Epidemiology in Community Health Care Chapter 7

Chapter outline Contents Historical Roots of Epidemiology (ch7) Concepts basic to epidemiology (ch7) Sources of information for epidemiologic study (ch7) Methods in the epidemiologic investigative process (ch7) Conducting epidemiologic research (ch7) 

Introduction Epidemiology concerned with the distribution and determinants of health and disease, morbidity, injuries, disabilities, and mortality in populations.

Introduction Epidemiologists ask questions as the following: • What is the occurrence of health and disease in a population? • Has there been an increase or decrease in a health state over the years? • Does one geographic area have a higher frequency of disease than another?

Introduction Epidemiologists ask questions as the following (cont): What characteristics of people with a particular condition distinguish them from those without the condition? • What factors need to be present to cause disease or injury? • Is one treatment or program more effective than another in changing the health of affected people? • Why do some people recover from a disease and others do not?

Historical roots of epidemiology An Epidemic refers to a disease occurrence that clearly exceeds the normal or expected frequency in a community or region. In past centuries, epidemics of cholera, bubonic plague, and smallpox swept through community after community, killing thousands of people, changing the community structure, and altering the lifestyle of masses of people. When an epidemic, such as acquired immunodeficiency syndrome (AIDS), is worldwide in distribution, it is called a pandemic

Cont… The continuing presence of a disease or infectious agent in a given geographic area, such as plague in Vietnam or malaria in the tropics of Brazil and Indonesia, means that the disease is endemic to that area. As the threat of great epidemic diseases declined; epidemiologist started to focus on other infectious diseases.

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

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

Host (cont) Physical factors include age, sex, race, and genetic influences on the host’s vulnerability or resistance. Psychological factors, such as outlook and response to stress, can strongly influence host susceptibility. Lifestyle factors also play a major role. Diet, exercise, sleep patterns and healthy or unhealthy habits all contribute to either increased or decreased vulnerability to the disease-causing agent.

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

Types of agents Agents vary considerably and include five types: biologic, chemical, nutrient, physical, and psychological. Biologic agents include bacteria, viruses, fungi, protozoa, worms, and insects. Some biologic agents are infectious, such as influenza virus or HIV.

Types of agents (cont) Chemical agents may be in the form of liquids, solids, gases, dusts, or fumes. Examples are poisonous sprays used on garden pests and industrial chemical wastes. The degree of toxicity of the chemical agent influences its impact on health.

Types of agents (cont).. Nutrient agents include essential dietary components that can produce illness conditions if they are deficient or are taken in excess. For example, a deficiency of niacin ( Vitamin B 3) can cause pellagra (marked by dermatitis, diarrhea, and disorder of the central nervous system), and too much vitamin A can be toxic.

Types of agents (cont)… Physical agents include anything mechanical (e.g., chainsaw, automobile), material (rockslide), atmospheric (ultraviolet radiation), geologic (earthquake), or genetically transmitted that causes injury to humans. The shape, size, and force of physical agents influence the degree of harm to the host. Psychological agents are events that produce stress leading to health problems.

Types of agents (cont)… Agents may also be classified as either infectious or noninfectious. Infectious agents cause diseases, such as AIDS or tuberculosis, that are communicable—that is, the disease can be spread from one person to another. Certain characteristics of infectious agents are important for community health nurses to understand.

Types of agents (cont)… Extent of exposure to the agent, the agent’s pathogenicity (capacity to cause disease), its infectivity (ability to enter the host and multiply), its virulence (severity of disease), and its structure and chemical composition all influence the effects of the agent on the host. Noninfectious agents have similar characteristics in that their relative abilities to harm the host vary with type of agent and intensity and duration of exposure

Environment (third component of Host, agent and Environment model) The environment refers to all the external factors surrounding the host that might influence vulnerability or resistance. The physical environment includes factors such as geography, climate, weather, safety of buildings, water and food supply, and presence of animals, plants, insects, and microorganisms that have the capacity to serve as reservoirs (storage sites for disease-causing agents) or vectors (carriers) for transmitting disease.

Environment (cont) The psychosocial environment refers to social, cultural, economic, and psychological influences and conditions that affect health, such as access to health care, cultural health practices, poverty, and work stressors, which can all contribute to disease or health. Host, agent, and environment interact to cause a disease or health condition.

Causality Causality refers to the relationship between a cause and its effect. A purpose of epidemiologic study has been to discover causal relationships, so as to understand why conditions develop and offer effective prevention and protection. Over the years, however, as scientific knowledge of health and disease has expanded, epidemiology has changed its view of causality

Chain of Causation As the scientific community’s thinking about disease causation and the tripartite model (host-agent-environment) grew more complex. Epidemiologists began to use the idea of a chain of causation (See the figure (next slide))

Causation in Noninfectious Disease With the availability of vaccines and antibiotics, attention shifted to the causes of noninfectious diseases such as cancer and diabetes. The linear linking embodied in models such as the chain of causation were insufficient in understanding the causes of the emerging health threats.

Multiple Causation Health care professionals realize that causation was never completely straightforward. They understand the complexity of many infectious and noninfectious threats In multiple causation the causative agent is only part of equation.

Web of causation Multiple causation concept emerged to explain the existence of health and illness states and to provide guiding principles of epidemiologic practice A causal paradigm that gained attention was referred to as web of causation The implications was intervention (or breaking of the web at any point nearest to the disease) could have an impact on the development of the disease.

Web of Causation in infant mortality

Association Association is a concept that is helpful in determining multiple causality. Events are said to be associated if they appear together more often than would be the case by chance alone. Such events may include risk factors or other characteristics affecting disease or health states. Examples are the frequent association of cigarette smoking with lung cancer, obesity with heart disease

Immunity Immunity: Refers to a host’s ability to resist a particular infectious disease–causing agent. This occurs when the body forms antibodies and lymphocytes that react with the foreign antigenic molecules and render them harmless. For community health nursing, this concept has significance in determining which individuals and groups are protected against disease and which individuals are vulnerable.

Types of immunity Passive immunity: short term resistance that is acquired either naturally or artificially. Active immunity: long term and sometimes lifelong resistance that is acquired either naturally or artificially. Cross immunity: person's immunity to one agent provides immunity to another related agent as well. Herd immunity: immunity level that is present in a population group.

Risk To determine the chances that a disease or health problem will occur, epidemiologists are concerned with risk, or the probability that a disease or other unfavorable health condition will develop. For any given group of people, the risk of developing a health problem is directly influenced by their biology, environment, lifestyle, and system of health care.

Risk (cont) A population at risk is a collection of people among whom a health problem Population at risk has the possibility of developing because certain influencing factors are present (e.g. exposure to HIV) or absent (e.g. lack of childhood immunizations, lack of specific vitamins in the diet), or because there are modifiable risk factors (e.g. with cardiovascular disease). A population at risk has a greater probability of developing a given health problem than other groups do.

Relative risk ratio Epidemiologists measure this difference using the relative risk ratio, which statistically compares the disease occurrence in the population at risk with the occurrence of the same disease in people without that risk factor. Relative risk ratio Incidence in exposed group = ------------------------------------- Incidence in unexposed group

Relative risk ratio (cont)… If the risk of acquiring the disease is the same regardless of exposure to the risk factor studied, the ratio will be 1:1, and the relative risk will be 1.0. A relative risk greater than 1.0 indicates that those with the risk factor have a greater likelihood of acquiring the disease than do those without it; for instance, a relative risk of 2.54 means that the exposed group is 2.54 times as likely to acquire the disease than the unexposed group.

Relative risk ratio (cont)…… This statistic may be used, for example, to compare the incidence of heart disease among smokers (smoking is a risk factor) with the incidence among nonsmokers, assuming that all other factors are the same. The relative risk ratio assists in determining the most effective points for community health intervention in regard to particular health problems.

Natural History of a Disease or Health Condition Any disease or health condition follows a progression known as its natural history; this refers to events that occur before its development, during its course, and during its conclusion. This process involves the interactions among a susceptible host, the causative agent, and the environment The natural progression of a disease occurs in four stages

Stages in natural history of a disease 1- Susceptibility stage: During this state, the disease is not present and individuals have not been exposed. 2- Subclinical disease stage; this stage begins when individuals have been exposed to a disease, but are asymptomatic, this followed by the incubation period , during which the organism multiplies to sufficient numbers to produce a host reaction and clinical symptoms

Stages in natural history of a disease (cont) Clinical disease stage; during this stage signs and symptoms of the disease or condition develop Resolution stage (Advanced stage); in this stage, the disease or health condition causes sufficient anatomic or functional changes to produce recognizable signs and symptoms.

Epidemiology of wellness The public health science of epidemiology has traditionally studied the occurrence of disease and health problems. However, the epidemiology of wellness grows increasingly, this help to examine and uncover new health promotion practices and encourage them, we can focus on wellness at ideal primary level of prevention.

Epidemiology of wellness (cont) Epidemiology has moved from concentrating only on illness to examining how host, agent, and environment are involved in wellness at various levels. Newer epidemiologic models are organized around four attributes that influence health: (1) the physical, social, and psychological environment; (2) lifestyle with its self-created risks; (3) human biology and genetic influences; (4) the system of health care organization.

Epidemiology of wellness (cont) CHN can play a primary role in the investigation and identification of factors that not only prevent diseases but also promote health

Causal Relationships One of the main challenges to epidemiology is to identify causal relationships in disease and health conditions in populations. As was previously suggested, the assessment of causality in human health is difficult at best; no single study is adequate to establish causality. Causal inference is based on consistent results obtained from many studies. Frequently, the accumulation of evidence begins with a clinical observation or an educated guess that a certain factor may be causally related to a health problem.

Types of studies that discover causality cross-sectional studies: It explores a health condition’s relation to other variables in a specified population at a specific point in time. It can show that the factor and the problem coexist.

Types of studies that discover causality (cont) Retrospective study: looks backward in time to find a causal relationship. It allows a fairly quick assessment of whether an association exists..

Types of studies that discover causality (cont) prospective study: looks forward in time to find a causal relationship. It is crucial to ensure that the presumed causal factor actually precedes the onset of the health problem. The prospective approach is concerned with current information and provides a direct measure of the variables in question.

Types of studies that discover causality (cont) Experimental study: in which the investigator controls or changes factors suspected of causing the condition and observes results. It is used to confirm the associations obtained from observational studies (in which the investigator merely observes data or people without controlling or changing any factors). It often requires many years to accumulate enough evidence to suggest a causal relationship.

SOURCES OF INFORMATION FOR EPIDEMIOLOGIC STUDY Epidemiologic investigators may draw data from any of three major sources: existing data, informal investigations, and scientific studies. The community health nurse will find all three sources useful in efforts to improve the health of aggregates.

Existing Data A variety of information is available nationally, by state, and by section, such as county, region, or urbanized area. This information includes vital statistics, census data, and morbidity statistics on certain communicable or infectious diseases; 1- Vital statistics; information gathered from ongoing registration of births, deaths, adoptions, divorces, and marriages. 2- Census data, data from population censuses taken every 10 years in many countries are the main source of population statistics. This could be a valuable assessment tool for the CHN who is taking part in planning for aggregates.

Existing Data (cont) 3- Reportable diseases; in USA, each state develop lows and regulations that require health organizations and practitioners to report to their local health authority cases of certain communicable and infectious diseases that can be spread through the community. 4- Disease Registries; some areas or states have disease registries for conditions with major public health impact

Existing Data (cont) 5- Environmental Monitoring States governments, through health departments or other agencies now monitor health hazards found in the environment ( e. g. pesticides, industrial wastes, radioactive or nuclear materials, chemical additives in foods, and medicinal drugs)

Existing Data (cont) 6- National Center for Health Statistics (NCHS); health surveys: NCHS furnishes valuable health prevalence data from surveys of Americans. The National Health Interview Survey (formerly known as National Health Survey)was established in 1956 and provides continual information about the health status and needs of the entire nation.

Informal Observational Studies A second information source in epidemiologic study is informal observation and description. Almost any client group encountered by the community health nurse can trigger such a study. If, for example, the nurse encounters an abused child at a clinic, a study of the clinic’s records to screen for additional possible instances of child abuse and neglect could lead to more case findings

Scientific Studies The third source of information used in epidemiologic inquiry involves carefully designed scientific studies. The nursing profession has recognized the need to develop a systematic body of knowledge on which to base nursing practice. Already, systematic research is becoming an accepted part of the community health nurse’s role

Methods in the epidemiologic investigative process The goals of epidemiologic investigation is to identify the causal mechanisms of health and illness states and to develop measures for preventing illness and promoting health  

1-Descriptive Epidemiology Descriptive epidemiology includes investigations that seek to observe and describe patterns of health-related conditions that occur naturally in a population. For example, a community health nurse might seek to learn how many children in a school district have been immunized for measles, how many home births occur each year in the county, how many cases of STDs have occurred in the city in the past month or how many automobile crashes have occurred near the community high school.  

Descriptive Epidemiology (cont) Descriptive studies almost always involve some form of broad-based quantification and statistical analysis.

Counts Counts ; the simple measure of description as a count. For example, an epidemiologic study to assess the impact of the varicella vaccine, data from the period prior to and following the vaccine availability showed that varecilla related deaths dropped by more than 45% from an average 145 per year between 1990-1994 to 66 per year during 1999-2001

Rates Rates are statistical measures expressing the proportion of people with a given health problem among a population at risk. The total number of people in the group serves as the denominator for various types of rates. To express a count as a proportion, or rate, the population to be studied must first be identified.

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

Rates (cont) Prevalence refers to all of the people with a particular health condition existing in a given population at a given point in time. The prevalence rate describes a situation at a specific point in time. If a nurse discovers 50 cases of measles in an elementary school, that is a simple count. If that number is divided by the number of students in the school, the result is the prevalence of measles. For instance, if the school has 500 students, the prevalence of measles on that day would be 10% (50 measles/500 population).

Rates (cont) Not everyone in a population is at risk for developing a disease, incurring an injury, or having some other health-related characteristic. The incidence rate recognizes this fact. Incidence refers to all new cases of a disease or health condition appearing during a given time. Incidence rate describes a proportion in which the numerator is all new cases appearing during a given period of time and the denominator is the population at risk during the same period. For example, some childhood diseases give lifelong immunity. The children in a school who have had such diseases would be removed from the total number of children at risk in the school population.

Computing Rates Computing Rates: to make comparisons between populations, epidemiologists often use a common base population in computing rates. For example, instead of merely saying that the rate of an illness is 13% in one city and 25% in another. The comparison is made per 100,000 people in the population. Computing rates include morbidity and mortality rates.

2-Analytic Epidemiology Analytic epidemiology, goes beyond simple description or observation and seeks to identify associations between a particular human disease or health problem and its possible causes. Analytic studies tend to be more specific than descriptive studies in their focus. Analytic studies test hypotheses or seek to answer specific questions and can be retrospective or prospective in design

Analytic Epidemiology (cont) Prevalence studies; describes patterns of occurrence, as in the study of vericella- related deaths Case-control studies ; compares people who have a health or illness condition (number of cases with the condition) with those who lack this condition (controls). Cohort studies: is a group of people who share a common experience in a specific time period.

3-Experimental Epidemiology Experimental epidemiology follows and builds on information gathered from descriptive and analytic approaches. It is used to study epidemics, the etiology of human disease, the value of preventive and therapeutic measures, and the evaluation of health services. In an experimental study, the investigator actually controls or changes the factors suspected of causing the health condition under study and observes what happens to the health state. In human populations Experimental studies should focus on disease prevention or health promotion rather than testing the causes of disease, which is done primarily on animals.

Experimental Epidemiology (cont) A community trial is a type of experimental study done at the community level. Geographic communities are assigned to intervention (experimental) or nonintervention (control) groups and compared to determine whether the intervention produces a positive change in the community.

Experimental Epidemiology (cont) Community trials can be extremely expensive and are not undertaken unless there is substantial evidence that the intervention will make a difference at the aggregate level.

Epidemiologic research involves seven steps. Everything from an informal study in the course of nursing practice to the most comprehensive epidemiologic research project can be undertaken with these steps: 1. Identify the problem. 2. Review the literature. 3. Design the study. 4. Collect the data. 5. Analyze the findings. 6. Develop conclusions and applications. 7. Disseminate the findings.