Principles of Epidemiology
Public health workers use epidemiologic principles as the foundation for disease surveillance and investigation activities.
Epidemiology is the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems.
Every public health worker should be familiar with the basic principles in this definition and how they are useful.
- Distribution - Epidemiology is concerned with the frequency and pattern of health events in a population. Frequency includes not only the number of events in a population, but also the rate or risk of disease in the population. Determining the rate of disease occurrences (number of events divided by size of the population) is critical for making valid comparisons across different populations.
- Determinants - Epidemiology is also used to search for causes and other factors that influence the occurrence of health-related events. The occurrence of a health-related event is usually related to multiple determinants that should be considered. Examples of determinants include host susceptibility to a disease, and opportunity for exposure to a microorganism, environmental toxin, insect vector or other infected individual that may pose a risk for acquiring disease.
- Specified populations - Epidemiologists are concerned with the collective health of people in a community or other area and the impact of health events on that population.
- Application - Epidemiology provides data for directing public health action. An epidemiologist uses the scientific methods of descriptive and analytic epidemiology in "diagnosing" the health of a community, but also must call upon experience and creativity when planning how to control and prevent disease in the community.
Disease surveillance usually begins with descriptive epidemiology -- defining the what, who, when and where of health-related events.
- what - Define the disease events and/or its determinants
- who - Descriptions of demographic characteristics are helpful in determining which groups are at risk for some outcome. The demographic characteristics usually include age, sex and race/ethnicity. Other categories include socioeconomic status, history of occupation, or smoking habits, which provide useful information about exposures that may present a risk. A history of underlying diseases may be useful for determining susceptibility to certain conditions.
- when - Following changes in disease rates over time, following long-term disease trends and knowledge of the seasonality of certain diseases helps identify unusual occurrences that may define epidemics. Temporal associations between particular exposures on illness give information about incubation periods and exposures posing a risk to others.
- where - Insight into the geographical extent of health-related events gives an idea of where the agent that causes a disease normally lives and multiplies, what may carry or transmit it and how it spreads.
The Primary Applications of Epidemiology in Public Health
To set policy and plan programs, public health officials must assess the health of the population they serve and must determine whether health services are available, accessible, effective and efficient. Epidemiology provides data for directing public health action. The information is used when planning how to control and prevent disease in the community. Through public health surveillance, a health systematically collects, analyzes, interprets and disseminates health data on an ongoing basis. By knowing the ongoing pattern of disease occurrence and disease potential, a health agency can effectively and efficiently investigate, prevent and control disease in the community.
Uses of Epidemiology
- Count health-related events
- Describe the distribution of health-related events in the population
- Describe clinical patterns
- Identify risk factors for developing diseases
- Identify causes or determinants of disease
- Identify control and/or preventive measures
- Establish priorities for allocating resources
- Select interventions for prevention and control
- Evaluate programs
- Conduct research
- risk factors and causes
- drug trials / vaccine trials
- operational research
Purposes of Communicable Disease Investigation and General Principles of Preventing Transmission
These guidelines have been written to assist public health staff engaged in investigating communicable diseases. Their intent is to provide basic, practical, up-to-date and easy-to-understand information which has been coalesced and interpreted from a variety of sources. These guidelines have been written as carefully as possible to balance the amount of work necessary to follow up a reported case against the probability of controlling spread of the disease.
Only certain diseases are included in these guidelines. This is based upon their frequency and complexity. For further information about these and other diseases, consult Control of Communicable Diseases in Man.
Remember, these are only guidelines. All situations differ and best judgment should prevail. Some situations require a more stringent approach. Some require a less stringent approach.
Purposes of Communicable Disease Investigation
Nothing is less self-fulfilling than going through the motions of a task without understanding its purpose. When investigating a report of communicable disease, always keep in mind the purposes are to:
- Prevent Transmission from Cases to Contacts: A full discussion of this topic begins below.
- Identify the Source of the Disease: The source may be in the environment, a food, or another person. Its identification can lead to control.
- Identify Other Cases: Each reported case should be regarded as a sentinel health event. Efforts to identify other cases which are not diagnosed and/or reported may uncover outbreaks and common sources. It is important to question the reported case about others who have been exposed or who have similar symptoms.
- Conduct Surveillance: Information collected when investigating a reported disease can be examined on a state or regional basis to identify outbreaks or trends and to formulate policy. This information is collected on standardized forms and often is sent to the Centers for Disease Control and Prevention for surveillance on a national basis.
- Ensure that the Patient has Adequate Medical Supervision: Our responsibilities include making sure the patient is well cared for and understands the consequences of his illness. Where medical care is inadequate, appropriate referrals should be made.
General Principles of Preventing Transmission
This section describes how cases and contacts can be managed to prevent transmission.
- Verify the Diagnosis: It is very important to promptly establish or verify the etiologic agent responsible for the disease. For many diseases reported by physicians or others, there are often little or no laboratory data initially available to verify the diagnosis. Since control efforts depend on the exact diagnosis, it is important to: (a) determine if this information exists; (b) arrange for laboratory tests if the information does not otherwise exist; or (c) make the best guess about diagnosis if it cannot be obtained.
- Determine if the Case is Infectious or Possibly Infectious: If the
case is still infectious, you must identify available measures for preventing transmission. There are
two ways to assess the case's infectivity:
- estimation--uses the date of onset of illness, dates of known treatment, and known periods of infectiousness for an illness. For example, hepatitis A is no longer infectious after 1 week of the onset of symptoms. Strep throat is no longer infectious after 24 hours of treatment.
- verification--requires laboratory testing of specimens for the case
(for example, a stool specimen for a foodhandler with salmonellosis). Which method you use depends upon
the disease (the period of communicability for some diseases is precise; for others it is not precise),
the quality of information about the case (e.g., is date of onset known?), and the need to know definitely
whether the case is infectious.
In practice, because of delays in diagnosis and reporting, many cases will have passed the infectious stage by the time they are reported. In this instance, the opportunity to prevent transmission is lost, but preventive efforts can be directed to the case's contacts, and other purposes of case investigation can be fulfilled. You may need to contact the physician to fill in information about the case, determine results of laboratory tests, or arrange for tests. Also ask about similar cases for which a report may be pending.
- Minimize the Duration of the Case's Infectivity: Drugs (antimicrobials) are the most important means to shorten the duration of infectivity for many diseases. Usually, decisions about drug treatment will be made by the time you investigate. However, sometimes it may be necessary to work with the patient and physician to ensure the patient is appropriately treated.
- Put into Effect Practices for Preventing Transmission: These practices
depend upon the disease and other circumstances. If the disease warrants, the case may have to employ
these practices while ill and for a period after recovery during which (s)he is still infectious:
- disinfect and dispose of contaminated material (e.g., blood, saliva, feces, urine, eating utensils, bedding, clothes, toys, etc.)
- disinfect the case, clothing, bedding, etc. (e.g., lice)
- encourage behavioral practices of the case (e.g., handwashing, covering the nose and mouth when coughing/sneezing, protecting lesions from contact with another person. Also, there may be a need for changes if individual is a foodhandler.)
- isolate the case -- the duration and degree depends upon the illness
- Determine the risk of infection to contacts: What is the frequency,
duration and dosage of the exposure? Often, these characteristics
can only be described vaguely. It is usually reasonable to assume persons living in the same household
had opportunity for exposure.
Is the contact susceptible? Has the person been vaccinated, or previously infected? Is there evidence of a compromised immune system (surgery, steroids, etc.)? Does the disease result in immunity or are people generally susceptible?
- Determine if the contact is ill or infected: Look for clinical signs or symptoms. It is important to know if the contact is in the early stages of an illness or asymptomatic infected, you must rely on the laboratory. If you determine that a contact is ill or infected, manage him/her as a case.
- Prevent illness in susceptibles: It may be useful for you to think
of contacts in two ways: Those who have been exposed, and those who might yet be exposed, either to the
original source or to a contact who subsequently acquires an infection.
The choice of the preventive measure depends upon the disease. Rapid identification of susceptibles is important. Remember, vaccines usually require a time period greater than one incubation period to develop antibody. Therefore, they rarely prevent illness in the first generation of contacts. They can, however prevent illness in later generations. Immune globulins must be given within a few days of contact to prevent or modify an infection, but they provide only short-term protection. Antimicrobials are particularly important for contacts exposed to strep, tuberculosis and meningitis.
Limit the activities of exposed persons -- sometimes it may be prudent to recommend cessation of contact between the contact and the source. Another example is quarantining exposed persons to protect others not yet exposed.
Conduct personal surveillance of the exposed person -- it may be important to call an exposed person by the end of the maximum incubation period to see if (s)he is ill (e.g., for hepatitis A), or simply ask the contact to call you if (s)he develops symptoms. The intensity of the surveillance depends upon the importance attached to preventing transmission from that person.
General Guidelines for Outbreak Investigations
The first action of a health agency when it receives a report of a case or a cluster of cases of a disease is to investigate. The investigation may be anything from a limited phone call to confirm reported information, or it may be as extensive as a field investigation coordinating the efforts of many people to determine the extent and cause of a large outbreak.
Outbreaks may be detected when routine, timely analysis of surveillance data reveals an increase in reported cases or an unusual clustering of cases. In South Carolina, for example, the weekly tabulation of disease reports by county is a useful way to monitor trends, evaluate geographical distribution throughout the state and identify possible clusters.
Why Do We Investigate Outbreaks?
The health agency has a responsibility to institute control and prevention measures. Outbreak investigations may be considered as opportunities to study disease events and are training exercises for those conducting the investigation. If cases are continuing to occur, the goal may be to prevent additional cases. If an outbreak appears to be almost over, the goal may be to prevent outbreaks in the future. The objective here would be to identify factors which contributed to the outbreak to implement measures that would prevent serious outbreaks in the future. Other concerns like public relations, political concerns and legal obligations may also need to be considered.
Steps in the Investigation
- Prepare for field work - talk to others with experience, review the literature.
- Establish the existence of an outbreak - An outbreak or epidemic is the occurrence of more cases of disease than expected in a given area or among a specific group of people over a particular period of time. A cluster is an aggregation of cases in a given area over a particular period without regard to whether the number of cases is more than expected. Defining an epidemic requires that one determine what the expected number of cases would be usually from existing surveillance data. Some reasons for a "false" epidemic may be reports of sporadic cases of unrelated cases of the same disease, reports of similar cases but unrelated diseases, or changes in surveillance or reporting procedures.
- Verify the diagnosis - Collect clinical information, contact the diagnosing physician, confirm results of laboratory tests done.
- Define and identify cases - Establish a case definition for the disease using a standard set of criteria for deciding whether an individual should be classified as having the health condition of interest.
- Perform descriptive epidemiology - Collect information to characterize an outbreak by time, place and person. Remember the what, who, when and where of descriptive epidemiology.
- Develop hypotheses - After collecting some of the initial information characterizing the outbreak, one should be able to formulate a hypothesis that addresses the source of the agent, the mode (and vehicle or vector) of transmission, and the exposures that caused the disease.
- Evaluate hypotheses - Compare your hypothesis with the established facts. Is it plausible and consistent? If not, the hypothesis should be revised.
- As necessary, reconsider/refine hypotheses and execute additional studies.
- Implement control and prevention measures - This is the primary goal of most outbreak investigations. Although mentioned far down on the list, prevention and control measures should be implemented as soon as possible in an outbreak setting. This may be accomplished by destroying contaminated foods, removing an infected foodhandler from the job, or immunizing and providing prophylaxis to a population at risk.
- Communicate findings - An oral briefing or a written report are always useful to summarize an investigation. Formally presenting recommendations provides a blueprint for action and may serve in the health agency as a reference if the health agency encounters a similar situation in the future.
Data Collection Guidelines
- Line listings. Traditionally we collect data on a standard case report form or questionnaire. A separate sheet of information from each person interviewed often makes synthesizing the data difficult. To create a line listing, critical items are selected from the data collection form and are organized so that data from multiple cases can be entered in columns on a single sheet to summarize the data. In a line listing, each column represents an important variable, such as name or identification number, age, sex, case classification, etc., while each row represents a different case.
- The epidemic curve. The "epi" curve serves as a map of an epidemic. It provides a simple visual display of the outbreak's magnitude and time trend. Although simple it provides a great deal of information about an epidemic. You will usually be able to tell where you are in the time course of an epidemic, and what the future course might be. If you have identified the disease and know its usual incubation period, you usually can deduce a probable time period of exposure and can develop a questionnaire focusing on that time period. Finally, you may be able to draw inferences about the epidemic pattern -- whether it is common source or propagated, or both. Drawing an epidemic curve, plotting time on the x axis and the number of cases on the y axis, will provide additional clues in furthering the investigation. Mapping of individual cases should be done by their date of onset. Changing the unit of time on the x axis may be necessary to best "see" the outbreak. This will depend on the incubation period of the disease you are dealing with.