the principal science of public health
the principal science of public health
Chapter 5
Epidemiology
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
Epidemiology Is …
… the study of the distribution and determinants of health and disease in human populations
(Harkness, 1995)
… the principal science of public health
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Historical Perspective
Investigations of disease pattern in the community; comparing people who had disease or who remained healthy
Person-Place-Time Model
Person: “Who” factors, such as demographic characteristics, health, and disease status
Place: “Where” factors, such as geographic location, climate and environmental conditions, political and social environment
Time: “When” factors, such as times of day, week, or month and secular trends over months and year
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Different Types of Epidemiology
Descriptive Epidemiology
Study of the amount and distribution of disease
Used by public health professionals
Identified patterns frequently indicate possible causes of disease
Analytic Epidemiology
Examine complex relationships among the many determinants of disease
Investigation of the causes of disease, or etiology
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Epidemiological Triangle
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Figure 5-1
Agent of Disease (Etiologic Factors)
Nutritive elements
Excesses, deficiencies
Chemical agents
Poisons, allergens
Physical agents
Ionizing radiation, mechanical
Infectious agents
Metazoa, protozoa, bacteria, fungi, rickettsia, viruses
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Host Factors–Intrinsic Factors (Susceptibility, or Response to Agent)
Genetic
Age
Sex
Ethnic group
Physiological state
Prior immunological experience
Active/, passive
Intercurrent or preexisting disease
Human behavior
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Environmental Factors— Extrinsic Factors…
… influence existence of the agent, exposure, or susceptibility to agent
Physical environment
Biological environment
Human populations, flora, fauna
Socioeconomic environment
Occupation, urbanization and economic development, disruption
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Wheel Model of Human-Environment Interaction
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Figure 5-2
Redrawn from Mausner JS, Kramer S: Mausner and Bahn epidemiology: an introductory text, ed 2, Philadelphia, 1985, Saunders.
Web of Causation
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Figure 5-3
From Friedman GD: Primer of epidemiology, ed 4, New York, 1994, McGraw-Hill.
Ecosocial Approach
Emphasize the role of evolving macro-level socioenvironmental factors along with microbiological process in understanding health and illness (Smith & Lincoln, 2011)
Challenges the more individually focused risk factor approach to understanding disease origins
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Calculation of Rates
Rates are arithmetic expressions that help practitioners consider a count of an event relative to the size of the population from which it is extracted
Number of health events in a specified period
Population in same area in same specified period
Proportion multiplied by a constant (k)
For example, the rate can be the number of cases of a disease occurring for every 1000, 10,000 or 100,000 people in the population
Can make meaningful comparisons
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Morbidity Rates
Incidence rates
New cases or conditions
Attack rate
Number of new cases of those
exposed to the disease
Prevalence rates
All cases of a specific
disease or condition at
a given time
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Prevalence Pot The relationship between incidence and prevalence
Figure 5-4
Redrawn from Morton RF, Hebel JR, McCarter RJ: A study guide to epidemiology and biostatistics, ed 3, Gaithersburg, MD, 1990, Aspen Publishers.
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Morbidity Rates (Cont.)
Incidence Rate
Prevalence Rate
Number of existing cases Total Population _____
Number of new cases _in given time period Population at risk in same time period ___75___ 4000–250
× 1000
= 0.02
0.02 × 1,000 = 20 per 1000 per time period
250
4000
= 0.0625
0.0625 × 1000 = 62.5 per 1000
Mortality Rates (routinely collected birth and death rates)
Other rates
Crude rates
Age-specific rates
Age-adjusted rates or standardization of rates
Proportionate mortality ratio (PMR)
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Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Number of deaths in year Total population size _1720_ 200,000
× 100,000
= 0.0086
Number of births in year Total population size _2900_ 200,000
× 100,000
= 0.0145
Concept of Risk
Risk—probability of an adverse event
Risk factor
Refers to the specific exposure factor
Often external to the individual
Attributable risk
Estimate of the disease burden in a population
Relative risk ratio
Divide the incidence rate of disease in the exposed population by the incidence rate of disease in the nonexposed population.
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Use of Epidemiology
Disease prevention
Primary prevention
Health promotion and specific prevention
Secondary and tertiary prevention
Establishing causality
Screening
Surveillance
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Use of Epidemiology (Cont.)
Health services
Used to describe the distribution of disease and its determinants in populations
Study population health care delivery
Evaluate use of community health services
Nurses must apply findings in practice
Incorporate results into prevention programs for communities and at-risk populations
Extend application into major health policy decisions
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Community health nurses should exercise “social responsibility” in applying epidemiological findings, but this will require the active involvement of the consumer.
Community health nurses collaborating with community members can combine epidemiological knowledge and aggregate-level strategies to affect change on the broadest scale.
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Epidemiological Methods
Descriptive epidemiology
Focuses on the amount and distribution of health and health problems within a population
Analytic epidemiology
Investigates the causes of disease by determining why a disease rate is lower in one population group than in another
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Analytic Epidemiology
Observational studies
Descriptive purposes
Etiology of disease
No manipulation by investigator
Cross-sectional studies
Sometimes called prevalence or correlational studies
Examine relationships between potential causal factors and disease at a specific time
Impossible to make causal inferences
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Analytic Epidemiology (Cont.)
Retrospective studies
Compare individuals with a particular condition or disease with those who do not have the disease
Data collection extends back in time
Prospective studies
Monitor a group of disease-free individuals to determine if and when disease occurs
Cohort shares a common experience within a defined time period
Monitors cohort for disease development
Copyright © 2015, 2011, 2007, 2001, 1997, 1993 by Saunders, an imprint of Elsevier Inc.
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Analytic Epidemiology (Cont.)
Experimental design
Also called a Randomized Clinical Trial (RCT)
Subjects assigned to experimental or control group
Apply experimental methods to test treatment and prevention strategies
Ethical considerations with human subject rights review
Also useful for investigating chronic disease prevention
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