Transcript EG3213 Spatial Science & Health

EG3246
Spatial Science & Health
Introduction to Basic Epidemiology
Dr Mark Cresswell
Topics
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Definition of Epidemiology
Statistical measures
Ideas of space & time
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Modelling and spatial analysis
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Definition of Epidemiology
Epidemiology is the study of the distribution and
determinants of diseases within human
populations. Research in this field is based
primarily upon observing people directly in their
natural environments.
Greenberg et al. 2005
Definition of Epidemiology
Epidemiology is the study of the distribution and
determinants of diseases within human
populations. Research in this field is based
primarily upon observing people directly in their
natural environments.
Greenberg et al. 2005
Definition of Epidemiology
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Epidemiologists often refer to a population at
risk
These are people who (regardless of their
state of health) would be regarded as a new
case if they contracted the disease being
studied
Can be country, region, town or GP list
Definition of Epidemiology
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A target population may be regarded as a
group studied who are thought to be at a
higher risk: e.g. people living within a 10 mile
radius of a nuclear power station
A study sample is (usually) a randomly
selected sample whose characteristics are
often extrapolated to be representative of the
larger population
Definition of Epidemiology
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The aetiology of a disease is the study of the
agent(s) of causality that causes that disease
Aetiology studies often examine levels of
exposure of some agent (e.g. incidence of
cancers between people who have lived close
to nuclear power stations compared to those
who have not)
Definition of Epidemiology
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Epidemiology also looks at temporal change
How have incidence levels changed over a
period of time?
Inherent in this type of analysis is discovery
of a rising or falling trend
This is achieved by active or passive
surveillance techniques
Relationship between Annual Absolute Humidity change, and a Specific Meningitis Epidemic
Case statistics, and humidity data refers to Ghana (Gold Coast)
22
5000
21
Number of Meningitis Cases
19
3000
18
CSM Cases
2000
17
Mean Absolute Humidity (g/m^3)
16
1000
15
0
14
September
October
November
December
January
February
March
April
Month (aggregated from 1992/93 for Absolute Humidity, and 1947/48 for Case Data)
Source: Cresswell, 1998
May
Mean Absolute Humidity (g/m^3)
20
4000
Statistical Measures - Incidence
This is the number of new cases in a particular time period:
N
I
P
I = Incidence
N = Number of new cases in a given time period
P = Person years at risk during same time period
Note that person years at risk means the total amount of time (in years)
that each member of the population being studied (the study
population) is at risk of the disease during the period of interest.
Statistical Measures - Prevalence
This is the proportion of current cases in a population at a given point in time:
Nc
P
P
P = Prevalence
Nc = Number of cases in the population at a given point in time
P = Total population at the same point in time
Statistical Measures – Absolute Risk
The probability of having a disease, for those individuals who were
exposed to a risk factor.
Ne
Ra 
Ie
Ra = Absolute Risk
Ne = Number of cases of disease in those exposed
Ie = Number of individuals exposed
Statistical Measures – Relative Risk
This is an indication of the risk of developing a disease in a group of
people who were exposed to a risk factor, relative to a group who were not
exposed to it.
Ie
RR 
In
RR = Relative Risk
Ie = Disease incidence in exposed group
In = Disease incidence in non-exposed group
Statistical Measures – Relative Risk
If RR=1, there is no association between the risk factor and the
disease
If RR>1, there is an increased risk of developing the disease if one
is exposed to the risk factor (eg. Disease=lung cancer; risk
factor=smoking). It suggests that exposure to the risk factor may
cause the disease.
If RR<1, there is a decreased risk of developing the disease if one
is exposed to the risk factor (e.g. disease=colon cancer; risk
factor=eating fresh fruit & veg). It suggests that exposure to the risk
factor may protect against the disease.
Statistical Measures – Other Rates
NLB
FR 
NW
FR = FERTILITY RATE
NLB = Number of live births
NW = Number of women aged 15 – 44 years
NLB
BR 
MYP
BR = BIRTH RATE
NLB = Number of live births
MYP = Mid-year population
NI
IMR 
NLB
IMR = INFANT MORTALITY RATE
NI = Number of infant deaths (<1 year old)
NLB = Number of live births
Source: BMJ, 2005
Ideas of Space & Time
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Diseases may typically exist within
geographical limits
If the exposure to a hazard is determined by
distance from the source OR
If exposure to the hazard is dependent upon
environmental/climatic factors
Ideas of Space & Time
Nuclear power station & cancers
Effects of time & age (source: BMJ, 2005)
Ideas of Space & Time
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For a disease to be regarded as endemic, it
must be habitually present in a community of
individuals
A sudden and great increase in the occurrence
of a disease within a population is referred to
as an epidemic.
A rapidly emerging outbreak of a disease that
affects a wide range of geographically
distributed people is described as a pandemic
Ideas of Space & Time
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Hypoendemic means little transmission where
effect on population is negligible
Mesoendemic means disease is found in small
isolated communities of varying intensity
Hyperendemic refers to intense but seasonal
transmission not affecting all age groups
Holoendemic refers to perennial and intense
transmission leading to adult immunity
Modelling & Spatial Analysis
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Epidemiological data may be modelled
spatially, temporally or spatio-temporally
Disease may be visualised using a GIS
Co-factors affecting epidemiological
characteristics (exposure or seasonal cycles
such as weather) may be modelled to
ascertain risk
Temporal change in Standardised Death Rate (cases per 100,000) due to Car Accidents
Source: WHO/Europe European Mortality Database, 2005
Temporal change in Standardised Death Rate (cases per 100,000) due to Heart Disease
Source: WHO/Europe European Mortality Database, 2005