Principles Of Disease Outbreak Pblic Heakth Flashcards
Steps of an Outbreak Investigation
Once the decision to conduct a field investigation of an acute outbreak has been made, working quickly is essential — as is getting the right answer.
In other words, researcher cannot afford to conduct an investigation that is “quick and dirty.”
They must conduct investigations that are “quick and clean.” Under such circumstances, researcher find it useful to have a systematic approach to follow. This approach ensures that the investigation proceeds without missing important steps along the way.
What are the 13 steps of an outbreak investigation
Step 1:Prepare for field work
Step 2: Establish the existence of an outbreak
Step 3: Verify the diagnosis
Step 4: Construct a working case definition
Step 5: Find cases systematically and record information
Step 6: Perform descriptive epidemiology
Step 7: Develop hypotheses
•Step 8: Evaluate hypotheses epidemiologically
Step 9: Reconsider, refine, and re-evaluate hypotheses
Step 10: Compare and reconcile with laboratory and environmental studies
Step 11: Implement control and prevention measures
Step 12: Initiate or maintain surveillance
Step13:Communicate findings
Explain the first step (preparing for field of work) of investigating an outbreak
This first step can be categorized into two,name them and explain each category
Step 1 Prepare for field work
The numbering scheme for this step is problematic, because preparing for field work often is not the first step. Only occasionally do public health officials decide to conduct a field investigation before confirming an increase in cases and verifying the diagnosis.
More commonly, officials discover an increase in the number of cases of a particular disease and then decide that a field investigation is warranted.
Sometimes investigators collect enough information to perform descriptive epidemiology without leaving their desks, and decide that a field investigation is necessary only if they cannot reach a convincing conclusion without one.
Regardless of when the decision to conduct a field investigation is made, you should be well prepared before leaving for the field.
The preparations can be grouped into two broad categories: (a) scientific and investigative issues, and (b) management and operational issues. Good preparation in both categories is needed to facilitate a smooth field experience.
•Management and operational issues
A good field investigator must be a good manager and collaborator as well as a good epidemiologist, because most investigations are conducted by a team rather than just one individual.
The team members must be selected before departure and know their expected roles and responsibilities in the field. Does the team need a laboratorian, veterinarian, translator/interpreter, computer specialist, entomologist, or other specialist? What is the role of each?
•Scientific and investigative issues
As a field investigator, you must have the appropriate scientific knowledge, supplies, and equipment to carry out the investigation before departing for the field. Discuss the situation with someone knowledgeable about the disease and about field investigations, and review the applicable literature. In previous similar outbreaks, what have been the sources, modes of transmission, and risk factors for the disease?
Explain the second step (establishing the existence of an outbreak ) of investigating an outbreak
What is an outbreak or an epidemic?
outbreak or an 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. Usually, the cases are presumed to have a common cause or to be related to one another in some way.
Many epidemiologists use the terms outbreak and epidemic interchangeably, but the public is more likely to think that epidemic implies a crisis situation. Some epidemiologists apply the term epidemic to situations involving larger numbers of people over a wide geographic area.
•One of the first tasks of the field investigator is to verify that a cluster of cases is indeed an outbreak. Some clusters turn out to be true outbreaks with a common cause, some are sporadic and unrelated cases of the same disease, and others are unrelated cases of similar but unrelated diseases.
•Even if the cases turn out to be the same disease, the number of cases may not exceed what the health department normally sees in a comparable time period.
Even if the current number of reported cases exceeds the expected number, the excess may not necessarily indicate an outbreak.
Reporting may rise because of changes in local reporting procedures, changes in the case definition, increased interest because of local or national awareness, or improvements in diagnostic procedures.
A new physician, infection control nurse, or healthcare facility may more consistently report cases, when in fact there has been no change in the actual occurrence of the disease. Some apparent increases are actually the result of misdiagnosis or laboratory error.
Finally, particularly in areas with sudden changes in population size such as resort areas, college towns, and migrant farming areas, changes in the numerator (number of reported cases) may simply reflect changes in the denominator (size of the population).
Explain the third step (verify the diagnosis ) of investigating an outbreak
Why is this third step important?
Verify the diagnosis
The next step, verifying the diagnosis, is closely linked to verifying the existence of an outbreak. In fact, often these two steps are addressed at the same time. Verifying the diagnosis is important:
(a) to ensure that the disease has been properly identified, since control measures are often disease-specific; and
(b) to rule out laboratory error as the basis for the increase in reported cases.
First, review the clinical findings and laboratory results. If you have questions about the laboratory findings (for example, if the laboratory tests are inconsistent with the clinical and epidemiologic findings), ask a qualified laboratorian to review the laboratory techniques being used.
Second, many investigators — clinicians and non-clinicians — find it useful to visit one or more patients with the disease.
If you do not have the clinical background to verify the diagnosis, bring a qualified clinician with you.
Talking directly with some patients gives you a better understanding of the clinical features, and helps you to develop a mental image of the disease and the patients affected by it.
In addition, conversations with patients are very useful in generating hypotheses about disease etiology and spread.
They may be able to answer some critical questions: What were their exposures before becoming ill? What do they think caused their illness? Do they know anyone else with the disease? Do they have anything in common with others who have the disease?
•Third, summarize the clinical features using frequency distributions. Are the clinical features consistent with the diagnosis ?
Frequency distributions of the clinical features are useful in characterizing the spectrum of illness, verifying the diagnosis, and developing case definitions.
•These clinical frequency distributions are considered so important in establishing the credibility of the diagnosis that they are frequently presented in the first table of an investigation’s report or manuscript.
Explain the fourth step (construct a working case definition ) of investigating an outbreak
What is a case definition
What does it include?
What must the case definition not include?
Step 4: Construct a working case definition
A case definition is a standard set of criteria for deciding whether an individual should be classified as having the health condition of interest.
A case definition is a standard set of criteria for deciding whether an individual should be classified as having the health condition of interest. A case definition includes clinical criteria and — particularly in the setting of an outbreak investigation — restrictions by time, place, and person.
The clinical criteria should be based on simple and objective measures such as “fever ≥ 40°C (101°F),” “three or more loose bowel movements per day,” or “myalgias (muscle pain) severe enough to limit the patient’s usual activities. ”
The case definition may be restricted by time (for example, to persons with onset of illness within the past 2 months), by place (for example, to residents of the nine-county area or to employees of a particular plant) and by person (for example, to persons with no previous history of a positive tuberculin skin test, or to premenopausal women).
Whatever the criteria, they must be applied consistently to all persons under investigation.
•The case definition must not include the exposure or risk factor you are interested in evaluating. This is a common mi mistake. For example, if one of the hypotheses under consideration is that persons who worked in the west wing were at greater risk of disease, do not define a case as “illness among persons who worked in the west wing with onset between…” Instead, define a case as “illness among persons who worked in the facility with onset between…”
•Then conduct the appropriate analysis to determine whether those who worked in the west wing were at greater risk than those who worked elsewhere.
•Diagnoses may be uncertain, particularly early in an investigation. As a result, investigators often create different categories of a case definition, such as confirmed, probable, and possible or suspect, that allow for uncertainty.
•To be classified as confirmed, a case usually must have laboratory verification. A case classified as probable usually has typical clinical features of the disease without laboratory confirmation. A case classified as possible usually has fewer of the typical clinical features.
Explain the fifth step (Find cases systematically and record information) of investigating an outbreak
Step 5: Find cases systematically and record information
As noted earlier, many outbreaks are brought t to the attention of health authorities by concerned healthcare providers or citizens.
However, the cases that prompt the concern are often only a small and unrepresentative fraction of the total number of cases.
Public health workers must therefore look for additional cases to determine the true geographic extent of the problem and the populations affected by it.
Usually, the first effort to identify cases is directed at healthcare practitioners and facilities — physicians’ clinics, hospitals, and laboratories — where a diagnosis is likely to be made. Investigators may conduct what is sometimes called stimulated or enhanced passive surveillance by sending a letter describing the situation and asking for reports of similar cases.
Alternatively, they may conduct active surveillance by telephoning or visiting the facilities to collect information on any additional cases.
In some outbreaks, public health officials may decide to alert the public directly, usually through the local media. In other situation the media may have already spread the word.
For example, in an outbreak of COVID 19, announcements in the media alerted the public and instructed them to see a physician if they developed symptoms compatible with the disease in question.
If an outbreak affects a restricted population such as persons in a school, or at a work site, and if many cases are mild or asymptomatic and therefore undetected, a survey of the entire population is sometimes conducted to determine the extent of infection.
A questionnaire could be distributed to determine the true occurrence of clinical symptoms, or laboratory specimens could be collected to determine the number of asymptomatic cases.
•In some investigations, investigators develop a data collection form tailored to the specific details of that outbreak. In others, investigators use a generic case report form. Regardless of which form i used, the data collection form should include the following types of information about each case.
State and define six types of information that can be recorded under the fifth step
Identifying information:
A name, address, and telephone number is essential if investigators need to contact patients for additional questions and to notify them of laboratory results and the outcome of the investigation. Names also help in checking for duplicate records, while the addresses allow for mapping the geographic extent of the problem.
•Demographic information. Age, sex, race, occupation, etc. provide the person characteristics of descriptive epidemiology needed to characterize the populations at risk.
•Clinical information. Signs and symptoms allow investigators to verify that the case definition has been met. Date of onset is needed to chart the time course of the outbreak. Supplementary clinical information, such as duration of illness and whether hospitalization or death occurred, helps characterize the spectrum of illness.
•Risk factor information. This information must be tailored to the specific disease in question.
For example, since food and water are common vehicles for hepatitis A but not hepatitis B, exposure to food and water sources must be ascertained in an outbreak of the former but not the latter.
•Reporter information. The case report must include the reporter or source of the report, usually a physician, clinic, hospital, or laboratory.
Investigators will sometimes need to contact the reporter, either to seek additional clinical information or report back the results of the investigation
Explain the sixth step (Perform descriptive epidemiology)of investigating an outbreak
What is descriptive epidemiology?
Why is this step critical?
Why is summarizing data by key demographic variables important?
Step 6: Perform descriptive epidemiology
Conceptually, the next step after identifying and gathering basic information on the persons with the disease is to systematically describe some of the key characteristics of those persons.
This process, in which the outbreak is characterized by time, place, and person, is called descriptive epidemiology. It may be repeated several times during the course of an investigation as additional cases are identified or as new information becomes available.
•This step is critical for several reasons.
•Summarizing data by key demographic variables provides a comprehensive characterization of the outbreak — trends over time, geographic distribution (place), and the populations (persons) affected by the disease.
After characterizing an outbreak by time, place, and person, it is useful to summarize what you know.
Specifically, the epidemic curve indicated that the outbreak was basically over, because no new case had been reported in the previous two weeks.
The affected population had a greater proportion of persons who were black, female, young, and less likely to smoke than persons in a typical Legionnaires’ outbreak. There appeared to be no clustering by either residence or worksite, and no connection with exposure to the town’s cooling towers.
Thus, the investigators were forced to develop new hypotheses about a source of Legionnaires’ disease to explain this outbreak.
Explain the seventh step (Develop hypothesis )of investigating an outbreak
Develop hypotheses
Although the next conceptual step in an investigation is formulating hypotheses, in reality, investigators usually begin to generate hypotheses at the time of the initial telephone call. Depending on the outbreak, the hypotheses may address the source of the agent, the mode (and vehicle or vector) of transmission, and the exposures that caused the disease.
The hypotheses should be testable, since evaluating hypotheses is the next step in the investigation.
In an outbreak context, hypotheses are generated in a variety of ways.
First, consider what you know about the disease itself: What is the agent’s usual reservoir? How is it usually transmitted? What vehicles are commonly implicated? What are the known risk factors? In other words, by being familiar with the disease, you can, at the very least, “round up the usual suspects.”
Explain the eighth step (Evaluate hypothesis epidemiologically) of investigating an outbreak
What are the two most common types of analytic epidemiology studies used in field investigations
Evaluate hypotheses epidemiologically
After a hypothesis that might explain an outbreak has been developed, the next step is to evaluate the plausibility of that hypothesis.
Typically, hypotheses in a field investigation are evaluated using a combination of environmental evidence, laboratory science, and epidemiology.
From an epidemiologic point of view, hypotheses are evaluated in one of two ways: either by comparing the hypotheses with the established facts or by using analytic epidemiology to quantify relationships and assess the role of chance.
The first method is likely to be used when the clinical, laboratory, environmental, and/or epidemiologic evidence so obviously supports the hypotheses that formal hypothesis testing is unnecessary.
For example, in an outbreak of hypervitaminosis D that occurred in Massachusetts in 1991, investigators found that all of the case-patients drank milk delivered to t their homes by a local dairy.
Therefore, investigators hypothesized that the dairy was the source and the milk was the vehicle. When they visited the dairy, they quickly recognized that the dairy was inadvertently adding far more than the recommended dose of vitamin D to the milk.
No analytic epidemiology was really necessary to evaluate the basic hypothesis in this setting or to implement appropriate control measures, although investigators did conduct additional studies to identify additional risk factors.
n many other investigations, however, the circumstances are not as straightforward, and information from the series of cases is not sufficiently compelling or convincing. In such investigations, epidemiologists use analytic epidemiology to test their hypotheses. The key feature of analytic epidemiology is a comparison group.
The comparison group allows epidemiologists to compare the observed pattern among case-patients or a group of exposed persons with the expected pattern among non cases or unexposed persons.
By comparing the observed with expected patterns, epidemiologists can determine whether the observed pattern differs substantially from what should be expected and, if so, by what degree.
In other words, epidemiologists can use analytic epidemiology with its hallmark comparison group to quantify relationships between exposures and disease, and to test hypotheses about causal relationships.
The two most common types of analytic epidemiology studies used in field investigations are retrospective cohort studies and case-control studies, as described in the following sections.
Explain retrospective cohort studies under the eight step
Retrospective cohort studies
A retrospective cohort study is the study of choice for an outbreak in a small, well-defined population, such as an outbreak of gastroenteritis among wedding guests for which a complete list of guests is available.
In a cohort study, the investigator contacts each member of the defined population (e.g., wedding guests), determines each person’s exposure to possible sources and vehicles (e.g., what food and drinks each guest consumed), and notes whether the person later became ill with the disease in question (e.g., gastroenteritis).
After collecting similar information from each attendee, the investigator calculates an attack rate for those exposed to (e.g., who ate) a particular item and an attack rate for those who were not exposed.
Explain the ninth step (Reconsider, refine, and re-evaluate hypotheses ) of investigating an outbreak
Reconsider, refine, and re-evaluate hypotheses
•Unfortunately, analytic studies sometimes are unrevealing or undisclose. This is particularly true if the hypotheses were not well founded at the outset.
•It is an axiom of field epidemiology that if you cannot generate good hypotheses (for example, by talking to some case-patients or local staff and examining the descriptive epidemiology and outliers), then proceeding to analytic epidemiology, such as a case-control study, is likely to be a waste of time.
•When analytic epidemiology is unrevealing, rethink your hypotheses. Consider convening a meeting of the case-patients to look for common links or visiting their homes to look at the products on their shelves. Consider new vehicles or modes of transmission
investigation of an outbreak of Salmonella Muenchen in Ohio illustrates how a reexamination of hypotheses can be productive.
In that investigation, a case-control study failed to implicate any plausible food source as a common vehicle. Interestingly, all case-households but only 41% of control households included persons aged 15–35 years.
The investigators thus began to consider vehicles of transmission to which young adults were commonly exposed.
By asking about drug use in a second case-control study, the investigators implicated marijuana as the likely vehicle. Laboratory analysts subsequently isolated the outbreak strain of S.Muenchen from several samples of marijuana provided by case-patients.
Finally, recall that one reason to investigate outbreaks is research.
An outbreak may provide an “experiment of nature” that would be unethical to set up deliberately but from which the scientific community can learn when it does happen to occur.
When an outbreak occurs, whether it is routine or unusual, consider what questions remain unanswered about that particular disease and what kind of study you might do in this setting to answer some of those questions.
The circumstances may allow you to learn more about the disease, its modes of transmission, the characteristics of the agent, host factors, and the like.
Explain the tenth and eleventh steps (Compare and reconcile with laboratory and environmental studies, Step 11: Implement control and prevention measures ) of investigating an outbreak
Step 10: Compare and reconcile with laboratory and environmental studies
•While epidemiology can implicate vehicles and guide appropriate public health action, laboratory evidence can confirm the findings. The laboratory was essential in both the outbreak of salmonellosis linked to marijuana and in the Legionellosis outbreak traced to the grocery store mist machine.
•Thus the epidemiologic, environmental, and laboratory arms of the investigation complemented one another, and led to an inescapable conclusion that the well had been contaminated and was the source of the outbreak.
•Step 11: Implement control and prevention measures
In most outbreak investigations, the primary goal is control of the outbreak and prevention of additional cases. Indeed, although implementing control and prevention measures is listed as Step 11 in the conceptual sequence, in practice control and prevention activities should be implemented as early as possible.
health department’s first responsibility is to protect the public’s health, so if appropriate control measures are known and available, they should be initiated even before an epidemiologic investigation is launched.
For example, a child with measles in a community with other susceptible children may prompt a vaccination campaign before an investigation of how that child became infected.
Confidentiality is an important issue in implementing control measures.
Healthcare workers need to be aware of the confidentiality issues relevant to collection, management and sharing of data. For example, in the treatment of tuberculosis (TB), the relationship between the patient and the healthcare worker is extremely important because of the serious consequences of treatment failure.
If patient information is disclosed to unauthorized persons without the patient’s permission, the patient may be stigmatized or experience rejection from family and friends, lose a job, or be evicted from housing.
Explain the twelfth step (Initiate or maintain surveillance ) of investigating an outbreak
Initiate or maintain surveillance
Once control and prevention measures have been implemented, they must continue to be monitored. If surveillance has not been ongoing, now is the time to initiate active surveillance.
If active surveillance was initiated as part of case finding efforts, it should be continued. The reasons for conducting active surveillance at this time are twofold.
First, you must continue to monitor the situation and determine whether the prevention and control measures are working.
Secondly, you must moniter the outbreak if the number of new cases slowing down or, better yet, stopping? Or are new cases continuing to occur? If so, where are the new cases?
Are they occurring throughout the area, indicating that the interventions are generally ineffective, or are they occurring only in pockets, indicating that the interventions may be effective but that some areas were missed?
Explain the thirteenth step (Communicate findings) of investigating an outbreak
Communicate findings
•As noted in Step 1, development of a communications plan and communicating with those who need to know during the investigation is critical. The final task is to summarize the investigation, its findings, and its outcome in a report, and to communicate this report in an effective manner.
•An oral briefing for local authorities. If the field investigator is responsible for the epidemiology but not disease control, then the oral briefing should be attended by the local health authorities and persons responsible for implementing control and prevention measures. Often these persons are not epidemiologists, so findings must be presented in clear and convincing fashion with appropriate and justifiable recommendations for action.
•This presentation is an opportunity for the investigators to describe what they did, what they found, and what they think should be done about it. They should present their findings in a scientifically objective fashion, and they should be able to defend their conclusions and recommendations.
