Epidemiology in Epidemiology in Community Health Community Health

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Epidemiology Epidemiology is the study of the is the study of the determinants and distribution of determinants and distribution of health, disease, and injuries in human health, disease, and injuries in human populations. It is a specialized form of populations. It is a specialized form of scientific research that can provide scientific research that can provide health care workers, including health care workers, including community health nurses, with a body community health nurses, with a body of knowledge on which to base their of knowledge on which to base their practice and methods for studying practice and methods for studying new and existing problems. new and existing problems.

The term is derived from the Greek The term is derived from the Greek words words epi (upon), Demos (the epi (upon), Demos (the people), and logos (knowledge)people), and logos (knowledge): the : the knowledge or study of what happens knowledge or study of what happens to people.to people.

Epidemiologists ask such questions Epidemiologists ask such questions as the following: as the following:

What is the occurrence of health and disease in a What is the occurrence of health and disease in a population?population?

Has there been an increase or decrease in a health Has there been an increase or decrease in a health state over the years? state over the years?

Does one geographic area have a higher frequency of Does one geographic area have a higher frequency of disease than another?disease than another?

What characteristics of people with a particular What characteristics of people with a particular condition?condition?

What factors need to be present to cause disease or What factors need to be present to cause disease or injury?injury?

Is one treatment or program more effective than Is one treatment or program more effective than another in changing the health of affected people?another in changing the health of affected people?

Why do some people recover from a disease and Why do some people recover from a disease and others do not?others do not?

Concepts Basic to Concepts Basic to EpidemiologyEpidemiology : :

Host, Agent, and Environment Host, Agent, and Environment Model: Model:

Through their early study of infectious Through their early study of infectious diseases, epidemiologists began to diseases, epidemiologists began to consider disease states generally in consider disease states generally in terms of the epidemiologic triad, or the terms of the epidemiologic triad, or the host, agent, and environment model. host, agent, and environment model. Interactions among these three elements Interactions among these three elements explained infectious and other disease explained infectious and other disease patternspatterns. .

HostHost

The host is a susceptible human or The host is a susceptible human or animal who harbors and nourishes a animal who harbors and nourishes a disease-causing agent. Many physical, disease-causing agent. Many physical, psychological, and lifestyle factors psychological, and lifestyle factors influence the host's susceptibility and influence the host's susceptibility and response to an agent. response to an agent.

Physical factors include age, sex, Physical factors include age, sex, race, and genetic influences on the race, and genetic influences on the host's vulnerability or resistance. host's vulnerability or resistance.

Psychological factors, such as Psychological factors, such as outlook and response to stress, can outlook and response to stress, can strongly influence host strongly influence host susceptibility.susceptibility.

Lifestyle factors also play a major Lifestyle factors also play a major role. Diet, exercise, sleep patterns, role. Diet, exercise, sleep patterns, and healthy or unhealthy habits all and healthy or unhealthy habits all contribute to either increased of contribute to either increased of decreased vulnerability to the decreased vulnerability to the disease-causing agent. disease-causing agent.

AgentAgent

An agent is a factor that causes or An agent is a factor that causes or contributes to a health problem or contributes to a health problem or condition. Causative agents can be factors condition. Causative agents can be factors that are present (e.g., bacteria that cause that are present (e.g., bacteria that cause tuberculosis, rocks on a mountain road tuberculosis, rocks on a mountain road that contribute to an automobile crash) or that contribute to an automobile crash) or factors that are lacking (e.g., lack of iron factors that are lacking (e.g., lack of iron in the body that causes anemia; lack of in the body that causes anemia; lack of seat belt use that contributes to the extent seat belt use that contributes to the extent of injury during an automobile crash)of injury during an automobile crash)

EnvironmentEnvironment : :

The environment refers to all The environment refers to all the external factors the external factors surrounding the host that surrounding the host that might influence vulnerability might influence vulnerability or resistance. or resistance.

The physical environment includes: The physical environment includes: geography, climate, weather, safety of geography, climate, weather, safety of buildings, water and food supply, and buildings, water and food supply, and presence of animals, plants, insects, and presence of animals, plants, insects, and microorganisms that have the capacity microorganisms that have the capacity to serve as reservoirs (storage sites for to serve as reservoirs (storage sites for disease-causing agents) or vectors disease-causing agents) or vectors (carriers) for transmitting disease.(carriers) for transmitting disease.

The psychosocial environment refers to The psychosocial environment refers to

social, cultural, economic, and social, cultural, economic, and psychological influences and conditions psychological influences and conditions that affect health, such as access to that affect health, such as access to health care, cultural health practices, health care, cultural health practices, poverty, and work stressors.poverty, and work stressors.

CausalityCausality::

Causality refers to the Causality refers to the relationship between a cause relationship between a cause and its effect. A purpose of and its effect. A purpose of epidemiologic study has been to epidemiologic study has been to discover causal relationships, so discover causal relationships, so as to understand why conditions as to understand why conditions develop and offer effective develop and offer effective prevention and protection.prevention and protection.

Chain of CausationChain of Causation::

ImmunityImmunity : : Immunity refers to a host's Immunity refers to a host's

ability to resist a particular ability to resist a particular infectious disease-causing agent. infectious disease-causing agent. This occurs when the body forms This occurs when the body forms antibodies and lymphocytes that antibodies and lymphocytes that react with the foreign antigen react with the foreign antigen and render them harmless.and render them harmless.

Passive ImmunityPassive Immunity : :

Passive immunity refers to short-term Passive immunity refers to short-term resistance that is acquired either resistance that is acquired either naturally or artificially.naturally or artificially. Newborns, through maternal antibody Newborns, through maternal antibody

transfer, have natural passive immunity transfer, have natural passive immunity that lasts about 6 months. that lasts about 6 months.

Artificial passive immunity is attained Artificial passive immunity is attained through inoculation with a through inoculation with a immunoglobulin that gives temporary immunoglobulin that gives temporary resistance during outbreaks. resistance during outbreaks.

Active ImmunityActive Immunity : :

Active immunity is long-term and Active immunity is long-term and sometimes lifelong resistance that is sometimes lifelong resistance that is acquired either naturally or artificially. acquired either naturally or artificially. Naturally acquired active immunity comes Naturally acquired active immunity comes

through host infection. That is, a person through host infection. That is, a person who contracts a disease often develops who contracts a disease often develops long-lasting antibodies that provide long-lasting antibodies that provide immunity against future exposures. immunity against future exposures.

Artificially acquired active immunity is Artificially acquired active immunity is attained through vaccination. attained through vaccination.

Cross-ImmunityCross-Immunity : :

Cross-immunity refers to a situation in Cross-immunity refers to a situation in which a person's immunity to one agent which a person's immunity to one agent provides immunity to another related provides immunity to another related agent as well. The immunity can be either agent as well. The immunity can be either passive or active. passive or active. Sometimes, infection with one disease, such Sometimes, infection with one disease, such

as cowpox, gives immunity to a related as cowpox, gives immunity to a related disease, such as smallpox (active). disease, such as smallpox (active).

The concept of cross-immunity has also been The concept of cross-immunity has also been useful in the development and administration useful in the development and administration of vaccines. Inoculation with a vaccine made of vaccines. Inoculation with a vaccine made from one disease-causing organism can from one disease-causing organism can provide immunity to related disease-causing provide immunity to related disease-causing organism (active). organism (active).

Herd ImmunityHerd Immunity : :

Herd immunity describes the Herd immunity describes the immunity level that is present in a immunity level that is present in a population group. population group.

RiskRisk

To determine the chances that a disease To determine the chances that a disease or health problem will occur, or the or health problem will occur, or the probability that a disease or other probability that a disease or other unfavorable health condition will unfavorable health condition will develop. develop.

For any given group of people, the risk For any given group of people, the risk of developing a health problem is of developing a health problem is directly influenced by their biology, directly influenced by their biology, environment, lifestyle, and system of environment, lifestyle, and system of health care. health care.

Epidemiologists measure this using the Epidemiologists measure this using the relative risk ratio, which statistically relative risk ratio, which statistically compares the disease occurrence in the compares the disease occurrence in the population at risk with the occurrence of population at risk with the occurrence of the same disease in people without that the same disease in people without that risk factor.risk factor.

Relative risk ratioRelative risk ratio = = Incidence in exposed groupIncidence in exposed group

Incidence in Incidence in

unexposed groupunexposed group

Natural History of a Disease or Health Natural History of a Disease or Health

ConditionCondition:: Any disease or health condition follows progression Any disease or health condition follows progression

known as its natural history; this refers to events known as its natural history; this refers to events that occur before its development, during its that occur before its development, during its course, and during its conclusion. course, and during its conclusion.

The natural progression of a disease occurs in four The natural progression of a disease occurs in four stages – two stages referred to as prepathogenesis stages – two stages referred to as prepathogenesis (before the detectability of the disease or (before the detectability of the disease or condition) and two referred to as pathogenesis condition) and two referred to as pathogenesis (while the disease or condition is present ). The (while the disease or condition is present ). The four stages are susceptibility, adaptation, early four stages are susceptibility, adaptation, early pathogenesis, and clinical disease (Fig. 8-6).pathogenesis, and clinical disease (Fig. 8-6).

Epidemiology of Epidemiology of WellnessWellness::

Epidemiology has moved from Epidemiology has moved from concentrating only on illness to examining concentrating only on illness to examining how host, agent, and environment are how host, agent, and environment are involved in wellness at various levels. In involved in wellness at various levels. In response to an escalating need for improved response to an escalating need for improved methods of health planning and health methods of health planning and health policy analysis, epidemiology has developed policy analysis, epidemiology has developed more holistic models of health. These newer more holistic models of health. These newer epidemiologic models are organized around epidemiologic models are organized around four attributes that influence health: four attributes that influence health:

1.1. The physical, social, and The physical, social, and psychological environment; psychological environment;

2.2. Lifestyle with its self-created risks; Lifestyle with its self-created risks;

3.3. Human biology and genetic Human biology and genetic influences; influences;

4.4. And the system of health care And the system of health care organization organization

Causal RelationshipsCausal Relationships::

One of the main challenges to One of the main challenges to epidemiology is to identify causal epidemiology is to identify causal relationships in disease and health relationships in disease and health conditions in populations. conditions in populations.

A A cross-sectional studycross-sectional study (which explores a (which explores a health condition's relation to other variables in a health condition's relation to other variables in a specified population at a specific point in time) specified population at a specific point in time) can show that the factor and the problem coexist. can show that the factor and the problem coexist.

A A retrospective studyretrospective study (which looks backward in (which looks backward in time to find a causal relationship) allows a fairly time to find a causal relationship) allows a fairly quick assessment of whether an association quick assessment of whether an association exists. exists.

A A prospective studyprospective study (which looks forward in (which looks forward in time to find a causal relationship) is crucial to time to find a causal relationship) is crucial to ensure that the presumed causal factor actually ensure that the presumed causal factor actually precedes the onset of the health problem.precedes the onset of the health problem.

Finally,Finally, if ethically possible, and if ethically possible, and experimental studyexperimental study (in which the (in which the investigator controls or changes factors investigator controls or changes factors suspected of causing the condition and suspected of causing the condition and observes results) is used to confirm the observes results) is used to confirm the associations obtained from observational associations obtained from observational studies. studies.

Epidemiologically, a causal relationship Epidemiologically, a causal relationship may be said to exist if two major may be said to exist if two major conditions are met: conditions are met:

1.1. The factor of interest (causal agent) is shown The factor of interest (causal agent) is shown to increase the probability of occurrence of to increase the probability of occurrence of the disease or condition as observed in many the disease or condition as observed in many studies in different populations, studies in different populations,

2.2. And evidence suggests that a reduction in the And evidence suggests that a reduction in the factor decreases the frequency of the given factor decreases the frequency of the given disease. disease.

Sources of Information for Sources of Information for Epidemiologic studyEpidemiologic study : :

Epidemiologic investigators may Epidemiologic investigators may draw data from any of three major draw data from any of three major sources: existing data, informal sources: existing data, informal investigations, and scientific studies. investigations, and scientific studies.

I. Existing DataI. Existing Data::

A variety of information is available A variety of information is available nationally, by state, and by section, nationally, by state, and by section, such as county, region, or urbanized such as county, region, or urbanized area. area.

This information includes This information includes vital vital statistics, census data, and statistics, census data, and morbidity statisticsmorbidity statistics::

Vital StatisticsVital Statistics : :

Vital statistics refers to the Vital statistics refers to the information gathered from ongoing information gathered from ongoing registration of births, deaths, registration of births, deaths, adoptions, divorces, and marriages. adoptions, divorces, and marriages. Certification of births, deaths, and Certification of births, deaths, and fetal deaths are the most useful vital fetal deaths are the most useful vital statistics in epidemiologic studies. statistics in epidemiologic studies.

Census DataCensus Data : :

Data from population censuses taken Data from population censuses taken every 10 years in many countries are every 10 years in many countries are the main source of population the main source of population statistics. statistics.

Population statistics can be analyzed Population statistics can be analyzed by age, sex, race, ethnic background, by age, sex, race, ethnic background, type of occupation, income, marital type of occupation, income, marital status, educational level, or other status, educational level, or other standards, such as housing quality. standards, such as housing quality.

Reportable DiseasesReportable Diseases : :

Each state has developed laws or Each state has developed laws or regulations that require health regulations that require health organizations and practitioners to organizations and practitioners to report to their local health authority report to their local health authority cases of certain communicable and cases of certain communicable and infectious diseases that can be spread infectious diseases that can be spread through the community. This reporting through the community. This reporting enables the health department to take enables the health department to take the most appropriate and efficient the most appropriate and efficient action. action.

Disease RegistriesDisease Registries::

Some areas or states have disease Some areas or states have disease registries with major public health registries with major public health impact. Cancer registries provide impact. Cancer registries provide useful incidence, prevalence, and useful incidence, prevalence, and survival data and assist the survival data and assist the community health nurse in community health nurse in monitoring cancer patterns within a monitoring cancer patterns within a community. community.

Environmental Environmental MonitoringMonitoring : :

State governments, through health State governments, through health departments, now monitor health departments, now monitor health hazards found in the environment. hazards found in the environment. Pesticides, industrial wastes, radioactive Pesticides, industrial wastes, radioactive or nuclear materials, chemical additives or nuclear materials, chemical additives in foods, and medicinal drugs have in foods, and medicinal drugs have joined the list of pollutants. joined the list of pollutants.

Concerned community members and Concerned community members and leaders view these as risk factors that leaders view these as risk factors that affect health at both community and affect health at both community and individual levels. individual levels.

II. Informal Observational II. Informal Observational StudiesStudies::

A second information source in epidemiologic A second information source in epidemiologic study is informal observation and description. study is informal observation and description. Almost any client group encountered by the Almost any client group encountered by the community health nurse can trigger such a community health nurse can trigger such a study. study.

If several cases of diabetes come to the If several cases of diabetes come to the attention of a nurse serving on a Navajo attention of a nurse serving on a Navajo reservation, a widespread problem might reservation, a widespread problem might come to light through informal inquiries about come to light through informal inquiries about the incidence and age at onset of the disease the incidence and age at onset of the disease among this Native American population. among this Native American population.

III. Scientific StudiesIII. Scientific Studies::

The third source of information used The third source of information used in epidemiologic inquiry involves in epidemiologic inquiry involves carefully designed scientific studies.carefully designed scientific studies.

→→ Systematic studies such as these, as Systematic studies such as these, as

well as informal studies and existing well as informal studies and existing epidemiologic data, can provide the epidemiologic data, can provide the community health nurse with valuable community health nurse with valuable information that can be used to information that can be used to positively affect aggregate health. positively affect aggregate health.

Methods in the Methods in the Epidemiologic Investigative Epidemiologic Investigative

ProcessProcess

The goals of epidemiologic investigation The goals of epidemiologic investigation are to identify the causal mechanisms of are to identify the causal mechanisms of health and illness states and to develop health and illness states and to develop measures for preventing illness and measures for preventing illness and promoting health. Epidemiologists employ promoting health. Epidemiologists employ an investigative process that involves an investigative process that involves sequence of three approaches that build sequence of three approaches that build on one another: descriptive, analytic, and on one another: descriptive, analytic, and experimental studies. experimental studies.

I. Descriptive I. Descriptive EpidemiologyEpidemiology

Descriptive epidemiology includes Descriptive epidemiology includes investigations that seek to observe investigations that seek to observe and describe patterns of health-and describe patterns of health-related conditions that occur related conditions that occur naturally in a population. naturally in a population.

For example,For example, how many children in a how many children in a school district have been immunized school district have been immunized for measles, how many home births for measles, how many home births occur each year in the country. occur each year in the country.

CountsCounts

The simplest measure of description The simplest measure of description is a count. is a count.

RatesRates : :

Rates are statistical measures expressing Rates are statistical measures expressing the proportion of people with a given the proportion of people with a given health problem among a population at health problem among a population at risk. The total number of people in the risk. The total number of people in the group serves as the nominator for various group serves as the nominator for various types of rates. types of rates.

Several rates have wide use in Several rates have wide use in epidemiology. Those most important for epidemiology. Those most important for the community health nurse to the community health nurse to understand are the prevalence rate, and understand are the prevalence rate, and the incidence rate. the incidence rate.

PrevalencePrevalence refers to all of the people refers to all of the people with a particular health condition with a particular health condition existing in a given population at a existing in a given population at a given point in time. given point in time.

The prevalence rateThe prevalence rate describes a describes a situation at a specific point in time. situation at a specific point in time.

If a nurse discovers 50 cases of measles in If a nurse discovers 50 cases of measles in an elementary school, that is a simple an elementary school, that is a simple count. If that number is divided by the count. If that number is divided by the number of students in the school, the number of students in the school, the result is the prevalence of measles, for result is the prevalence of measles, for instance, if the school has 500 students, instance, if the school has 500 students, the prevalence of measles on that day the prevalence of measles on that day would be 10% (50 measles /500 would be 10% (50 measles /500 population) population)

Prevalence rate = Prevalence rate = number of persons with a number of persons with a characteristiccharacteristic

Total number in Total number in populationpopulation

IncidenceIncidence refers to all new cases of refers to all new cases of a disease or health condition a disease or health condition appearing during a given time. appearing during a given time. Incidence rateIncidence rate describes a describes a proportion in which the numerator is proportion in which the numerator is all new cases appearing during a all new cases appearing during a given period of time and the given period of time and the denominator is the population at risk denominator is the population at risk during the same period. during the same period.

To describe The To describe The morbidity ratemorbidity rate, , which is the relative incidence of which is the relative incidence of disease in a population, the ratio of disease in a population, the ratio of the number of sick individuals to the the number of sick individuals to the total population is determined. The total population is determined. The mortality ratemortality rate refers or sum of refers or sum of deaths in a given population at a deaths in a given population at a given time. given time.

II. Analytic EpidemiologyII. Analytic Epidemiology

Case-Control StudiesCase-Control Studies::

Case-control studies compare people Case-control studies compare people who have a health or illness who have a health or illness condition (number of cases with the condition (number of cases with the condition) with those who lack this condition) with those who lack this condition (controls). These studies condition (controls). These studies begin with the cases and look back begin with the cases and look back over time for presence or absence of over time for presence or absence of the suspected causal factor in both the suspected causal factor in both cases and controls. cases and controls.

Cohort StudiesCohort Studies::

Cohort studies, rather study the Cohort studies, rather study the development of a condition over development of a condition over time. A cohort study begins by time. A cohort study begins by selecting a group of people who selecting a group of people who display certain defined display certain defined characteristics before the onset of characteristics before the onset of the condition being investigated the condition being investigated

III. Experimental III. Experimental EpidemiologyEpidemiology

Is used to study epidemics, the etiology of human Is used to study epidemics, the etiology of human disease, the value of preventive and therapeutic disease, the value of preventive and therapeutic measures, and the evaluation of health services. measures, and the evaluation of health services.

Experimental studies are carried out under Experimental studies are carried out under carefully controlled conditions. The investigator carefully controlled conditions. The investigator exposes an experimental group to some factors exposes an experimental group to some factors thought to cause disease, improve health, thought to cause disease, improve health, prevent disease, or influence health in some way prevent disease, or influence health in some way (as in the Women's Health Study). (as in the Women's Health Study). Simultaneously, the investigator observes a Simultaneously, the investigator observes a control group that is similar in characteristics to control group that is similar in characteristics to the experimental group but without the exposure the experimental group but without the exposure factor. factor.