Dr mathias introduction to epidemiology

MPH 5101: Epidemiology
Introduction
BSU-MPH 1
Dr Mathias Tumwebaze
Mathiastumwebaze@gmail.com
07002916618

Session objectives
 Define epidemiology
 Summarize the historical evolution of
epidemiology
 List the key features and uses of descriptive
epidemiology
 List the key features and uses of analytic
epidemiology
 List 3components of the epidemiologic triad
 List and describe primary applications of
epidemiology in public health practice
 List & describe modes of transmission of

Epidemiology
Originates from the Greek words:
EPI DEMOS LOGOS
upon people study
epi (upon, on, befall)+
 demos (people, population, man)+
 logy (study of)
 Literal translation: “That which befalls man” (epidemics)

Epidemiology
 It is the study of the nature, cause, control and
determinants of the frequency and distribution of
disease, disability and death in human populations.
(Timreck, 1994)
 It is the study of the distribution and determinants of
disease frequency in human populations. (MacMahon
and Trichopoulos, 1996)

Epidemiology: Definition
The study of the distribution and
determinants of health-related states or
events in specified populations and the
application of this study to control health
problems. Last, Dictionary of Epidemiology, 1988.
 Epidemiologists study sick and well people to
determine the crucial difference between those
who get disease and those who are spared

Anatomy of the definition
 Study. Epidemiology is a scientific discipline, sometimes
called “the basic science of public health.” It has, at its
foundation, sound methods of scientific inquiry.
 Population of interest are human populations
Distribution.
 Epidemiology is concerned with the frequency and
pattern of health events in a population. Distribution of
health and disease (by person, place and time
variables).
 Determinants. Epidemiology is also used to search for
causes and other factors that influence the occurrence of
health-related events(risk factors, causes)

 Pattern refers to the occurrence of health-
related events by time, place, and personal
characteristics.
• Time characteristics include annual
occurrence, seasonal occurrence, and daily or
even hourly occurrence during an epidemic.
• Place characteristics include geographic
variation, urban-rural differences, and location
of worksites or schools.
• Personal characteristics include demographic
factors such as age, race, sex, marital

 Analytic epidemiology attempts to provide
the Why and How of such events by
comparing groups with different rates of
disease occurrence and with differences in
demographic characteristics
 Health-related states or events. Originally,
epidemiology was concerned with epidemics
of communicable diseases.
 Epidemiology has been extended to endemic
communicable diseases and non
communicable infectious diseases

 . More recently, epidemiologic methods have
been applied to chronic diseases, injuries,
birth defects, maternal-child health,
occupational health, and environmental
health.
 Specified populations
epidemiologists and physicians in clinical
practice are both concerned with disease and
the control of disease in human beings

 Clinicians are concerned with the health of an
individual; epidemiologists are concerned
with the collective health of the people in a
community or other area.
Application.
 Epidemiology is more than “the study of.” As
a discipline within public health, epidemiology
provides data for directing public health
action

History of Epidemiology
 Hippocrates (circa 400 B.C.) attempted to explain
disease occurrence from a natural viewpoint. “On
Airs, Waters, and Places,” he suggested that
environmental and host factors such as behaviors
might influence the development of disease.:
 1662 - John Graunt. Published his landmark
analysis of mortality data in 1662. He was the first
to quantify patterns of birth, death, and disease
occurrence, noting male-female disparities, high
infant mortality, urban-rural differences, and
seasonal variations.- regarded as first
demographer

 1747 - Lind used an "experimental" approach to prove
the cause of scurvy by showing it could be treated
effectively with fresh fruit.
 1839 - William Farr took responsibility for medical statistics in
the Office of the Registrar General for England and Wales. He
extended the epidemiologic analysis of morbidity and mortality
data, looking at effects of marital status, occupation, and
altitude
 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". According to literatures, Snow removed
the handle of that pump and aborted the cholera epidemic.

 John Snow - the father of field epidemiology
 Twenty years before the development of the
microscope, conducted studies on cholera
outbreaks both to discover the cause of
disease and to prevent its recurrence.
 His work illustrates the sequence from
descriptive epidemiology to hypothesis
generation to hypothesis testing (analytic
epidemiology) to application,

John Snow
 His work featured various techniques in
epidemiologic inquiry such as spot map of
cases, tabulations of cases and deaths,
development and testing of hypotheses that
contaminated water may be associated with
cholera outbreaks.
 Snow began his investigation by determining
where in this area persons with cholera lived
and worked
 He marked the location of water pumps on his
spot map, and then looked for a relationship
between the distribution of cholera case
households and the location of pumps.

John snow
 He noticed that more case households
clustered around Pump A, the Broad Street
pump, than around Pump B or C, and he
concluded that the Broad Street pump was
the most likely source of infection
 Snow removed the handle of that pump and
aborted the outbreak

Spot map of deaths from cholera in
Golden Square area, London, 1854

Two Holy Trinities of
Epidemiology
Disease characteristics: agent
host
environment
Health is a state of equilibrium between:
Agent Host

Environment

Two Holy Trinities of
Epidemiology (2)
Disease descriptors: Time
Place
Person
Epidemiologists describe disease/states in
terms of time, place and person

Determinants of Disease
 Individual
 Gender
 Age
 Diet
 Exercise
 Immunity level
 Existing diseases
 Genetic makeup
 Environmental
 Housing conditions
 Work exposures
 Built environment
 Air quality
 Social
 Neighborhood
 Peers
 Family

Objectives of Epidemiology
1. To determine the extent of disease in populations
2. To study the natural history and prognosis of disease
3. Identify the etiology or cause of disease and the risk factors
4. To evaluate both existing and new preventive and therapeutic
measures and modes of health care delivery
5. To provide the foundation for developing public policy

Purpose of Epidemiology
 To provide a basis for developing
disease control and prevention
measures for groups at risk.
 This translates into developing
measures to prevent or control
disease.

Uses of Epidemiology
 To study the cause (or etiology) of
disease(s), or conditions, disorders,
disabilities, etc.
 determine the primary agent responsible
or ascertain causative factors
 determine the characteristics of the agent
or causative factors
 define the mode of transmission
 determine contributing factors
 identify and determine geographic patterns

 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

 Community diagnosis; i.e., what are
the major health problems occurring
in a community
 Establishing the history of a disease
in a population; e.g., identifying the
periodicity of an infectious disease

Uses of Epidemiology (2)
 Describing the natural history of
disease in the individual; e.g., natural
history of HIV infection in the individual
(infection-acute syndrome-asymptomatic
phase-clinical disease-death)
 Describing the clinical picture of
disease; i.e., who gets the disease,
who dies from the disease, and what the
outcome of the disease is

Uses of Epidemiology (3)
 Estimating risk; e.g., what factors
increase the risk of heart disease,
automobile accidents, and violence
 Identifying syndromes and precursors;
e.g., the relationship of high blood
pressure to stroke, kidney disease, and
heart disease
 Evaluating prevention/intervention
programs; e.g., vaccine and clinical trials
 Investigating epidemics/diseases of
unknown etiology

Two Broad Types of Epidemiology
 Descriptive Epidemiology
 Examining the distribution of
disease in a population, and
observing the basic features of its
distribution
 Analytic Epidemiology
Testing a hypothesis about the cause
of disease by studying how
exposures relate to the disease

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
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

Three essential characteristics of
disease that we look for in
descriptive studies are...
Person
Place
Time

Person
 Age, gender, ethnicity
 Genetic predisposition
 Concurrent disease
 Diet, exercise, smoking
 Risk taking behavior
 SES, education,
occupation

Place
 Geographic place
 presence or 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

Example
 You have been asked to investigate
an event in which 2,220 people were
exposed and 1,520 of them died.
 Your role as an epidemiologist is to
ask questions about person, place
and time.

How do we ask questions?
Surveys
-of survivors
-of next-of-kin
-of other related persons

with questions you learn that
...
 Person: Men, women and children
were all exposed and at risk. The
majority of people who died were
wealthy and young men between 18-
50 years (when compared to
survivors).
 Place: All those exposed were within
1 block of one another, the climate
was cold.

Three essential characteristics examined to study the
cause(s) for disease in analytic epidemiology are...
Host
Agent
Environment

Host
EnvironmentAgent
Epidemiologic Homeostasis

The Epidemiologic Triangle
Host Factors
Personal traits
behaviors
genetic
predisposition
immunologic
factors
Agent
Host
Environment
• Influence the chance
for disease or its severity

Agents
Biological
Physical
Chemical
Agent
Host
Environment
• Necessary for
disease to occur

Environment
External conditions
Physical or biologic
or social
Agent
Host
Environment
• Contribute to the
disease process

The Basic Triad Of
Descriptive Epidemiology
THE THREE ESSENTIAL CHARACTERISTICS
OF DISEASE WE LOOK FOR IN DESCRIPTIVE
EPIDEMIOLOGY:
 TIME
 PLACE
 PERSON

Time
 Changing or stable?
 Seasonal variation.
 Clustered (epidemic) or evenly
distributed (endemic)?
 Point source or propagated.

Place
 Geographically restricted or
widespread (pandemic)?
 Relation to water or food supply.
 Multiple clusters or one?

Person
 Age
 Socio-economic status
 Gender
 Ethnicity/Race
 Behavior

Agents
 Nutrients
 Poisons
 Allergens
 Radiation
 Physical trauma
 Microbes
 Psychological experiences

Host Factors
 Genetic endowment
 Immunologic state
 Age
 Personal behavior

Environment
 Crowding
 Atmosphere
 Modes of communication – phenomena
in the environment that bring host and
agent together, such as:
 Vector
 Vehicle
 Reservoir

Epidemics arise when host, agent, and
environmental factors are not in balance
 Due to new agent
 Due to change in existing agent
(infectivity, pathogenicity, virulence)
 Due to change in number of
susceptibles in the population
 Due to environmental changes that
affect transmission of the agent or
growth of the agent

Epidemiologic Activities
…are often framed under the mantle of
descriptive and analytic epidemiology
 Descriptive epidemiology – person, place &
time
 Demographic distribution
 Geographic distribution
 Seasonal patterns etc.
 Frequency of disease patterns
 Useful for:
 Allocating resources
 Planning programs

Epidemiologic Activities
 Analytic epidemiology
 built around the analysis of the
relationship between two items
 Exposures
 Effects (disease)
 looking for determinants or
possible causes of disease
 useful for
 hypothesis testing

Dynamics of
Disease
Transmission

Disease transmission
 Endemic
 The usual existence of a disease in certain areas.
 Malaria in African Countries
 Epidemic
 any unusual occurrence of disease, generally first noticed
by an unexpected number of cases occurring over a
particular amount of time or in a particular place.
 Measles outbreak
 Pandemic
 an epidemic occurring over a very wide area, crossing
international boundaries and usually affecting large
numbers of people. A global epidemic.
 HIV/AIDS

Dynamics of Disease
Transmissible
 Infectious diseases
 Genetic diseases
Non-Transmissible
 Cardiovascular disease
 Cancer
 Diabetes
 Reproductive outcomes

Dynamics of Disease
 Clinical disease
 Characterized by signs and symptoms
 Nonclinical (inapparent disease)
 Preclinical disease
 Disease not yet clinically apparent, but will become so
 Subclinical disease
 Disease not clinically apparent and will not become so
 Persistent (chronic) disease
 Manifestations of disease many years after disease
 Latent disease
 An infection with no active multiplication of the agent
 Carrier status
 Individual harbors organism, but is not infected

Epidemiology Triad
Host
Agent Environment
Vector

Epidemiology Triad
Host
Agent Environment
Vector
A factor whose presence, absence or
relative amount is essential for the
occurrence of disease.

Epidemiology Triad
Host
Agent Environment
Vector
Person or animal that provides a suitable place
for an infectious agent to grow and multiply
under natural conditions. The suitability is
dependent upon the host characteristics.

Epidemiology Triad
Host
Agent Environment
Vector
Surroundings and conditions external to the
person or animal that cause or allow disease
transmission.

Epidemiology Triad
Host
Agent Environment
Vector
Any nonhuman carrier of
disease that transports and
serves the process of
disease transmission. A
vector spreads infection
from an infected human or
animal to other susceptible
humans or animals through
its waste products, bite,
body fluids or indirectly.

Epidemiology Triad
Host Characteristics Agent Environment Vector/Fomite
Age Biologic Temperature Insect
Gender Chemical Humidity Water
Genetic profile Physical Altitude Food
Immune status Nutritional Crowding Door knob
Health status Housing
Smoking status Neighborhood
Alcohol consumption Workplace
Socio-cultural

Epidemiology Triad
Child
Malaria
Parasite
Rainy Season
Mosquito

Methods of Transmission
 Direct
 Touching
 Kissing
 Sexual intercourse
 Childbirth
 Airborne (short
distance)
 Droplets
 Coughing
 Sneezing
 Transfusion
 Transplacental
 Indirect
 Vehicle borne
 Food
 Water
 Vector borne
 Insects
 Animals
 Clothing
 Airborne (long
distance)
 Dust
 Droplets
 Parenteral
 Injection with
contaminated needle

Natural history of a disease
 Natural history of disease refers to the
progress of a disease process in an individual
over time, in the absence of intervention. The
process begins with exposure to or
accumulation of factors capable of causing
disease.
 Without medical intervention, the process
ends with recovery, disability, or death
 For leukemia from exposure to the atomic
bomb blast in Hiroshima, the IP range was 2
to 12 years with a peak at 6 to 7 years

Natural History of The disease

Infectivity
Infectivity =
ILL
Number of Exposed
Ability of an agent to cause infection in a
susceptible host

Pathogenicity
Pathogenicity =
ILL
Number Infected
Ability of a microbial agent to induce
disease
(Illness Rate)

Virulence
Virulence =
Severe/Fatal Cases
Total Cases
The severity of disease after infection
occurs
(Case Fatality Rate)

Biological Characteristics of
Infectious Agents
Incubation
period
Infectivity Pathogenicity Virulence
Smallpox 7-17 days High High High
Ebola 2-21 days High High High
Measles 10 days High High Intermediate
Rubella 12-23 days Intermediate Intermediate Very low
Common Cold 24-72 hours Intermediate High Very low
TB 4-12 weeks Low Low High

Herd Immunity
The resistance of a group to attack by an infectious
disease to which a large proportion of the
members of the group are immune, thus lessening
the likelihood of a patient with disease will come in
contact with a susceptible person.

Herd Immunity
Infected
Susceptible
Immune

Herd Immunity
Infected
Susceptible
Immune
New infection

Determinants of Herd Immunity
 Disease agent must be restricted to a single host
species within which transmission occurs
 Transmission must be relatively direct
 Infections must produce solid immunity
 Probability of an infected person encountering
every other person in the population is the same
(random mixing)

Herd Immunity
Herd immunity level is determined by the basic
reproduction rate of the epidemic
Reproductive Rate (R0)
• Average number of new persons directly infected by 1
infectious case during the infectious period in a
population of totally susceptible persons.
R0 > 1 – Epidemic
R0 < 1 – Extinction
R0 = 0 – Endemic

1. Define the epidemic
 Define cases (numerator)
 What is the population at risk (denominator)
 Calculate the attack rates
Investigating an acute outbreak

Attack Rate
 Overall attack rate
 Food Specific attack rate
Attack Rate =
Number of people at risk in whom a
certain illness develops
Total number at risk
Food Specific Attack
Rate
=
Number of people who ate a specific food
and became ill
Total number of people who ate
that food

Salmonella Food-to-Night
Outbreak (hypothetical data)
Among 85 people who ate at a particular
restaurant, 60 became ill with the salmonella
Food-to-Night strain
=
60
85
70.6%

Salmonella Food-to-Night
Outbreak (hypothetical data)
Ate Did Not Eat
Food Total Ill Attack rate Total Ill Attack rate
Salad 65 54 20 6 30%
Chips 56 40 71% 29 20
Katogo 63 49 22 11 50%
Salad
Yes No
Katogo
Yes 46/53 = 87% 3/10 =
No 8/12 = 3/10 = 30%

Secondary attack rate
 This is an important measure of spread of
disease among contacts of an index case.
 AR = New cases among the population
during the specified period /Population at risk at
the beginning of the period .expressed as a
percent.
 Secondary AR = New cases among contacts of
index cases during the period/ Total number of
contacts with the index cases

2. Examination the distribution of cases
 Person
 Place
 Time

Epidemic Curve
 Describe magnitude and pattern of epidemic
 Calculate mean incubation time
 Identify probable cause
 Food-borne, person-to-person transmission
 Evaluate effect of intervention on epidemic

Single Exposure, Common
Source Epidemic
 Sudden and rapid increase in number of cases
 Cases limited to people who share common
exposure
 Rarely secondary cases

Continuous, Common Source Epidemic
 Cases limited to people who share common
exposure
 Rarely secondary cases
 Source is not eliminated immediately

Propagating Epidemic
 Secondary cases

Infectivity
Infectivity =
ILL
Number of Exposed
(Secondary Attack Rate)

2. Examination the distribution of cases
 Person
 Place
 Time
3. Develop hypotheses
 Existing knowledge
 Analogy to diseases of known etiology
4. Test hypotheses
 Case-control studies
 Cohort studies
5. Recommend control measures
 Control of current outbreak
 Preventive for future outbreaks

Review
 Dynamics of disease transmission
 Agent, host, environment, and vector
 Measures of disease transmission
 Infectivity, pathogenicity, virulence
 Herd immunity
 Reproductive rate
 Outbreak investigations
 5 steps
 Attack rates
 Epidemic curves

Dr mathias introduction to epidemiology

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