Epidemiology in Community Health Care�Lecture # 3

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Epidemiology in Community Health Care

Objectives:

At the end of this lecture, students will be able to:

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• Explain the host, agent, environment, and other important concepts to epidemiology.
• Define immunity and compare passive immunity, active immunity, cross-immunity, and herd immunity.
• Identify the four stages of a disease or health condition.
• List the major sources of epidemiologic information.
• List the types of epidemiologic studies that are useful for researching aggregate health.

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Definition of Epidemiology

• The term epidemiology originates from the Greek terms logos (study), demos (people), and epi (upon) → means the study of what is upon the people.
• The focus of study is disease occurrence among population groups

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Definition of Epidemiology

• Is the study of the determinants and distribution of health, disease, and injuries in human populations.

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• “the study of the distribution and determinants of disease frequency” (MacMahon, 1970)

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• “the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control of health problems” (Last, 1995)

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Definition of Epidemiology

• It is a specialized form of scientific research that can provide health care workers, including CHN, with a body of knowledge on which to base their practice and methods for studying new and existing problems.

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Concepts Basic to Epidemiology

Host, Agent, and Environment Model:

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:
• Physical factors: age, sex, race, and genetic influences on the host's vulnerability or resistance.
• Psychological factors: response to stress (can strongly influence host susceptibility).
• Lifestyle factors: Diet, exercise, sleep patterns, and unhealthy habits all contribute to increased of vulnerability to the disease-causing agent.

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Concepts Basic to Epidemiology

Agent:

• An agent is a factor that causes or contributes to a health problem or condition.
• Causative agents;
• factors that are present (eg, bacteria that cause tuberculosis, a rocks that contribute to an automobile crash) or
• factors that are lacking/absent (eg, lack of iron causes anemia; lack of seat belt use contributes to the extent of injury during RTA)

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Concepts Basic to Epidemiology

Environment:

• Refers to all the external factors surrounding the host that might influence vulnerability or resistance.
• The physical environment: factors such as geography, climate, weather, 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.
• The psychosocial environment: social, cultural, economic, and psychological influences and conditions that affect health (e.g. access to health care, poverty, and work stressors)

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• The epidemiologic triangle

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Concepts Basic to Epidemiology

Causality refers to the relationship between a cause and its effect.

• A purpose of epidemiologic study has been to discover causal relationships to understand why conditions develop and offer effective prevention and protection.
• Causation in infectious Diseases
• Causation in Noninfectious Disease
• Multiple Causation
• Web of Causation

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Concepts Basic to Epidemiology

Chain of Causation of infectious diseases

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Basic Concepts

• Communicable Disease: is the disease that can be transmitted from one person to another, is caused by an agent that is infectious (capable of producing infection), and is transmitted from a source, or reservoir, to a susceptible host

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• Communicable Diseases control is one of the major threat to public health and are of significant concern to CHN.

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Basic Concepts

• Modes of Transmission

1. Direct Transmission: occurs by immediate transfer of infectious agents (direct contact with the source, through touching, biting, kissing, or sexual intercourse) from a reservoir to a new susceptible host e.g coughing or sneezing secretions into the face of a susceptible individual can directly transmit respiratory infections, such as measles or pertussis.

2. Indirect Transmission (vehicle-borne transmission): occurs when the infectious agent is transported within contaminated inanimate materials such as air, water, or food.

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Basic Concepts

• Modes of Transmission

3. Food- and Water-Related Illness: Food- or water-related illness can be caused by viruses, toxins, bacteria, or parasites: such as Salmonella

4. Airborne Transmission: Sneezing and coughing of droplet nuclei inhaled into the respiratory system of a host.

5. Vector Transmission: transmission that occurs through a vector (a nonhuman carrier such as an animal or insect such as Rabies

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Concepts Basic to Epidemiology

Immunity:

• Immunity refers to a host's ability to resist a particular infectious disease-causing agent.
• It occurs when the body forms antibodies and lymphocytes that react with the foreign antigenic molecules and render them harmless.

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Concepts Basic to Epidemiology

Types of immunity:

1. Passive Immunity: refers to short-term resistance that is acquired either naturally or artificially.

• Natural: newborns, through maternal antibody transfer, have natural passive immunity that lasts about 6 months .
• Artificial: is attained through inoculation with a vaccine that gives temporary resistance.

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Concepts Basic to Epidemiology

Types of immunity:

2. Active Immunity: is long-term and sometimes lifelong resistance that is acquired either naturally or artificially.

• Naturally acquired active immunity comes through host infection/exposures.
• Artificially acquired active immunity is attained through vaccine inoculation.

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Concepts Basic to Epidemiology

Types of immunity:

3. Cross-Immunity: refers to a situation in which a person's immunity to one agent provides immunity to another related agent as well such as infection with cowpox gives immunity to a related disease; smallpox.

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4. Herd Immunity: it describes the immunity level that is present in a population group.

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Concepts Basic to Epidemiology

Risk:

• the probability that a disease or other unfavorable health condition will develop.
• Relative risk ratio = Incidence in exposed group/Incidence in unexposed group

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Basic concepts

• 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.

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• Prevalence rate: refers to all of the people with a particular health condition existing in a given population at a given point in time.

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Basic concepts

• Epidemic refers to a disease occurrence that clearly exceeds the normal or expected frequency in a community or region.
• Pandemic occurs when an epidemic disease is world widely distributed.
• Endemic the continuing presence of a disease or infectious agent in a given geographic area (such as malaria in Brazil and Indonesia)

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Natural History of a Disease or Health Condition

1. Susceptibility Stage:

• During this state, the disease is not present and individuals have not been exposed but host and environmental factors could very likely influence people’s susceptibility to a causative agent and lead to development of the disease

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Natural History of a Disease or Health Condition

2. Subclinical Disease Stage

• The stage of subclinical disease begins when individuals have been exposed to a disease but are as yet asymptomatic.
• It is followed by an incubation period, during which the organism multiplies to sufficient numbers to produce a host reaction and clinical symptoms.

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Natural History of a Disease or Health Condition

3. Clinical Disease Stage: when signs and symptoms of the disease or condition develop

4. Resolution Stage:

• The disease or health condition causes sufficient anatomic or functional changes to produce recognizable signs and symptoms.
• Disease severity may vary from mild to severe.
• The disease may conclude with a return to health, a residual or chronic form of the disease with some disabling limitations, or death

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Sources of information or data for epidemiology

• Epidemiologic investigators may draw data from the 3 major sources:
• Existing data
• Informal investigations
• Scientific studies

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Sources of information or data for epidemiology

1. Existing data:
1. Vital Statistics refers to the 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.
3. Reportable Diseases
4. Disease Registries

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Sources of information or data for epidemiology

E. Environmental Monitoring: government through health departments or other agencies monitors health hazards found in the environment such as pesticides, industrial wastes, radioactive or nuclear materials, chemical additives in foods, and medicinal drugs

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F. Other sources of data such as data from MOH

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Sources of information or data for epidemiology

2. Informal investigations: observing cases

3. Scientific studies

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Contribution of Epidemiology:

1. Understanding the factors that contribute to health and disease
2. The development of health promotion and disease prevention measures
3. The detection and characterization of emerging infectious agents
4. The evaluation of health services and policies
5. Important for the practice of community /public health nursing

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Methods in the Epidemiologic Investigative Process

• The goals of epidemiologic investigation are to identify the causal mechanisms of health and illness states and to develop measures for preventing illness and promoting health.

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• Epidemiological investigative process involves a sequence of three approaches that build on one another:
• Descriptive epidemiology
• Analytic epidemiology
• Experimental epidemiology

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Methods in the Epidemiologic Investigative Process

1. Descriptive epidemiology
1. Includes investigations that seek to observe and describe patterns of health-related conditions that occur naturally in a population
2. Example: a CHN wanted to learn how many home births occur each year in the county, or how many automobile crashes have occurred near the community high school
3. Descriptive epidemiology can be:
1. retrospective descriptive study:(identify cases and controls, then go back to review existing data)
2. prospective descriptive study:(identify groups and exposure factors and then follow them forward in time)

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Methods in the Epidemiologic Investigative Process

2. 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.
• Can be:
• Case-control studies
• Retrospective cohort studies
• prospective cohort studies

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1. Case-control studies

• Studies in which patients who already have a specific condition are compared with people who do not have the condition. The researcher looks back to identify factors or exposures that might be associated with the illness.  They often rely on medical records and patient recall for data collection.
• These types of studies are often less reliable than randomized controlled trials and cohort studies because showing a statistical relationship does not mean than one factor necessarily caused the other.
• The outcome is always identified before the exposure.
• Considered retrospective study in nature.

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Case-control study

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2. Cohort studies �

• Cohort studies identify a group of patients who are already taking a particular treatment or have an exposure, follow them forward over time, and then compare their outcomes with a similar group that has not been affected by the treatment or exposure being studied. Cohort studies are observational and not as reliable as randomized controlled studies, since the two groups may differ in ways other than in the variable under study.

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a. Prospective:

• A prospective study watches for outcomes, such as the development of a disease, during the study period and relates this to other factors such as suspected risk or protection factor(s). The study usually involves taking a cohort of subjects and watching them over a long period. The outcome of interest should be common; otherwise, the number of outcomes observed will be too small to be statistically meaningful (indistinguishable from those that may have arisen by chance). All efforts should be made to avoid sources of bias such as the loss of individuals to follow up during the study. Prospective studies usually have fewer potential sources of bias and confounding than retrospective studies.

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b. Retrospective �

• A retrospective study looks backwards.

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• Retrospective cohort (or historical cohort) Outcome data is collected and exposure data is collected retrospectively.
• This study design uses information that has been collected in the past and kept in files or databases.  

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Methods in the Epidemiologic Investigative Process

3. 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

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Experimental study

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