Lesson 3: Epidemiology April 11, 2006
ENV H 311: Intro. to Environmental
Health 1
ENV H 311: Lesson 3 1
Lesson 3. Epidemiology
Human Disease:
Identification &
Control Strategies
Chuck TreserChuck TreserUniversity of WashingtonUniversity of Washington
Dept. of Environmental andDept. of Environmental and
Occupational Health SciencesOccupational Health Sciences
April 11, 2006April 11, 2006
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Definition of Health
“Health is the state of completephysical, mental and social wellbeing and not merely the absence ofdisease or infirmity.”
World Health Organization1970
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EnvironmentalExposures
IntrinsicGenetic
Susceptibility
Age / Time
Determinants of Health
Lesson 3: Epidemiology April 11, 2006
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Health 2
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“Genetics loads the gun . . .
but the environment pulls thetrigger.”
Dr. Judith Senn
Professor of Nutrition & Internal MedicineUniv. Of California, Davis
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HostAgent
Environment
Disease Causation
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Disease Causation
HostAgent
Environment
Lesson 3: Epidemiology April 11, 2006
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Health 3
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Disease & Injury
Acute Effects
Infectious Diseases
Injuries
Chronic Effects
Chronic Disease
Disability
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Environment
Agent Host
Epidemiological Model
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Origins
John Snow(1813-1858)
John Snow's original 1854 map on the location of 578 deaths from Cholera, from An Introduction to VisualisationSoftware for Astronomy, Starlink Guide 8.1, A C Davenhall, 9th February 1996 CCLRC / Rutherford Appleton
Laboratory Particle Physics & Astronomy Research Council
Lesson 3: Epidemiology April 11, 2006
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Epidemiology
Looks for patterns of diseaseoccurrence
Geographically
Demographically
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Definition
Epidemiology is the study of
the distribution and
determinants of
health effects (disease & injuries)
in human populations
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Distribution Factors
Person
Place
Time
Lesson 3: Epidemiology April 11, 2006
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Distribution Factors
PersonAge
Race
Sex
Occupation
Education
Hobbies
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Population Differences
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Distribution Factors
Person
Age
Race
Sex
Time
Episodic
Cyclical
Secular
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Temporal Distribution
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Distribution Factors
Person
Age
Race
Sex
Time
Episodic
Cyclical
Secular
Place
Geographic
Longitude &
Latitude
Geologic
Climatic
Geo-political
Urban / Rural
Industry
Pollution
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Spatial Distribution
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Determinants
Determinants
Agent
Host
Environment
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Determinants
Agent Factors
Biological
Chemical
Physical
Host Factors
GeneticPredisposition
Exposure
EnvironmentFactors
NaturalEnvironment
Built Environment
Socio-culturalEnvironment
TemporalEnvironment
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Balance BeamAffects equilibrium through:1. Interaction with agents2. Individual habits & group customs3. Age, sex & race characteristics4. Defense Mechanisms5. Constitution and heredity6. Psychologic characterisitics
Changes upset balance
Human Host
Affect equilibrium through:1. Their basic nature and character2. Resistance and lability3. Reservoir and Sources4. Conditions of dissemination
These determine the balance of health and
preventive attack
Disease AgentsParasitic, Nutrient, Chemical,
Physical & Mechanical
Environment
PhysicalBiological
Economic &Social-Political
At Equilibrium
Variable fulcrum position
Variable fulcrum position
Simplified diagram of factors influencing equilibrium. Adapted from Leavell HR and Clark EG. Preventive Medicine for the Doctor in his Community.
Lesson 3: Epidemiology April 11, 2006
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The Disease Process
Natural History of Disease
Problems with Detection & Reporting
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DEATH
Chronic
Defect
Disability
Illness
Signs &Symptoms
Tissue &Physiologic Changes
STIMULUS or AGENTbecomes established and increases by multi- plication or incrementIn the
HumanHost
Clinical Horizon
The course of disease in man
RECOVERY
EarlyPathogenesis
DiscernableEarly
Lesions
AdvancedDisease
Convalescence
THE NATURAL HISTORY OF ANY DISEASE IN MAN
Prepathogenesis Period Period of Pathogenesis
Interaction ofHOST & STIMULUS HOST
REACTION
Adapted from: Leavell, Hugh R. and E. Gurney Clark, Preventive Medicine for the Doctor in His Community .
Before man isinvolved
Interrelationship of the various
AGENT / HOST / ENVIRONMENTFactors
(Known and Unknown)
Whichwhich bring together the
AGENT and HOST
Or
Produce a disease producing
STMULUS
ENV H 311: Lesson 3 24Infection terminated,
become latent or intermittently patent
Disease in host
Agent being shed
Total period of infection (agent present)
InfectionLatent
Latentperiod Period of
communicability
Infectionpatent
m: Fox, Hall, Elveback. Epidemiology: Man and Disease, The Macmillan Company, 1970:57.
Schematic Diagram of the Stages of Infection in a Host
Incubationperiod
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THE NATURAL HISTORY OF ANY DISEASE IN MAN
LEVELS OF APPLICATION OF PREVENTIVE MEASURESPrimary Prevention Secondary Prevention Tertiary Prevention
Period of PathogenesisPrepathogenesis Period
HEALTH PROMOTION
Use of specific immunizations
Attention to personal hygiene
Use of environmental sanitation
Protection against occupational hazards
Protection from accidents
Use of specific nutrients
Protection from carcinogens
Avoidance of allergens
Health Education
Good standard of nutritionadjusted to the develop-mental phase of life
Attention to personalitydevelopment
Provision of adequatehousing, recreationand agreeable workingconditions
Marriage Counseling and sex education
Genetics
Periodic selective examinations
Case-finding measures,individual and mass
Screening surveys
Selective examinations
Objectives• To cure and prevent disease processes• To prevent the spread of communicable disease• To prevent complications and sequelae• To shorten the period of disbility
Adequate treatment toarrest the diseaseprocess and to preventcomplications and sequelae
Provision of facilities tolimit disability and toprevent death
Provision of hospital andcommunity facilities forretraining and educationfor maximum use of re-maining capacities
Education of the publicand industry to utilizethe rehabilitated
As full employment aspossible
Selective placement worktherapy in hospitals
Interrelations of Agent, Host andEnvironment Factors
Production of STIMULUS
Reaction of the HOST to the STIMULUS
Early Pathogenesis
DiscernableEarly Lesions
AdvancedDisease
Conva-lescence
SPECIFIC PROTECTIONEARLY DIAGNOSIS &PROMPT TREATMENT DISABILITY
LIMITATIONS REHABILITATION
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Contingencies of Morbidity Reporting
Symptoms? Complaints?• Stoical• Ashamed
ConsultationSought?• Lack of funds• Indifference• "Don't believe in doctors"
DiagnosisSuspected?• MD's acumen• Nature of Complaint• Communication• Rarity of disease
DiagnosisEstablished?• Too early• Too late• Follow up
Report Made?• Ignorance• Indifference• Forgot• Rarity of disease
POPULATION
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
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Concepts & Methods
Disease Prevalence – the proportionof a population with the disease, at achosen point in time. (snap shot)
Rp = CT / P (x 100,000)(at that time)
E.g., 10% of the population of KingCounty has respiratory asthma atpresent.
Lesson 3: Epidemiology April 11, 2006
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Prevalence Rate
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Prevalence Rate
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Concepts & Methods Continued
Incidence Rate – the proportion of apopulation with newly-diagnoseddisease per given unit of time.(New cases over time)
Ri = Cn / P (x 100,000)(at the midpoint of the unit of time)
E.g., the total mortality rate (alldeaths) is 0.89% per year among thepopulation of Seattle
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Incidence Rates
That is, in a given year there were 4450deaths reported among residents ofSeattle, a population of 500,000.
Incidence rate = 4450 / (500,000 x 1year) = 0.0089/year
= 890 per 100,000 persons per year
= 2.4 per 100,000 persons per day
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Incidence Rate
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Analytic Techniques
Stratification – dividing the sampleaccording to some characteristic,e.g. age:
Age-specific deaths from heart disease among non-smokingBritish male doctors
Age Deaths/104 persons per year
35-44 1.064
45-54 11.23
55-64 49.04
65-74 96.71
75-84 212.04
Total 25.75
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Analytic Techniques Continued
Association of disease with anenvironmental factor – comparingdisease prevalence or incident ratesbetween groups with and withoutexposure to the environmental factor
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Analytic Techniques Continued
Age-specific deaths from heartdisease among smoking British maledoctors.
Relative RiskAge Deaths/104 persons per year (RR)
35-44 6.106 5.74
45-54 24.05 2.14
55-64 72.00 1.47
65-74 146.88 1.52
75-84 191.84 0.91
Total 44.29 1.72
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Analytic Techniques Continued
What if smoking British male doctorsdrink more ethanol, compared tonon-smoking British male doctors?Since from other studies we knowthat ethanol is associated with heartdisease, can we argue that smoking isthe cause of heart disease mortalityin this group?
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Analytic Techniques Continued
Confounding factor: a factor that isassociated both with exposure andoutcome, and thus interferes indetermining the relationshipbetween exposure and outcome.Ethanol in this case is a confounder
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Limitations
Most environmental diseases havemultiple contributing causes – e.g. lungcancer, heart disease – so multipleexposures must be measured.
Smoking, age, diet, and genetic make-up are powerful interfering factors
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Limitations Continued
Measurement of exposure can behighly inaccurate, especially whenpast exposures are needed. Theusual result is called misclassifica-tion, and any underlying associationbetween exposure and illness islikely to be missed or underesti-mated
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Limitations Continued
Latency of many (most?) environmentaldiseases is years to decades.
Thus exposures from the distant past aremost relevant, and least likely to be knownquantitatively.
Longitudinal epidemiology, in whichexposed persons are followed overyears, is most precise.
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Limitations Continued
Examples of longitudinal studies:
Framingham, Mass. heart disease;
Fluoridation of water and dental caries;
Salk vaccine and polio incidence;
Smoking and several diseases.
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Limitations Continued
An observed association betweenenvironmental agent and disease shouldnot be termed a cause-effect relation-ship until a biological mechanism hasalso been demonstrated.
Otherwise, the observed epidemiologicoutcome could easily be a coincidence.
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HostAgent
Environment
Disease Causation
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Disease Causation
HostAgent
Environment
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HostAgent
Environment
Disease Control
Lesson 3: Epidemiology April 11, 2006
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Prevention & Control
Modify the environment
Engineering Controls
Modify Behavior
Legal/Regulatory Controls
Administrative Controls
Education
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Engineering Controls Tactics
Substitution
Treatment
Isolation
Shielding
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Control Tactics Continued
HostAgent
Substitution
Newcompound or
process
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Control Tactics Continued
HostAgent
Treatment
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Control Tactics Continued
HostAgent
Isolation
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Control Tactics Continued
Host
Shielding
Agent
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Regulatory Controls
Statutes
Rules and Regulations
Enforcement Programs
Private Sector Control
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Administrative Controls
Planning
Supervision
Biological Monitoring
Work Scheduling
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Education
Education
Training
Safety campaigns
Administrative priority
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Control Strategies Continued
In order of effectiveness
Engineering Control Tactics
Legal / Regulatory Controls
Administrative Controls
Education
How do we operationalize these?
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Summary
Epidemiology is the study of thedistribution and determinants ofhealth effects in human populations
Distribution
Person
Place
Time
Determinants
Agent
Host
Environment
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