Transcript Lecture 1 - some small revisions Powerpoint

Introduction to Epidemiology
Dr. Mary Rose Sweeney
February 2013
Email:[email protected]
Room HG19
Office no. HG269
Learning Objectives of NS270
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On completion of this module and following a period of personal study
the student will be able to………
Outline the contribution of epidemiology to the health of populations
Display an understanding of the measures of disease frequency
Display an understanding of communicable disease epidemiology
Display an understanding of nutritional epidemiology
Illustrate an understanding of the levels of prevention
Demonstrate an understanding of the contribution of epidemiology to
planning and evaluation of health care activities.
Understand the principles underlying “health screening”
Outline the criteria for causality – the cause of illness or health
Objective of today’s session
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Introduction to Epidemiology
Epidemiology – an historical perspective
Modern day epidemiology
Principles of causation
Reading List
1. Beaglehole, R., Bonita R. Kjellstrom T. (1993).
Basic Epidemiology.
Geneva, World Health Organization.
2. Gordis, L. (2004)
Epidemiology.
Philadelphia, Elsevier Saunders.
3. Coggon, D., Barker, D., Rose, G. (2003)
Epidemiology for the Uninitiated,
(5th ed). UK: Blackwell BMJ Books.
4. Policy Documents will be recommended with each session also.
• See online Beaglehole WHO book
What is epidemiology?
Definition of Epidemiology
Epidemiology is the study of the distribution and
determinants of health related states or events in
specified populations, and the application of this
study to the control of health problems
(Last 1988)
Greek origin epi= on or upon, demos= people, logos= the study of
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Distribution – Refers to the analysis of disease patterns according to the characteristics
of person, place and time. In other words, who is getting the disease, where it is
occurring and how is it changing over time? (See obesity slides)
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Determinants- are factors which bring about a change in a person’s health –that is,
factors that either cause a healthy individual to become sick or cause a sick person to
recover. Thus determinants include both causal and preventive factors (see heath
determinants model)
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Populations - are at the heart of all epidemiological activities because epidemiologists
are concerned with disease occurrence in groups of people rather than individuals. The
term population refers to groups of persons with a common characteristic such as place
of residence gender, age, or use of common services. Determining the size of the
population in which the disease occurs is as important as counting the cases of the
disease, because it is only then that we can know the frequency of the disease (refer to
AITHS)
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Control – Epidemiologists accomplish disease control through epidemiological research
and through surveillance. Surveillance data are essential for formulating and evaluating
programs to reduce the spread of diseases (See Cancer Control Strategy, 2006)
What are our health
determinants? Or what
determines our health?
How do each of these play a role?
• Expressed another way, epidemiology seeks to describe the
distribution of disease in the human population and then to
undertake studies that address specific hypothesis arising
from these descriptive studies.
• Ultimately it is concerned with understanding how an
event causes a disease. We may refine this further by
asking how a a particular exposure affects a particular
outcome and the exposure can be to anything eg, nutrition
breastfeeding and obesity in offspring, smoking and lung
disease, , radiation exposure and cancer risk, etc.
Epidemiology is considered the basic or
underlying science of public health.
What is public health?
Public health is defined as
“the process of promoting health, preventing
disease, prolonging life and improving the
quality of life through the organised efforts
of society”
How promotes public health?
Public Health Practice includes
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Surveillance
Audit
Service Planning
Health Promotion/Prevention
Aetiology (cause)
Epidemiology is based on two
fundamental assumptions
1.That human disease does not occur at
random
2.That human disease has causal and
preventive factors that can be identified
through systematic investigation
Epidemiological Information is used
to
• Describe a population’s health status (Can be
national level or smaller group e.g. prisoners, Irish
Travellers)
• Search for the cause of disease or health related
state in an attempt to identify ways to prevent the
disease in the first place (neural tube defects with
giving folic acid)
• Evaluate health related interventions (fluoridation
of water, fortifying the food supply with folic acid,
immunization programmes)
• Gather information on how a disease progresses
Uses of Epidemiology
Historical events in disease
management
• Throughout history, humanity has been challenged by
outbreaks of infectious illnesses and other health
emergencies that have spread.
• With no better solution, people’s response was to respond
by removing the sick person from the healthy population
and wait until the illness ran its course i.e. the person either
recovered naturally or died.
• This practice dates back to the 5th and 6th century with both
biblical and koranic references to the isolation of lepers.
• By the 7th century China had a well-established policy of
detaining sailors and foreign travellers suffering from
plague.
Quarantine and plague
• The term quarantine dates back to the 14th century
when people were isolated upon arrival from
plague-infected areas to the port of Ragusa
(Venetian Republic).
• The term quarantine is derived from the Italian for
“40” because in 1397 the period was set at 40 days
• Similar public health measures were implemented
in many other European ports of the time.
Picture of 14th century Doctor wearing protective clothing, a
mask and a beak containing strong herbal medicine
• The continuing devastation wrought by plague and
other diseases demonstrated that crude quarantine
control measures where largely ineffective. In the
17th century it had made it way across continental
Europe to England and in 1965 caused
devastation.
• In recent years the most serious outbreak of plague
occurred in India (1994) where 700 suspected
bubonic plague cases and 56 deaths were reported
to the WHO.
Epidemiology- Historical Context
• Epidemiology has its origins in the idea, first expressed
over 2000 years ago by Hippocrates and others, that
environmental factors can influence the occurrence of
disease.
• However it was not until the 19th century that the
distribution of disease in specific human population groups
was measured to any great extent.
• This work marked not only the formal beginnings of
epidemiology but also some of its most spectacular
achievements, for example, the findings of John Snow that
the risk of cholera in London was related, among other
things, to the drinking of water supplied by a particular
company.
John Snow: 1850s, formulated and tested a hypothesis
concerning the origins of an epidemic of cholera in London,
often called the “Father of Field Epidemiology”.
• Snow located the home of each person who
died from cholera in London during 184849 and 1853-54 and noted an apparent
association between the source of the
drinking water and the deaths – see
mapping exercise
This is a portion of the original map created by Dr. John Snow. Through
plotting the deaths (signified by a line parallel to the building front in
which the people died), Dr. Snow was able to trace the spread of Cholera
to the pump at the corner of Cambridge and Broad Street.
• He prepared a statistical comparison of
cholera deaths in districts with different
water supplies and showed that both the
number of deaths and the mortality rate
were higher in the people supplied by the
Southwork company (table 1)
Snow, 1855
Water
Population No. of Cholera death
supply
deaths rate per 1000
company
population
Southwark 167,654
844
5.0
Lambeth
19,133
18
0.9
So called “death dispensary”
• From this Snow constructed a theory about the
communication of infectious diseases in general
and suggested that cholera was spread by faecal
contaminated water. He was thus able to
encourage improvements in the water supply long
before the discovery of the organism responsible
for cholera – his work had an enormous direct
impact of public health.
• A new sewage system was constructed in London
in the 1880’s
• Cholera continues to be a major health risk
all over the world. In addition to human
suffering and death it causes panic, disrupts
social and economic structures and
threatens development.
• The need to provide sanitation both for
drinking-water and hygiene remains a huge
challenge today for developing countries.
Smallpox and Immunization
• Smallpox is one of the oldest
known human diseases. There
is evidence of its existence over
3000 years ago in Egypt – in a
mummified head in 1157 –
shows a pustular eruption that
was most likely caused by
smallpox.
• During the 18th century,
smallpox killed every 7th child
in Russia and every 10th child in
France and Sweden.
• Edward Jenner’s experiment
brought hope that it could be
controlled.
• Jenner an English physician, realized that many of
his patients who had been exposed to cowpox, the
much milder related disease were immune to
smallpox. He inoculated an eight year old boy
with cowpox virus and after a period inoculated
him with smallpox. The boy did not get the illness
demonstrating that the cowpox inoculation
protected him from against smallpox.
• Jenner’s procedure was soon widely accepted and
practised and resulted in sharp falls in smallpox
deaths.
Smallpox immunisation scene
• Through the success of a global 10-year
eradication campaign that began in 1967, the
global eradication of smallpox was certified in
1979.
• Since this some terrorist groups are alleging that
they have the smallpox virus in frozen storage and
could cause a whole new epidemic potentially
undoing the work of one of the world’s greatest
public health achievements.
By the end of the 19th century it was
recognised that
• Quarantine
• Sanitation
• Immunization
Were powerful tools in disease control
Other events in history
• Hippocrates - 5th century, first suggestion that
human disease is related to the environment as
well as the individual
• John Graunt-1662, quantified patterns of birth,
death and disease in populations, he noted
seasonal variations and urban rural differences in
disease patterns
• William Farr-1839, compared mortality patterns in
different populations and linked mortality to
occupation and marital status
Other triumphs in history
• Identification of arthropod vectors for many
diseases - including malaria, yellow fever
(1905-1909)
Other triumphs in history
Cigarette smoking found to be a
major cause of lung cancer,
emphysema and cardiovascular
disease (1951-1965)
Other triumphs in history
• Identification of the AIDS
syndrome prediction, that
the cause was a sexually
transmitted virus (19811983) and development of
prevention measures were
advocated to populations
at risk before the virus
was identified.
Modern Epidemiology
The more recent events in epidemiology can be illustrated
by the work of Dull, Hill and others who studied the
relationship between cigarette smoke and lung cancer in the
1950s.
This work which was preceded by clinical observations
linking smoking to lung cancer expanded epidemiological
interest to chronic diseases. A long-term follow-up of
British doctors indicated a strong association between
smoking habits and the development of lung cancer.
• It soon became clear that, for many diseases, a
number of factors contributed to causation. Some
factors were essential for the development of a
disease while some just increased the risk of
developing it.
• Today communicable disease epidemiology
remains of vital importance in developing
countries where malaria, leprosy and other
diseases remain prevalent.
Definitions & Scope of modern day
epidemiology
• Modern day epidemiologists are concerned
not only with death, illness and disability
but also with more positive health states and
with the means to improve health.
• In the broad field of modern day public
health, epidemiology is used in a number of
ways
Modern day uses
• In identifying causes of illness/disease/ill health – what are they?
• In identifying preventative measures – what are they?
• To describe the health status of population groups – how do we do
this?
• Knowledge of disease burden in populations is essential for health
authorities, which seek to use limited resources to the best possible
effect by identifying priority health programmes for prevention and
care.
• In some specialist areas, such as environment and occupational
exposure, the emphasis is on studies of populations with particular
types of environmental exposure.
• Epidemiology is also concerned with the course and outcome of
diseases in individuals and groups.
• Recently epidemiologists have become
involved in evaluating the effectiveness and
efficiency of health services, by
determining the appropriate length of stay
for a particular treatment, or condition etc.
REVISION Lecture 1 – Mary
Rose
• Define Epidemiology
• What is the modern day scope of
epidemiology (uses)?
• What are the landmark events historically in
epidemiology?
• Describe John Snow’s work in London
Revision lecture 2 Anthony
Staines
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Define prevalence
Define Incidence
How does one measure health and illness?
What is health?
What are the main measures of health?
What is the importance of measuring rates
of health?
What causes disease?
How do we establish the cause of
disease?
Question
• How did scientists work out that folic acid
prevents Neural tube defects in early
pregnancy?
Question
• Or how did we find out that a drug taken to
prevent morning sickness causes absent
limbs?
Question
• Or how do we know that co-sleeping with
an infant increases the risk of cot death?
Todays first task
• Pretend you are Bill Gates (or his wife)
• Somebody close to you has lost a baby to
cot death
• You have decided to spare no money in
getting to the bottom of the cause of cot
death
• What do you do? – get into groups and
decide
Causation
• Causation of some diseases if linked exclusively
to genetic factors, such as phenylketonuria, but
more commonly is linked to an interaction
between genetic and environmental factors.
• Behaviour and lifestyle are of great importance
too and epidemiology is increasingly used to study
both their influence and preventative intervention
through health promotion.
Causation
• An understanding of the causes of disease is
important in the health field not only for
prevention but also in the diagnosis and the
application of correct treatments.
• However the concept of cause is the source
of much controversy in epidemiological as
it is in many other sciences.
• The cause of a disease is an event,
condition, characteristic or a combination of
these factors which plays an important role
in producing the disease. Logically a cause
must precede a disease. A cause is termed
sufficient when it inevitably produces or
initiates a disease and is termed necessary if
a disease cannot develop in its absence.
• A sufficient cause is not usually a single factor, but often
comprises several components. In general, it is not
necessary to identify all the components of a sufficient
cause because effective prevention can take place, since
the removal of one component may interfere with the
action of the others and thus prevent the disease –ie. John
Snow’s water pump removal was sufficient to remove the
cause even though he had no idea of the nature of the
organism. Or for example cigarette smoking is one
component of the sufficient cause of lung cancer. Smoking
is not sufficient in itself to produce the disease; some
people smoke for 50 years without developing lung cancer
implying that other factors (mostly unknown) are required.
• And cessation of smoking reduces the number of
cases of lung cancer in a population even if the
other component causes are not altered.
• Each sufficient cause has a cause as a component,
e.g. in an outbreak of foodborne infection it may
be found that chicken salad and creamy dessert
were both sufficient causes of salmonella. The
ingestion of salmonella are the necessary cause of
the illness.
• The usual approach in epidemiology is to begin
with a disease and start with a potential cause (e.g.
Air pollution, contaminated water) and then search
for its effect.
• Epidemiology encompasses a whole set of
relationships e.g. social class is associated with a
whole range of health problems, income,
education, housing and occupation are all linked to
health outcomes.
Important factors in causation
• 4 types of factor play a part in the causation of disease.
• Predisposing factors – such as age, sex, previous illness
may all create a susceptibility to a disease
• Enabling factors – such as low income, poor nutrition, bad
housing, and inadequate medical care may assist the
development of disease.
• Conversely factors that assist in recovery are also referred
to as enabling factors.
• Precipitating factors – such as exposure to a specific
disease agent
Interactions
• The effect of two or more causes acting
together is often greater than would be
expected on the basis of summing the
individual effects for e.g. the risk of lung
cancer in people who smoke cigarettes and
become exposed to asbestos is much higher
than would be indicated by a simple
addition of the risks from smoking and
exposure to asbestos dust,
Establishing the cause of the illness
• Causal inference is the term used for the process
of determining whether observed associations are
likely to be causal. Before an association is
assessed for the possibility that is casual, other
explanations, such as chance, bias, and
confounding, have to be excluded (lighter in
pocket example).
• A systematic approach to determining the nature
of an association must be utilised.
Hill’s guidelines for assessing
Causation
The best known criteria for assessing causation were proposed by Sir
Austin Bradford Hill in 1965. Hill’s nine criteria are listed below
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Strength of association
Consistency
Specificity
Temporality
Biological gradient
Plausibility
Coherence
Experimental
Analogy
Temporal Relationship
• This is crucial – the cause must precede the effect.
Repeated measures of the exposure at different time-points
and in different locations may strengthen the evidence
• This is illustrated very nicely in figure 5.4 a graph showing
a time series of measurements of exposure and effect. It
shows the impact of a sudden compulsory use of seat belts
in the UK in January 1983, which are highly suggestive of
a protective effect of seat-belts. It would add weight to this
study if previous years data were also included.
Consistency
• Consistency is demonstrated by several
studies giving the same result. This is
particularly important when a variety of
designs are used in different settings since
the likelihood that all studies are making the
same mistake is thereby minimized. When
results of various different studies are being
interpreted the ones with the best design
should carry the greatest weight.
Strength of association
• A strong association between possible cause and effect, as
measured by the size of the risk ratio, is more likely to be
causal that a weak association which could be influenced
by confounding or bias. Relative risks greater than 2 can
be considered strong. For e.g. cigarette smokers have an
approximately twofold increase in the risk of acute MI
compared with non-smokers. The risk of lung cancer in
smokers, compared with non-smokers, has been shown in
various studies to be increased fourfold and twentyfold.
However, such very strong associations are rare in
epidemiology.
Dose-response relationship
• This occurs when changes in the level of a
possible cause are associated with changes
in the prevalence or incidence of the effect.
Table 5.2 illustrates this dose-response
relationship very nicely between noise level
and hearing loss: the prevalence of hearing
loss increases with noise level and
exposure time
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• Pg 79 Beaglehole
Plausibility
• Hill asserted that there should be an existing
biological or social model to explain the
association. For e.g. it is biologically plausible that
smoking causes lung cancer because cigarettes
contain many carcinogenic substances.
• Can neural tube defects really be caused by low
intake of folic acid in pregnancy? Yes because
folic acid is incorporated into every dividing cell
so if it is in low supply defective cell division can
occur.
Specificity
• This concept was first developed for
infectious diseases – the notion that a cause
could lead to a single effect and vice versa.
Experimental evidence
Not a causal guideline in the strictest sense
but rather is a method used for testing a
causal hypothesis if well designed and
executed.
Analogy
• Hill suggested that epidemiologists use
analogies or similarities between the
observed association and any other
associations.
Noteworthy
• Hill never intended these guidelines to be
used as rigid criteria for assessing cause more as guidelines to follow and various
other epidemiologists had picked holes in
these guidelines.
REVISION
• What causes disease?
• How do we determine the cause of disease?
• Provide an example of one disease and how
the cause of it was established
• Outline Hill’s criteria for establishing cause
of disease
Self-directed activities
Read
Chapters 1 “What is epidemiology?”
and
Chapter 5 “Causation in Epidemiology”
“Basic Epidemiology” Beaglehole R. WHO.
Next week
Lecture by Professor Anthony Staines on
• Measuring health
• Descriptive Epidemiology – monitoring the
health of the population, trends, birth rates,
death rates, determinants of health,
incidence and prevalence, population
pyramids