OBJECTIVES:

1. Discuss the application of epidemiologic principles to nursing practice and to the improvement of the health status of populations across the life span.
2. Explain the concepts of agent/host/environment and levels of prevention as related to epidemiology.
3. Explain the significance of the following terms in epidemiology: rate, risk, mortality rate, morbidity rate, prevalence, incidence, epidemic, endemic, pandemic.
4. Discuss the major study designs utilized in epidemiologic studies.
5. Discuss reliability and validity as related to tests in epidemiology.

Reference Text: Comprehensive Community Health Nursing by S. Clemen-Stone, S. McGuire,and D. Eigsti. (2002). 6^th edition. St. Louis : C.V. Mosby.
      Read Chapter 11(pp. 332-369).

Additional Reference: Epidemiology in Nursing Practice by G. Harkness, (1995) , St. Louis: C.V. Mosby
 

Internet Sites:

Healthy People 2010 initiative-- see    http://www.healthypeople.gov/

National Statistics and info. Re: CDC, surveillance, etc.
  http://www.cdc.gov/nchs/
  For statistics ,  click on fast stats A-Z---- this is the site for incidence and prevalence rates for various diseases, as well as other stats which are required in your community assessment later in the program.

MS STATE BOARD OF HEALTH--- info/statistics for MS ---  http://www.msdh.state.ms.us/phs/statisti.htm
 

Definition

Epidemiology is the science of health events affecting populations, investigating the distribution and determinants of those events, and focusing on characterizing a health outcome in terms of what, who, where, when, and why---What is the disease? Who is affected? Where is this occurrence? When does it occur? Why does it occur? Epidemiology once included primarily infectious diseases, but has expanded to include chronic diseases, e.g. cancer, cardiovascular disease; and also mental health problems, accidents, injuries/violence, occupational/environmental exposures and their effects, and positive health states. Epidemiological methods are used now to study health-related behaviors, such as physical activity, and in health services research. (Stanhope/Lancaster, 1996) Epidemiology can be considered both as a methodology used to study human health conditions and as the body of knowledge that results from the study of a specific health condition.

Epidemiologic studies provide information about the distribution of illness and wellness, and that information can be used to investigate the factors that determine why that particular state of health exists. Through the study of health problems as they occur in groups or populations, many characteristics of specific illnesses or disabilities can be identified that may not be evident in the study of individuals. For example, the cause of lung cancer probably would not have been discovered in a study of individuals with lung cancer. Groups of people with lung cancer have been compared with groups of people without lung cancer. It has been demonstrated clearly that more people with lung cancer had smoked cigarettes than those without lung cancer. It is only by observing large groups that similarities and differences between people who have, or do not have, a particular condition can be identified. Often people have certain characteristics or engage in certain activities that increase their potential for becoming ill or for developing a health problem. When these associations are found, preventive health measures can be instituted for those populations at risk, even if the physiologic or environmental causes have not yet been identified.

In recent years, epidemiologists have begun to question basic tenets of the field. Two forces have been identified which are resulting in a new era in epidemiology: 1) a transformation in global health patterns and 2) new technology.  1) An example of the change in global health patterns is the HIV in Africa, where AIDS is the leading cause of death despite current knowledge about the spread and prevention of this disease. Epidemiologists know what behaviors need to change, but they don't know how to change them. This is due to lack of understanding of societal forces which are active in determining the status of health and disease for GROUPS of people.
Factor 2-new technology-is being used to help epidemiologists to clarify disease processes and not just causal factors. Also, advances in global communication networks provide epidemiologists with immediate access to vital data. This technology assists epidemiologists to recognize patterns of health and disease, large-scale events, and systems in the social context of their occurrence.

When caring for individual clients, nurses use the nursing process to guide their decision-making. In a similar way, the epidemiologic process should guide problem-solving activities when focusing on groups of people or populations. In epidemiology, the individual patient is replaced by the community as the primary focus of concern. Nurses use the body of epidemiologic knowledge about various states of health and illness in clinical decision-making; in assessing individuals and populations at risk; and in planning, implementing and evaluating nursing care and health services. (Harkness,1995) The Community Assessment you will do over the course of the next three semesters is an example of use of the nursing process and epidemiologic knowledge.

Basic Concepts in Epidemiology

To use the epidemiological process effectively, community health nurses need to have an understanding of the basic concepts, tools, and terms of epidemiology.

Concept: Aggregates at Risk-
A key concept of epidemiology is that the study of disease in populations is more significant than the study of individual cases of disease.  The identification of commonalities and differences among groups focuses attention on the essential or contributory factors that produce illness or promote health. A preventive health philosophy has led professionals in community health to emphasize the study of groups. Identification of aggregates at risk is one goal of epidemiologic study and allows for the initiation of preventive health measures. Aggregates at risk are defined as those who engage in certain activities or who have certain characteristics that increase their potential for contracting a disease, injury, or health problem, i.e., the presence of risk factors. Risk factors are determined by a risk estimate process. Risk estimates are derived by contrasting the frequency of a disease or health condition in persons exposed to a specific trait or risk factor and the frequency in another group not exposed to a risk factor.  Risk factors fall into three major categories: 1) behavioral or lifestyle patterns; 2) inborn or inherited characteristics; and 3) environmental factors.  These factors increase one's susceptibility to death, disease, and injury.

Concept: Epidemiologic Triangle- Agent, Host, Environment

Disease results from complex relations among causal agents(agent), susceptible persons(hosts), and environmental factors. Changes in one of these elements can influence the occurrence of disease by increasing or decreasing a person's risk-- the probability a person will become ill-- for disease. (See p. 342-345 in text for discussion of agent, host, etc.) The term web of causation(p. 337) is used to describe the complex interrelationships of numerous factors interacting to increase, or decrease, risk of disease. The theory of multiple causation(p. 337) illustrates and confirms that it is the interactions and relationships between host-agent-environment that actually cause a disease or condition.

Concept:Natural Life History of Disease

The natural life history of disease is defined as "the course of a disease from onset (inception) to resolution." Many diseases have certain well-defined stages that, taken all together, are referred to as its natural history. Historically, there are 2 distinct periods identified in the natural history of a disease:prepathogenesisand pathogenesis. In the prepathogenesis period, the disease has not developed but interactions are occurring between the host, agent, and environment that produce disease stimulus and increase the host's potential for disease. The pathogenesis period begins when disease-producing stimuli start to produce changes in the tissues of humans. Preventive interventions can alter the natural life history of many diseases in man.
(See textbook, p. 343)
 

Levels of Prevention

The goal of epidemiology is to understand causal factors well enough to devise interventions to prevent adverse events before they begin. There are three levels of prevention described in public health: primary, secondary, and tertiary. These levels are tied to stages of the course of a disease process from onset to resolution:
Primary-- intervention in the period before disease occurs; involves interventions that promote health and prevent diseases from developing; aimed at individuals who are susceptible to disease-example: immunizations.
Secondary--intervention during the preclinical or clinical disease state; purpose is to detect disease in early stages in order to reverse or reduce the severity or provide a cure; example: screening tests for cancer such as PAP or PSA, BP checks.
Tertiary-- intervention during the resolution phase; directed toward persons with active disease; aim is to shorten the course of the disease, reduce disability, or rehabilitate; examples: physical therapy, corrective surgery.
(See chart in textbook, p. 343)
 

BIOSTATISTICS/VITAL STATISTICS

Collection of data in epidemiologic investigations is from three sources: 1) routinely collected data, such as census data, vital records, and surveillance data from the CDC; 2) data collected for other purposes but useful for epidemiology, such as medical records; and 3) original data collected for specific epidemiologic studies. Statistical concepts and methods are the undergirding for data collection and analysis regardless of the source of the data.

Epidemiologic studies are designed to investigate a particular question in epidemiology. One of the first steps in the epidemiologic investigation of the distribution and the determinants of a health care problem is to describe the problem statistically in terms of person, place, and time: BIOSTATISTICS.Following are some terms used commonly in epidemiologic studies(also see chart in text, p. 351.):

* rate= a measure of the frequency of a health event in a defined population during a specified period of time; consists of a numerator, a denominator, and an indication of the time period applicable; example: 50 /100,000/yr , 50=no. of cases(numerator), 100,000= population at risk(denominator), yr.= time period.

*endemic= refers to the constant presence of a disease within a geographic area or a population

*epidemic= the rate of disease, injury, or other condition is clearly in excess of the usual level of that condition

*pandemic= refers to an epidemic occurring worldwide and affecting large populations

HIV/AIDS can be referred to as both epidemic and pandemic, since the no. of cases is growing rapidly across various regions of the world as well as in the U.S.

*risk= the probability an event will occur within a specified time period

*mortality rate=no. of deaths for a specified population over a particular time period

In giving rates for a small population, the rates are multiplied by a scaling factor to avoid a small fraction. Mortality rates are usually calculated for a specific population group, i.e., a specific age group. Variables such as age can be taken into account by making adjustments based on standard populations.
Note : the infant mortality rate is used as an international indicator of overall health and measure of health services availability.

*morbidity rate= no. of persons experiencing disease/disability at a particular time ; similar to* prevalence rate, which is a measure of existing disease/condition in a population at a particular time( no. of existing cases divided by the current population) . Prevalence rate is not an estimate of the risk of developing disease because it is a function of both the rate at which new cases of the disease develop and how long those cases last. (prevalence = incidence x duration) Prevalence is useful in planning health care services since it is an indication of the level of disease existing in the population and therefore indicates those in need of health care services. Morbidity statistics are difficult to evaluate because there is no comprehensive surveillance system that monitors the incidence of all conditions contributing to morbidity. Hospital discharge records, cancer registry data, and communicable disease surveillance data are used to obtain select information about the extent and distribution of important health problems.

*incidence rate=no. of new cases developing in a population at risk during a specified time; also called occurrence rate. It estimates the risk of developing the disease in the observed population within a specified time. Note that existing cases are excluded from the population for this calculation since they already have the condition. Incidence rates are useful in determining disease etiology, since incidence is affected only by factors related to the risk of developing disease and not to survival or cure.
Priority rates that are actively reviewed on a regular basis are morbidity, mortality, incidence and prevalence rates. Morbidity and mortality rates are used to determine the effectiveness of health care programs. These data provide us with the who, where, and when characteristics of the problem and can be used to examine the reasons why and how the problem occurs. This is done by examining the data for differences, areas of agreement, and variations in frequency of the characteristics between groups.

On a national level, the Centers for Disease Control and Prevention (CDC) is the major agency responsible for public health surveillance. "Epidemiologic surveillance is the ongoing and systematic collection, analysis, and interpretation of health data in the process of describing and monitoring a health event. This information is used for planning, implementing, and evaluating public health interventions and programs...." Each state has a department responsible for morbidity reporting to the CDC. The CDC in turn reports to the World Health Organization.
Although reporting by states to the CDC is voluntary, there are some conditions/diseases which are categorized as "notifiable" disease(52 infectious diseases, 3 noninfectious conditions, and 1 health risk behavior).
 

THE EPIDEMIOLOGIC PROCESS AND INVESTIGATION

The previously discussed concepts in epidemiology aid in identifying variables that are considered when community health professionals describe the distribution patterns and determinants of health, disease, and conditions in populations. These concepts help to analyze causal relationships in diseases or conditions. To establish these causal relationships, health professionals use the epidemiological process.

The epidemiological process is a systematic course of action taken to identify (1) who is affected:
(2) where the affected persons reside; (3) when the persons were affected; (4) causal factors of health and disease occurrence; (5) prevalence and incidence of health and disease; and (6) prevention and control measures in relation to the natural life history of the disease/condition.

There are eight basic steps in the epidemiological process:

Step 1: Determine the nature, extent, and possible significance of the problem-this is accomplished through data collection from multiple sources and then determining the extent and possible significance of the problem. Data collection is done via clinical observations, laboratory studies, and lay reporting (from the community). Analysis of the data in terms of person, place, and time help to quantify the problem(prevalence and incidence rates) and to identify possible causal relationships. This leads to the second step of the process:

Step 2- Formulation of tentative hypothesis(es). This step occurs rather rapidly and the hypotheses are tested out using laboratory tests and further data collection may be done.

Step 3- Collect and analyze further data to test hypothesis . Attack rates, chain of transmission and environmental conditions are studied. Epidemiological health histories are used to collect individual data and additional laboratory tests may be done.

Step 4- Plan for control. Preventive activities based on knowledge of the natural history of the disease in question are identified. Host-agent-environment factors are analyzed to determine populations at risk and the preventive measures directed toward breaking the chain of transmission are identified.

Step 5- Implement the control plan. This plan must involve the community. Goals and objectives must be set before the program begins. Control measures involve primary, secondary, and/or tertiary preventive activities, and include such activities as disease reporting, quarantine, environmental control, health education, immunizations, and technological advances. Some conditions do not lend themselves to control, e.g. those of unknown etiology, those with no known treatment . There are many other barriers to the success of control plans. Community health nurses must be aware of these and work to overcome them, if possible.

Step 6- Evaluate the control plan. Determines how well the objectives were met. Also looks at how the situation exists in comparison to the situation before the investigation. It also evaluates the control measures to see if they have been effective.

Step 7- Make appropriate report. Prompt, accurate, and concise epidemiological reporting of what was involved in the entire epidemiologic process: diagnosis, factors leading to the epidemic, control measures, process evaluation, and recommendations for preventing similar situations.

Step 8- Conduct research. Essential to the improvement of health services to the community.

Draws from the data accumulated from varied situations/conditions. Is aimed at the prevention and control of infectious diseases, chronic conditions, and other health-related phenomena in populations.
 
 
 

Types of Epidemiological Investigations

Epidemiologic investigators draw data from three major sources of information: 1) existing data from the census bureau and reportable disease statistics; 2) informal investigations and inquiry and 3) carefully designed scientific studies. Two basic methods of investigation for epidemiologic studies are observational and experimental. Observational studies can be either descriptive studies, which seek to describe health-related conditions as they naturally occur, and analytic studies, which look for causal factors.

The nature of epidemiological investigation, because it involves the health of human beings, tends to use observational rather than experimental approaches to research design. Thus those persons engaged in epidemiological research frequently observe rather than manipulate variables believed to influence the health of the human population. This means that the researcher has far less control of the factors under investigation . Epidemiological studies do, however, identify nonrandom patterns of health and disease, and serve as the basis for determining the circumstances in which experimental studies would be beneficial.

Three major types of epidemiological studies have been identified:

1) Descriptive Studies - These studies usually describe the amount and distribution of disease within a population. This approach relies primarily on the use of existing data and answers the following questions:

• •Who is affected?(Person)
• •Where is the disease distributed in the human population? (Place)
• •When is the disease present ?(Time)
• •What is the overall effect of the disease? (Population)

3) Experimental Trials-These trials begin with carefully designed questions, hypotheses, and research protocols that specify the criteria for selection of the subjects to be studied, the procedures for random assignment of the experimental and control groups, the treatment procedure, the follow-up of subjects, and the details of the data collection and analysis. In experimental studies the researcher always manipulates variables, such as a nursing intervention or a health teaching approach.
 

Causality

One of the first steps in assessing the relation of some factor with a health outcome is determining whether a statistical association exists-- the statistical element of probability. If a statistical test indicates a probability of statistical significance that is positive, then a statistical association is present; if there is no statistically significant amount of difference, then no association is assumed. Remember that sample size affects statistical significance.
One may also observe a statistically significant result because of bias, a systematic error due to the study design. Bias may occur during the selection procedure for study subjects, during the collection of data, and/or during the interpretation of data.
The existence of a statistical association does not necessarily mean there is a causal relation.
The observed association may be a random event due to chance or may be due to bias in the study. Statistical associations, although necessary to an argument for causality, are not sufficient proof.
 

Screening
One major activity at the primary and secondary levels of prevention of illness is screening. Screening is performed to either detect individuals who are at risk for disease or to identify those with early or asymptomatic disease. Remember that screening tests are not diagnostic--they must be followed up by more specific diagnostic procedures. Screening programs, therefore, must include provision for follow-up and referral for further diagnosis/treatment.

Reliability and Validity-- in reference to screening tests:
You will see these terms again in Research. They mean essentially the same thing here as in other areas of tests/ measurement. Reliability of a measure is its consistency or repeatability and its accuracy: Does it measure what we say it measures and does it do that accurately?
There are three major sources of error affecting the reliability of tests:
1) variation inherent in the tested element itself (i.e. if BP is the tested element, there are normal variations in BP throughout the day so that readings taken at different times of day may vary normally).
2) observer variation-- variations consistently by the same observer(intraobserver) and variations by different observers in the same situation(interobserver).
3) inconsistency in the instrument-- internal consistency( the instrument does not measure the same thing every use) or stability of an instrument over time is lacking(the instrument does not measure the same way over time).

Validity in a screening test is measured in terms of the probability of correctly classifying an individual with regard to the disease or outcome in question. It is usually measured by sensitivity-- how accurately the test identifies those with the condition/trait( the proportion of those who do have the trait and are identified as positive by the test) and specificity-- how accurately the test identifies those without the disease ( the proportion of those who do not have the trait and are identified as negative by the test). High degrees of sensitivity and specificity are needed in screening tests but are not always achievable-- increasing one sometimes decreases the other, i.e. increasing the sensitivity of a test may increase the no. of false positives obtained in test subjects, thereby decreasing the specificity.
 

Application of Epidemiology in Nursing

Epidemiology provides information needed by nurses in assessing needs, identifying problems, formulating appropriate strategies for study of problems, setting priorities in developing a plan of care, and evaluating the effectiveness of care. Nurses in all areas of clinical practice and function rely on epidemiologic information as the basis of their nursing practice.
 
 

MATERIALS IN THIS MODULE PREPARED FROM THE FOLLOWING REFERENCES:

Comprehensive Community Health Nursing by Clemen-Stone/McGuire/Eigsti;
Community Health Nursing by Stanhope/Lancaster;
Epidemiology in Nursing Practice by Harkness;
Community Health Nursing :Theory and Practice. C. M. Smith and F. A. Maurer. 1995. Philadelphia: W. B. Saunders Company.