References:
- Community and Public Health Nursing, 2nd Philippine Edition, ISBN 978-981-48-6503-6, by Earl Francis R. Sumile (Ch. 7, pp. 139–145)
Epidemiology is defined as “the study of the distribution and determinants of health-related states of events in specified populations, and the application of this study to the prevention and control of health problems” (Last, 2001). This definition clearly states that the focus is the population (not individuals), whether at the global level, country, city, school or neighborhood. It also shows that the scope covers not only diseases, but also health-related states and events; described in terms of distribution (frequency or pattern) including their determinants (risk factors or causes), and their prevention and control.
The Father of Epidemiology
John Snow is considered as the father of field epidemiology, having conducted studies of cholera outbreaks to discover the cause of disease and to prevent its recurrence in 1854. He was able to infer that water was a source of infection for cholera by determining where the persons with cholera lived and worked.
Epidemiology in Public Health Nursing
Epidemiology is the basic science of public health, providing the foundation for directing practical and appropriate public health action based on this science and causal reasoning. Public health nurses use epidemiology to describe the distribution and extent of health condition or disease in the population, identify the risk factors in studying natural history of diseases or models of disease causation, monitor health condition or disease through screening and surveillance, and evaluate approaches or interventions that reduce disease risk and promote health.
Definition of some terms:
- Frequency: the number of health events, such as the number of cases of pneumonia in a population. Rate (over time) and ratio (over population) may also be used to represent the data.
- Pattern refers to the occurrence of health conditions or disease by time (annual, seasonal, weekly, daily, etc.), place (urban/rural, schools, workplace, etc.), and person (demographics, age, sex, marital status, etc.). Classifying health events by time, place and person is also called descriptive epidemiology.
- Determinants include demographic characteristics, genetics, behavior, or environmental exposures. Analytic epidemiology provides answers to assess whether groups with different rates of health conditions or diseases differ with these determinants. These findings provide evidence for public health control and prevention measures.
- Communicable Diseases (CDs) are those spread from one person to another through a variety of ways; such as, physical contact with an infected person, contaminated food or water, bites from insects or animals capable of transmitting the disease, and breathing in an airborne virus.
- Noncommunicable Diseases (NCDs) are chronic diseases which are the result of a combination of genetic, physiological, environmental, and behavioral factors. The main types of NCDs are cardiovascular diseases (heart attacks and stroke), cancers, chronic respiratory diseases (asthma, chronic obstructive pulmonary diseases) and diabetes.
- Emerging Infectious Diseases are infections recently appearing within a population or those whose incidence or geographic range is rapidly increasing or threatens to increase in the near future.
- Disease Occurrence: the relative incidence, location, and frequency of disease allows it to be classified as the following:
- Sporadic: infrequent, irregularly occurring disease.
- Endemic: constant presence of a disease within a locality.
- Hyperendemic: persistent, high levels of disease occurrence within a locality.
- Epidemic: an increase, often suddenly, in the number of cases of a disease above what is normally expected in a locality.
- Outbreak: similar to an epidemic, but limited within a smaller geographic boundary.
Natural History of Disease
In studying the course and outcome of diseases, epidemiology gives a picture of a natural history of diseases in individuals and groups. It describes the sub-clinical changes to signs and symptoms of the clinical disease until its resolution to either recovery or death.
- An exposure is when a pathogen enters a susceptible host. The pathogen then invades the target organ or tissue and begins multiplying.
- When the host’s immune system responds, there is already an acknowledged infection whether symptomatic or not. Between the exposure and infection, the duration is called the latent period. As soon as infection occurs, the possibility of infecting others begins; the disease is in its infectious period. When this ceases, then disease enters its non-infectious period.
- Between the exposure and the appearance of symptoms, the duration is called the incubation period.
Models of Disease Causation
It is important to understand the risk factors or any attribute, characteristic, or exposure of an individual that increases the likelihood of developing a disease or health condition. This is imperative for providing preventive and control measures. There are three models primarily used by epidemiology:
- Epidemiologic Triad or Triangle: the traditional model for infectious (communicable) diseases, consisting of a susceptible host, an external agent, and an environment that brings the host and agent together. Disease results from the interaction between the susceptible host and the agent in an environment that supports transmission of the agent from a source to that host.
- Agent refers to infectious microorganism or pathogen present for the disease to occur. It could also be chemical contaminants or physical forces. The presence of an agent is not always sufficient to cause disease. Hosts may not be susceptible to a particular agent, or the environment may not be conducive for the agent to thrive.
- Iceberg Principle: this model states that diagnosed cases of a disease is only “the tip of the iceberg”, and that a large number of cases at a given time are subclinical. The public health goal is to find out through screening and early detection those who are exposed or affected so that proper interventions can be done. For example, the recorded cases of suicide are the tip of the iceberg, and that efforts can be pointed towards those with depression before they resort to suicide.
- Web of Causation: a web-like illustration of multiple factors that contribute to the cause of a disease. This model proposes that disease are caused by the interaction between genetic factors and environmental factors.
Screening and Surveillance
Screening is the active search or process of detection for disease or disorders among apparently healthy people. The primary aim of screening is to identify risk factors and diseases in their earliest stage. Screening of whole populations aims to detect those at risk of a specific disorder or condition who are likely to benefit from further investigations and possible treatment or action to prevent the disorder or condition or its consequences.
- Some examples include breast cancer screening, cervical cancer screening, lung cancer, and newborn screening.
Surveillance is the systematic, on-going, and analytic process of monitoring to scrutinize disease condition. Surveillance involves investigating the distribution and possible causes of disease and conditions within the population. Information from surveillance inform planning ang improving health services and understanding the factors that contribute towards disease, as well as the long-term effects of these diseases.
- Event-based surveillance is used for rapid detection, notification, verification and assessment of public health events such as clusters of disease, or rumors of unexplained deaths
- Indicator-based surveillance involves routine reporting of cases of disease such as notifiable disease surveillance systems.
Levels of Prevention in Epidemiology
- Primordial Prevention: at a very early stage, this is prevention of even risk factors to exist in the first place. This includes activities that promote healthy lifestyles, and avoiding the development of behaviors and exposure patterns that contribute to increased risk of disease. For example, advising parents to quit smoking for the sake of their child’s health.
- Primary Prevention: the control of exposure to risk factors, before disease development. This relies on epidemiological information to indicate those behaviors that are either protective, and those that are associated with increased risk. Actions designed to promote healthy lifestyle and environment are called health promotion activities; while those aimed at reducing the risk of specific diseases are called health protection.
- Secondary Prevention: the application of available measures to detect diseases early or health conditions before the onset of signs and symptoms. Screening and physical examinations that are aimed at early diagnosis (e.g., mammography, cervical screening, regular blood pressure testing) are examples of secondary prevention.
- Tertiary Prevention: managing disease after diagnosis to slow or stop disease progression and limit disability or complications. Epidemiological studies examine risk factors affecting function and suggest strategies in the care of patients such as chemotherapy, rehabilitation, and screening for complications.
Evaluating Interventions
Epidemiologic methods are also used in evaluating the effectiveness and efficiency of interventions. There are population-based approach interventions which attempt to change socio-cultural or structural factors in the community, cities, or even at a national level; an example of which is smoking bans in public places. There are also high-risk approach interventions, which targets high-risk persons; those likely to have increased incidence of disease based on the presence of risk factors.