1. Basic Epidemiology.ppt

Basic Epidemiology
1/29/2024 1

Definition
Epidemiology
 Greek
 Epi – about or upon
 Demos – populace or people of districts
 Logos – study
 “Study of that which is upon the people”
 “The study of disease in populations”
 “The study of how often diseases occur in different
groups of people and why”.
1/29/2024 2

John Snow (1813-1858)
 A well known anesthesiologist who
administered chloroform to Queen Victoria
in child birth
 Snow’s true love was the epidemiology
of cholera.
1/29/2024 drnauman.arif@gmail.com 3

Snow & Cholera
 Cholera was a major problem in England in the mid
19th century
 Snow hypothesized that cholera was transmitted
through contaminated water
 Broad Street Pump – 600 deaths in one week
 Water was supplied via water supply companies
with intakes from the polluted part of the river
 Lambeth water company moved intake upstream
 Mortality should then be lower in people getting
water from L.

 Cholera was a major problem
in England in the mid 19th
century
 Snow hypothesized that
cholera was transmitted
through contaminated water
 The risk of cholera in London
was related to the drinking of
water supplied by a particular
company
Water
supply
company
Populati
on 1851
Cholera
deaths
(n)
Cholera
death rate
(per 1000
population)
Southwar
k
167654 844 5.0
Lambeth 19133 18 0.9
John Snow’s findings
(1854)

THE FATHER OF MODERN
EPIDEMIOLOGY
 "For his persistent efforts to determine how
cholera was spread and for the statistical mapping
methods he initiated, John Snow is widely
considered to be the father of [modern]
epidemiology."
David Vachon: Old News 16(8), 8-10, May & June, 2005

Definitions of Epidemiology
Oxford English Dictionary
THE BRANCH OF MEDICAL SCIENCE WHICH TREATS
EPIDEMICS
Kuller LH: Am J Epid 1991;134:1051
EPIDEMIOLOGY IS THE STUDY OF "EPIDEMICS" AND
THEIR PREVENTION
Anderson G,quoted in Rothman KJ: Modern Epidemiology
THE STUDY OF THE OCCURRENCE OF ILLNESS

“ The study of the distribution and
determinants of health related states and
events in populations, and the application of
this study to the prevention and control of
health problems”
John M. Last, Dictionary of Epidemiology
Epidemiology?

Study
 Includes;
 Surveillance
 Observation
 Hypothesis testing
 Analytic research
 experiments

Distribution
 Epidemiology studies where diseases are
found in the population
 who gets them
 (e.g., young people, older people, women, men),
 where they occur
 (e.g., in urban areas, rural areas, industrialized
countries, developing countries), and
 When they occur: the patterns of disease
occurrence by season and over time.

Descriptive Epidemiology
 Examining the distribution of disease in
a population and observing the basic
features of its distribution in terms of
person, place and time.

Time – When?
 Changing or stable?
 pertusis (whooping cough)
 Seasonal variation?
 influenza
 Clustered (epidemic) or evenly
distributed (endemic)?
 cancer

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

Person – Whom?
 Age
 Gender
 Ethnicity
 Race
 Socio-economic status
 Behavior
 Genetics
 Occupation
 Religion
 Personal habits
 Marital status
 Travel

Determinants
 Epidemiology is interested in the factors that
influence the occurrence of disease –
 risk factors,
 environmental factors, and
 preventive factors.
 Biological, chemical, physical, social, cultural,
economic, genetic and behavioral
 The object is to prevent disease, and in order
to do that we need to identify the factors
that affect its occurrence.

Analytic Epidemiology
 Focus on causation of disease by testing
a specific hypothesis about the
relationship of a disease to a cause (risk
factor).

Health related states and
events
 Epidemiology can be used to study any phenomena
 Diseases, causes of death, reactions to preventive
regimes, use of health services
 Besides disease, epidemiology can study
 positive conditions e.g. immunity
 adverse ones, e.g. use of tobacco

 We see the world through our
perspectives.
 In public health, two perspectives:
 Individual perspective –
 focus on health, risk factors,
exposures, causal mechanisms in
people as individuals
 Population perspective –
 focus on disorders (“mass disease”),
exposures, causal mechanisms in
people as a group
Epidemiology emphasizes the
population perspective

Application to prevention and
control
 It involves applying the knowledge gained
by the studies to community-based
practices.
 To promote health
 To maintain health and
 To restore health

Below are four key terms taken from the definition of
epidemiology, followed by a list of activities that an
epidemiologist might perform. Match the term to the
activity that best describes it. You should match only one
term per activity.
A. Distribution
B. Determinants
C. Application
1. Compare food histories between persons with Staphylococcus food
poisoning and those without
2. Compare frequency of brain cancer among anatomists with frequency
in general population
3. Mark on a map the residences of all children born with birth defects
within 2 miles of a hazardous waste site
4. Graph the number of cases of congenital syphilis by year for the
country
5. Recommend that close contacts of a child recently reported with
meningococcal meningitis receive Rifampin
6. Tabulate the frequency of clinical signs, symptoms, and laboratory
findings among children with chickenpox in Cincinnati, Ohio

1. Compare food histories between persons with
Staphylococcus food poisoning and those without
Determinants
2. Compare frequency of brain cancer among anatomists with
frequency in general population
Determinants
3. Mark on a map the residences of all children born with birth
defects within 2 miles of a hazardous waste site
Distribution
4. Graph the number of cases of congenital syphilis by year for
the country
Distribution
5. Recommend that close contacts of a child recently reported
with meningococcal meningitis receive Rifampin
Application

Uses
 To identify the etiology or cause of a disease
and the risk factors (characteristics that
increase an individual’s risk for a disease).
 Ultimate goal is to intervene to reduce morbidity
and mortality
 To determine the extent of disease found in
the community.
 Help planning programs, obtain resources, etc…
 To study the natural history and prognosis of
disease.
 Define the baseline of a disease for comparisons
post intervention

Uses
 To provide the foundation for developing
public policy and regulatory decisions
relating to environmental problems.
 Occupational risk in workers and required
regulations

Core Functions of Epidemiology
 Public health surveillance
 Field Investigation
 Analytic studies
 Linkages
 Evaluation
 Policy development

Public health surveillance
Surveillance Cycle

Field Investigation
 Identification of unreported or
unrecognized ill persons who might
otherwise continue to spread infection to
others.
 For example, one of the hallmarks of
investigations of persons with sexually
transmitted disease is the identification of
sexual partners or contacts of patients.
 Identification and treatment of these
contacts prevents further spread.

Analytic studies
 Design
 Conduct
 Analysis
 Interpretation

Evaluation
 Focus on
 Plans (Formative evaluation)
 Operations (Process evaluation)
 Impact (Summative evaluation)
 Example : Evaluation of immunization
program

Linkages
 Epidemiologist
 Laboratorians
 Sanitarians
 Infection control personnel
 Nurses
 Computer information specialists

Match the appropriate core function
to each of the statements below.
A. Public health surveillance
B. Field investigation
C. Analytic studies
D. Evaluation
E. Linkages
F. Policy development
1. Reviewing reports of test results for Chlamydia trachomatis from
public health Clinics
2. Meeting with directors of family planning clinics and college health
clinics to discuss Chlamydia testing and reporting
3. Developing guidelines/criteria about which patients coming to the
clinic should be screened (tested) for Chlamydia infection
4. Interviewing persons infected with Chlamydia to identify their sex
partners
5. Conducting an analysis of patient flow at the public health clinic to
determine waiting times for clinic patients
6. Comparing persons with symptomatic versus asymptomatic Chlamydia
infection to identify predictors

1. Reviewing reports of test results for Chlamydia
trachomatis from public health Clinics:
Public health surveillance
2. Meeting with directors of family planning clinics
and college health clinics to discuss Chlamydia
testing and reporting : Linkages
3. Developing guidelines/criteria about which patients
coming to the clinic should be screened (tested)
for Chlamydia infection: Policy development
4. Interviewing persons infected with Chlamydia to
identify their sex partners: Field investigation
5. Conducting an analysis of patient flow at the
public health clinic to determine waiting times for
clinic patients: Evaluation
6. Comparing persons with symptomatic versus
asymptomatic Chlamydia infection to identify
predictors: Analytic studies

Approaches in Medicine and in
Epidemiology

The Epidemiologic Approach
 How does the epidemiologist proceed to
identify the cause of a disease? Via a
multi-step process.
 First – determine if an association exists
between a factor (e.g., sun exposure) or a
characteristic of a person (e.g., moles, fair
skin) and the development of disease (e.g.,
skin cancer).
 Second – is this association a causal one?
(not all associations are causal)

Contd….
 Counts cases or health events and
describes them in terms of time, place
and person.
 Divides the number of cases by an
appropriate denominator to calculate
rates.
 Compares these rates.

Usual Sequence of Epi.
Method
Theory or Observation
Review Existing Information
Define/Refine Hypothesis
Descriptive Studies
Analytic Studies
Collect & Analyze Data
Formulate Conclusions
Preventive
Action

In Summary
 Epidemiology is the study of the distribution
and determinants of disease in populations.
 Public health uses epidemiologic study findings
to prevent and control health problems in
human populations.
 Major causes of mortality have changed
radically over the current century.
 Epidemiology is an invaluable tool in the control
of disease and the human suffering associated
with it.

Dynamics of Disease
Transmission

Disease causation
 Human disease results from an interaction of
the
 Host
 Agent
 Environment
 Disease causation is usually described in
terms of two models:
 Epidemiologic triad/triangle
 Web of causation

Epidemiological Triad

Disease
Agent
• virus
•Bacteria
•Parasite
•Other
microbes
Environment
• Weather
• Housing
• Geography
• Occupational setting
• Air quality
• Food
Host
• Age
• Sex
• Behaviour
• Nnutritional

Host Characteristics
 Include personal characteristics
and behaviors, genetic
predispositions, immunologic
related factors that increase the
likelihood of disease.
 Examples: Age, sex, race,
religion, occupation, genetics,
marital status, immune status.

Agent Characteristics
 Biological, physical, or chemical factors
whose presence, absence, or relative
amount (too much or too little) are
necessary for the disease to occur.
 Examples: Bacteria, viruses, fungi, or
other microbes.

Environmental Characteristics
 External conditions, other than the
agent, that contribute to the disease
process.
 Examples: Temperature, humidity,
crowding, housing, water, milk, food,
air pollution, noise.

Web of Causation
 This model de-emphasizes “agent” and
stresses the multiplicity of interactions
between the host and environment.
 Multiple actions and reactions occur
between promoters and inhibitors of
disease.
 Example: Diabetes, cancer

Natural History and Spectrum
of Disease

Exposure Clinical Effect Incubation/
Latency Period
Organ phosphorus Nausea, vomiting
,convulsion
few minutes-few
hours
SARS-associated
Severe Acute
Respiratory corona
virus
Severe Acute
Respiratory
corona virus Syndrome
(SARS)
3–10 days, usually 4–
6 days
Varicella-zoster virus Chickenpox 10–21 days,
usually 14–16 days
Treponema palladium Syphilis 10–90 days,
usually 3 weeks
Hepatitis A virus Hepatitis 14–50 days, average
4 weeks
Hepatitis B virus Hepatitis 50–180 days, usually
2–3 months
HIV AIDS <1 to 15+ years

Iceberg concept
 “Iceberg” concept – the tip is only visible
much like the clinical appearance of
disease…bulk of the problem may be
hidden from view.
 Example – tuberculosis cases are not always
clinically visible

“Iceberg” Concept of
Infectious Diseases

Stages of Disease & Prevention

 Infectivity
 Proportion of exposed persons who become
infected.
 Pathogenicity
 Proportion of infected individuals who develop
clinically apparent disease.
 Virulence
 Proportion of clinically apparent cases that are
severe or fatal.

Chain of Infection

Reservoir
 Habitat in which an infectious agent
normally lives, grows, and multiplies.
 Reservoirs include:
 Humans.
e.g. Typhoid Mary
 Animals
e.g. Brucellosis (cows), Plague
(rodents).
 Environment
 e.g.plants,soil,water.

Portal of Exit
 The path by which an agent leaves its host.
 Examples
 Influenza viruses & Myco TB – Resp. Tract
 Cholera vibrios – feces
 Syphilis- crossing placenta from mother to child
 Hepatitis B- cuts in skin,secretions.

Modes of Transmission
 Direct
 Person to person e.g. STDs
 Droplet spread e.g. pertussis
 Indirect
 Airborne
 dust, droplet nuclei e.g. Measles
 vehicles
 contaminated food, water, blood & fomites (Hep B)
 Vectors
 Mosquito (West Nile Virus), deer tick (Lyme’s Disease)

Portal of Entry
 Skin (Hookworm)
 Mucous membrane (syphilis)
 Blood (Hep B, HIV)

Chain of Infection

Herd Immunity
 Resistance of a group to an attack by a
disease to which large proportions of the
group are immune.
 If a large percent of the population is
immune, the entire population is likely to
be protected, not just those who are
immune.

Why is herd immunity so
important?
 It is not necessary to carry out 100%
immunization rates .
 Achieve highly effective protection by
immunizing large part of population; the
remaining will be protected by herd
immunity.

Epidemic Disease Occurrence
 Sporadic refers to a disease that occurs infrequently and
irregularly.
 Endemic habitual presence or usual occurrence of disease in
a geographic area e.g. Malaria
 Hyper endemic refers to persistent, high levels of disease
occurrence.
 Epidemic refers to an increase, often sudden, in the number
of cases of a disease above what is normally expected in that
population in .e.g. cholera

 Outbreak carries the same definition of epidemic,
but is often used for a more limited geographic area.
 Cluster refers to an aggregation of cases grouped
in place and time that are suspected to be greater
than the number expected, even though the
expected number may not be known.
 Pandemic refers to an epidemic that has spread
over several countries or continents, usually
affecting a large number of people.e.g HIV

For each of the following situations,
identify whether it reflects:
A. Sporadic disease
B. Endemic disease
C. Hyper endemic disease
D. Pandemic disease
E. Epidemic disease
 22 cases of legionellosis occurred within 3 weeks among residents of
a particular neighborhood (usually 0 or 1 per year)
 Average annual incidence was 364 cases of pulmonary tuberculosis
per 100,000 population in one area, compared with national average of
134 cases per 100,000 population
 Over 20 million people worldwide died from influenza in 1918-1919
 Single case of histoplasmosis was diagnosed in a community
 About 60 cases of gonorrhea are usually reported in this region per
week, slightly less than the national average

Epidemic Patterns
 Common-source outbreak
 Group of persons exposed to an infectious agent
from the same source.
 Point source (e.g.) food served only once
 Periodic or continuous (e.g.) a water supply is
contaminated with sewage b/c of leaky pipes.

Propagated outbreaks
 Results of transmission from one person to
another.
 Direct i.e. from person to person
 Indirect
 vehicle borne (Hep B by sharing needles)
 Vector borne (yellow fever by mosquitoes)

For each of the following situations,
identify the type of epidemic spread
with which it is most consistent.
A. Point source
B. continuous common source
C. Propagated
 21 cases of shigellosis among children and workers at a day
care center over a period of 6 weeks, no external source
identified incubation period for shigellosis is usually 1-3 days)
 36 cases of giardiasis over 6 weeks traced to occasional use of
a supplementary reservoir (incubation period for giardiasis 3-25
days or more, usually 7-10 days)
 43 cases of norovirus infection over 2 days traced to the ice
machine on a cruise ship (incubation period for norovirus is
usually 24-48 hours)

Point source
(common-vehicle outbreak)
 Such outbreaks are explosive – a
sudden and rapid increase in the # of
cases of a disease
 The cases are limited to those who
share the common exposure.

Graphical Representation of an
Outbreak
Classic epi. curve

Outbreak Investigations
 Three critical variables in investigating an
outbreak or epidemic are:
 When did the exposure take place?
 When did the disease begin?
 What was the incubation period for the
disease?

Outline of an Epidemic
Investigation
 Preliminary Analysis
 Verify the diagnosis
 Verify the existence of an epidemic
 Learn about the disease using existing
information
 Describe the epidemic with respect to time,
place and person (cases = numerator)
 What population is at risk? (denominator)
 Formulate and test hypotheses

Investigation
 Further Investigation and Analysis
 Search for additional cases
 Collect additional data
 Analyze the data
 Make a decision about the hypotheses
considered
 Intervention and follow-up

Investigation
 Report of the Investigation
 Discussion of factors leading to the epidemic
 Evaluation of measures used for control of
present epidemic
 Recommendations for future prevention of
outbreaks

1. Basic Epidemiology.ppt

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