Introduction_to_Epidemiology

8/7/2019 Introduction_to_Epidemiology

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Introduction to Epidemiology

ENVE 569

Environmental Risk Assessment

What is Epidemiology? Study of disease in populations

Study of patterns, causes and control ofdisease in populations

The branch of medicine that deals with thestudy of the causes, distribution, andcontrol of disease in populations.

The study of the spread of diseases withinand between populations.


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Definitions (from JAWWA Sept. 1996,

Craun et al.)

Association The dependence between two or more events,

characteristics or variables (e.g., exposure anddisease)

Association DOES NOT necessary imply cause-effectbetween the events and variables.

Carrier One who harbors a specific infectious agent and is a

potential source of infection

Case Person who has a particular disease or health

condition

Definitions (from JAWWA Sept. 1996,Craun et al.)

Causality Relating causes to tehir effects

Cohort People assembled on the basis of a common

characteristics and followed over time (maybe retrospectively or prospectively followed)to determine a health-related outcome

Control A person who does not have the condition

being studied.


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Craun et al.)

Endemic

The constant low-level presence of a disease orinfection.

Environmental epidemiology

Use of epidemiologic methods to assess humanhealth effects associated with environmentalcontaminants

Epidemic Occurrence of disease clearly in excess of thenumber of cases normally found or expected; ifconfined to a limited population, it is an outbreak.

Definitions (from JAWWA Sept. 1996,Craun et al.)

Etiologic agent Agent responsible for the postulated cause of

a disease.

Etiology Postulated cause(s) of disease, specifically

those that initiate the pathogenic medium.

Incubation period Time between first contact with an infectiousagent and the appearance of the first diseasesymptom.


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Craun et al.) Latency or Latent Period

Delay between exposure and disease The period between initiation of infection or disease to its

detection.

Risk Factor An exposure, characteristic, or aspect of personal

behavior (e.g., age, occupation) associated with anincreased probability of a health-related condition

Not necessarily a causal explanation

Secondary transmission Transmission among familial, institutional, or othercontacts.

Person who is sick did not have primary contact with thesource

Basic Measures of Disease

Frequency Incidence

Rate at which new cases of disease occur.

Prevalence

Measures both new and existing cases in apopulation with and without disease.

Proportion of people who have a specific condition atany specified time.

Attack rate Measures the cumulative incidence of disease during

an epidemic or outbreak.


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Geographic-Specific Attack Rates from aWaterborne Outbreak of Giardiasis in

Massachusetts

12.650,265635Total

12.434,351427Mixed

30.04,200126Reservoir C

6.12,30914Reservoir B

7.29,40568Reservoir A

Attack Rateper 1,000

PopulationCasesWater Source

Summary of John Snows Data

27/10,000173,748473Lambeth

160/10,000266,5164,267Southwarkand Vauxhall

Mortality RatePopulationCholeraDeaths

Water SupplyCompany


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Dose-response attack rates from a

waterborne outbreak of chronic diarrhea

57.1>30

37.511 30

12.51 10

Attack Rate per 100Water consumption duringoutbreak (glasses)

Morbidity and Mortality Morbidity

The relative incidence of a particular disease

Mortality

A fatal outcome


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Study Types

Descriptive Epidemiological Studies Information is only available about the occurrence of

disease or associations among exposures,demographic characteristics, and disease rates inpopulation groups

Analytical Studies Test hypotheses to evaluate exposure-disease

information

Ecological Studies Explore associations between routinely gathered

health and demographic statistics and otherinformation (such as environmental parameters).

Reporting of Study Results The likelihood of a positive association is

caused by random error can be estimatedby calculating the level of statisticalsignificance (p value) or the confidenceinterval (CI).

Epidemiologists typically use theconfidence interval. CI provides a range of possible values of the

risk estimate.


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Guide to the Strength of an Epidemiologic

Association

Very strong/infinite> 10.0

Strong3.1 10.0

Moderate1.5 3.0

Weak>1.0 -


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Confounding Bias

Confounders

Characteristics inherent in the population

studied that may affect the data leading to anerroneous interpretation of an association orcause

Characteristic must be associated with theexposure being evaluated, a risk factor for the

disease, and not part of the exposure-diseasepathway.

Evaluating Causality Temporal association

Study precision and validity

Strength of association

Consistency

Specificity

Biological plausibility

Dose-response relationship

Reversibility

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