the principal science of public health

Chapter 5

Epidemiology

Epidemiology Is …

… the study of the distribution and determinants of health and disease in human populations

(Harkness, 1995)

… the principal science of public health

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

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

Epidemiological Triangle

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

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

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

Wheel Model of Human-Environment Interaction

Figure 5-2

Redrawn from Mausner JS, Kramer S: Mausner and Bahn epidemiology: an introductory text, ed 2, Philadelphia, 1985, Saunders.

Web of Causation

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

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

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

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.

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)

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.

Use of Epidemiology

Disease prevention

Primary prevention

Health promotion and specific prevention

Secondary and tertiary prevention

Establishing causality

Screening

Surveillance

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

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.

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

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

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

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

the principal science of public health

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