EG3246Spatial Science & HealthIntroduction to Basic Epidemiology

Dr Mark Cresswell

Topics Definition of Epidemiology Statistical measures Ideas of space & time

Modelling and spatial analysis

Definition of EpidemiologyEpidemiology 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 EpidemiologyEpidemiology 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 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 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 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 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 EpidemicCase statistics, and humidity data refers to Ghana (Gold Coast)

0

1000

2000

3000

4000

5000

September October November December January February March April May

Month (aggregated from 1992/93 for Absolute Humidity, and 1947/48 for Case Data)

Nu

mb

er o

f M

enin

git

is C

ases

14

15

16

17

18

19

20

21

22

Mea

n A

bso

lute

Hu

mid

ity

(g/m

^3)

CSM Cases

Mean Absolute Humidity (g/m^3)

Source: Cresswell, 1998

Statistical Measures - IncidenceThis is the number of new cases in a particular time period:

P

NI

I = IncidenceN = Number of new cases in a given time periodP = 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 - PrevalenceThis is the proportion of current cases in a population at a given point in time:

P = PrevalenceNc = Number of cases in the population at a given point in timeP = Total population at the same point in time

P

NcP

Statistical Measures – Absolute RiskThe probability of having a disease, for those individuals who were exposed to a risk factor.

Ie

NeRa

Ra = Absolute RiskNe = Number of cases of disease in those exposedIe = Number of individuals exposed

Statistical Measures – Relative RiskThis 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.

In

IeRR

RR = Relative RiskIe = Disease incidence in exposed groupIn = Disease incidence in non-exposed group

Statistical Measures – Relative RiskIf 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

MYP

NLBBR

NW

NLBFR

NLB

NIIMR

FR = FERTILITY RATENLB = Number of live birthsNW = Number of women aged 15 – 44 years

BR = BIRTH RATENLB = Number of live birthsMYP = Mid-year population

IMR = INFANT MORTALITY RATENI = Number of infant deaths (<1 year old)NLB = Number of live births Source: BMJ, 2005

Ideas of Space & Time 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 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 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 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 AccidentsSource: WHO/Europe European Mortality Database, 2005

Temporal change in Standardised Death Rate (cases per 100,000) due to Heart DiseaseSource: WHO/Europe European Mortality Database, 2005