical features
and laboratory investigation
• Probable – a case with typical clinical features without
laboratory confirmation
• Possible a case with fewer of typical clinical features
163

Steps in epidemic investigation….
5. Describe the epidemic with respect to person, place and
time
• Each case must be defined according to standard
epidemiologic parameters:
• the date of onset of the illness,
• the place where the person lives or became ill,
• and the socio-demographic characteristics (age, sex, education
level, occupation).
• The tools to be used when characterizing the epidemic are
• Epidemic curve,
• spot map and
• attack rates.
164

Steps in epidemic investigation….
• Epidemic curve is an important tool for the
investigation of disease outbreaks.
• In epidemic curve the distribution of cases is plotted
over time, usually in the form of histogram, with the
date of onset of cases on the horizontal axis, and the
number of cases corresponding to each date of onset
on the vertical axis.
165

• Spot map is a map of locality where the outbreak has
occurred, on which the location of cases is plotted.
• The spot map is often helpful in detecting the source of
an outbreak
• Mapping disease can be done at kebele, woreda,
regional, and national level
• One limitation of spot map is that it does not take into
account underlying geographic differences in population
density
• Thus the spot map needs to be supplemented by
calculation of place specific attack rates.
166

6. Formulate hypothesis
• The hypothesis should addressed
• Source of the agent
• Mode of transmission
• Exposure that cause the disease
• All factors that can contribute to the occurrence of the
epidemic should be assessed.
• The epidemic investigating team should try to answer
questions like:
• Why did this epidemic occur?
• Are there many susceptible individuals?
• Is the temperature favorable for the transmission
of the diseases?
• Are there breeding sites for the breeding of
vectors? Etc
167

Steps in epidemic investigation….
7. Search for additional cases
• Using active and passive case detection
• Investigation of in apparent asymptomatic person
8. Analyze the data
Interpret findings
9. Make a decision on the hypothesis tested
10. Intervention and follow up
• Intervention must be as soon as possible
• Control
• Mode of transmission
• Destroying contaminated food
• Sterilizing
168

Steps in epidemic investigation….
11.Reporting
• Comprehensive report to concerned agencies
• Factor leading to epidemics
• Evaluation of measures
• Recommendation for prevention of similar
episodes
169

Managing Outbreak/Epidemic
• Management of epidemics requires an urgent and
intelligent use of appropriate measures against the
spread of the disease.
• Action to be taken is dependent on the type of the
disease as well as the source of the outbreak.
However, the actions can be generally categorized
as presented below to facilitate easy understanding
of the strategies.

A. Measures Directed Against the Reservoir
Domestic animals as reservoir:
 Immunization. Example – giving anti-rabies vaccine for
dogs
Destruction of infected animals e.g anthrax
Wild animals as reservoir:
post-exposure prophylaxis for human beings- Example:
rabies
Humans as reservoir
Isolation of infected persons.
is not suitable in the control of diseases in which a large
proportion are unapparent infection(without sign and
symptom)

Humans as reservoir…
Treatment to make them noninfectious- e.g.,
tuberculosis.
Quarantine- is the limitation of freedom of
movement of apparently healthy persons or
animals who have been exposed to a case of
infectious disease.

B. Measures that interrupt the transmission of
organisms
i. Purification of water
ii. Pasteurization of milk
iii. Inspection procedures to ensure safe food supply.
iv. Improve housing conditions
Actions to reduce transmission of respiratory
infections
include ventilation of rooms.

C. Measures that reduce host susceptibility
immunization (vaccination). Example vaccination
for meningitis
Chemoprophylaxis: for example, use of chloroquine
to persons traveling to malaria endemic areas.
After the epidemic is controlled, strict follow up
mechanisms should be designed so as to prevent
similar epidemics in the future.

Thank you!!