Question

What is considered an outbreak?

How to investigate a disease outbreak?

PLEASE EXPLAIN THE ANSWER IN DETAILS.

Answer 1

A disease is considered as outbreak, when a disease occurs in greater numbers than expected in a community or region or during a season.

An outbreak may occur in one community or even extend to several countries.

It can last from days to years.

Sometimes a single case of a contagious disease is considered an outbreak.

This may be true if it is an unknown disease, is new to a community, or has been absent from a population for a long time.

A disease outbreak is the occurrence of disease cases in excess of normal expectancy. The number of cases varies according to the disease-causing agent, and the size and type of previous and existing exposure to the agent.

Disease outbreaks are usually caused by an infection, transmitted through person-to-person contact, animal-to-person contact, or from the environment or other media.

Occasionally the cause of an outbreak is unknown, even after thorough investigation.

Communicable disease outbreaks

Environmental factors influencing the spread of communicable diseases

Water, sanitation, food and air quality are vital elements in the transmission of communicable diseases and in the spread of diseases prone to cause epidemics.

Disease outbreaks caused by chemicals:

Outbreaks may also occur following exposure to chemicals or to radioactive materials. For example, Minamata disease is caused by exposure to mercury.

Disease outbreaks of unknown etiology:

Occasionally, an outbreak is seen in a population for which the cause is unclear.

Such an outbreak may be due to a new or modified pathogen, a natural toxin, or it may be due to an initially undetected release of a chemical agent or over-exposure to ionizing radiation.

INVESTIGATION OF DISEASE OUTBREAK:

Investigations are essential to determine source of outbreaks , Be systematic , Follow the steps to investigate an outbreak.

Reasons to Investigate an Outbreak:

• Identify the source (and eliminate it)

• Develop strategies to prevent future outbreaks

• Evaluate existing prevention strategies

• Describe new diseases and learn more about known diseases

• Address public concern

• It’s your job

When to Investigate:

Consider the following factors:

• Severity of illness

• Transmissibility

• Unanswered questions

• Ongoing illness/exposure

• Public concern

Principles of Outbreak Investigations:

• Be systematic

– Follow the same steps for every type of outbreak

– Write down case definitions

– Ask the same questions of everybody

• Stop often to re-assess what you know

– Line list and epi curve provide valuable information; many investigations never go past this point

• Coordinate with partners (e.g., environmental and epidemiology)

Steps of an Outbreak Investigation

there are 10 steps of an Outbreak Investigation
1. Identify investigation team and resources

2. Establish existence of an outbreak

3. Verify the diagnosis

4. Construct case definition

5. Find cases systematically and develop line listing

6. Perform descriptive epidemiology/develop hypotheses

7. Evaluate hypotheses/perform additional studies as necessary

8. Implement control measures

9. Communicate findings

10. Maintain surveillance

1. Identify investigation team and resources

Investigation Resources include

• Local

– Epi teams

• State

– CD Branch epidemiologists / subject matter experts

– Nurse Consultants

– PHRST teams

– Disease Investigation Specialists (DIS)

• Other

– Team Epi-Aid (UNC)

– CDC

2. Establish existence of an outbreak :

Outbreak is an increase in cases above what is expected in that population in that area

Four kids with cough and runny nose in a child care center in January?

Woman vomiting after eating at Restaurant A?

10 members of the swim team vomiting after eating at Restaurant A?

One case of smallpox?

3. Verify the diagnosis

• Obtain medical records and lab reports – Contact Public Health Epidemiologist in Hospital & Infection Preventionists

• Conduct clinical testing if needed – Consult with CD Branch, State Lab

4. Construct case definition

Components of Case Definition

• Person...... Type of illness (e.g., “a person with...”)

• Place......... Location of suspected exposure

• Time.......... Based on incubation (if known)

Sample Outbreak Case Definition

Hepatitis A outbreak:

• Person: An acute illness involving jaundice or elevated liver function tests

• Place: Occurring after visiting or residing on Property A

• Time: During May–August 2006

5. Find cases systematically and develop line listing:

What to Put on a Line List

1. Clinical information

• Symptoms (type, duration)

• Onset dates and/or times

2. Demographic information

3. Exposure information Use line list to summarize information

6. Perform descriptive epidemiology/develop hypotheses:

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.
• From this characterization you can identify or infer the population at risk for the disease.
• The characterization often provides clues about etiology, source, and modes of transmission that can be turned into testable hypotheses (see Step 7).
• Descriptive epidemiology describes the where and whom of the disease, allowing you to begin intervention and prevention measures.
• Early (and continuing) analysis of descriptive data helps you to become familiar with those data, enabling you to identify and correct errors and missing values.

• Time

  Traditionally, a special type of histogram is used to depict the time course of an epidemic. This graph, called an epidemic curve, or epi curve for short, provides a simple visual display of the outbreak's magnitude and time trend.

• Epidemic curves are a basic investigative tool because they are so informative (see Lesson 6).

• The epi curve shows the magnitude of the epidemic over time as a simple, easily understood visual. It permits the investigator to distinguish epidemic from endemic disease. Potentially correlated events can be noted on the graph.
• The shape of the epidemic curve may provide clues about the pattern of spread in the population, e.g., point versus intermittent source versus propagated.
• The curve shows where you are in the course of the epidemic — still on the upswing, on the down slope, or after the epidemic has ended. This information forms the basis for predicting whether more or fewer cases will occur in the near future.
• The curve can be used for evaluation, answering questions like: How long did it take for the health department to identify a problem? Are intervention measures working?
• Outliers — cases that don't fit into the body of the curve —may provide important clues.
• If the disease and its incubation period are known, the epi curve can be used to deduce a probable time of exposure and help develop a questionnaire focused on that time period.

• Drawing an epidemic curve. To draw an epidemic curve, you first must know the time of onset of illness for each case. For some diseases, date of onset is sufficient. For other diseases, particularly those with a relatively short incubation period, hour of onset may be more suitable

  Occasionally, you may be asked to draw an epidemic curve when you don't know either the disease or its incubation time. In that situation, it may be useful to draw several epidemic curves with different units on the x-axis to find one that best portrays the data.

  • For example figure sho wsOutbreak of SalmonellaEnteritidis Gastroenteritis — Maryland, 2003 (Epidemic Curve by 6-Hour Intervals)

    

    Source: Castel AD, Blythe D, Edwards L, Totaro J, Shah D, Moore M. A large outbreak of Salmonella Enteritidis infections associated with crabcakes at a church fundraiser–Maryland, 2003. Presented at 53rd Annual Epidemic Intelligence Service Conference, April 19–23, 2004, Atlanta.

  • Interpreting an epidemic curve.

  • The first step in interpreting an epidemic curve is to consider its overall shape. The shape of the epidemic curve is determined by the epidemic pattern (for example, common source versus propagated), the period of time over which susceptible persons are exposed, and the minimum, average, and maximum incubation periods for the disease.

    An epidemic curve that has a steep upslope and a more gradual down slope (a so-called log-normal curve) is characteristic of a point-source epidemic in which persons are exposed to the same source over a relative brief period. In fact, any sudden rise in the number of cases suggests sudden exposure to a common source one incubation period earlier (Figure 6.3).

    In a point-source epidemic, all the cases occur within one incubation period. If the duration of exposure is prolonged, the epidemic is called a continuous common-source epidemic, and the epidemic curve has a plateau instead of a peak. An intermittent common-source epidemic (in which exposure to the causative agent is sporadic over time) usually produces an irregularly jagged epidemic curve reflecting the intermittence and duration of exposure and the number of persons exposed. In theory, a propagated epidemic — one spread from person-to-person with increasing numbers of cases in each generation — should have a series of progressively taller peaks one incubation period apart, but in reality few produce this classic pattern.

  • Place

    Assessment of an outbreak by place not only provides information on the geographic extent of a problem, but may also demonstrate clusters or patterns that provide important etiologic clues. A spot map is a simple and useful technique for illustrating where cases live, work, or may have been exposed.

    Some spot maps indicate each patient's residence. If the map shows a cluster or other pattern (such as cases along a road), the investigator must consider possible explanations — perhaps water supplies, wind currents, or proximity to a restaurant or grocery. A spot map, like that used by John Snow in London in 1854 can give clues about mode of spread.( For example, clustering of cases in a wing of a nursing home is consistent with either a focal source or person-to-person spread, whereas scattering of cases throughout the facility is more consistent with a widely disseminated vehicle or a source common to the residents that is not associated with room assignment, such as a common dining hall or water supply. In an outbreak of pneumococcal pneumonia in a nursing home in New Jersey, cases were more common in the north wing than in the south wing . Nursing home staff did report that the 2 residents of the south wing who developed pneumonia did spend much of their time in the north wing.

    Figure shows cases of Pneumonia by Room, Nursing Home A — New Jersey, 2001

    

    South wing North wing Case-resident Dining PT Kit. TV TV Main entrance

    Adapted from: Tan C. A preventable outbreak of pneumococcal pneumonia among unvaccinated nursing home residents in New Jersey during 2001. Infect Control Hosp Epidemiol 2003;24:848–52.

  • Person

    Characterization of the outbreak by person provides a description of whom the case-patients are and who is at risk. Person characteristics that are usually described include both host characteristics (age, race, sex, and medical status) and possible exposures (occupation, leisure activities, and use of medications, tobacco, and drugs). Both of these influence susceptibility to disease and opportunities for exposure.

    The two most commonly described host characteristics are age and sex because they are easily collected and because they are often related to exposure and to the risk of disease. Depending on the outbreak, occupation, race, or other personal characteristics specific to the disease under investigation and the setting of the outbreak may also be important. For example, investigators of an outbreak of hepatitis B might characterize the cases by intravenous drug use and sexual contacts, two of the high risk exposures for that disease. Investigators of a school-based gastroenteritis outbreak might describe occurrence by grade or classroom, and by student versus teacher or other staff.

    Early in an investigation, investigators may restrict the descriptive epidemiology to numbers of cases. However, in many circumstances the investigators also calculate rates (number of cases divided by the population or number of people at risk). Numbers indicate the burden of disease and are useful for planning and service delivery. Rates are essential for identifying groups with elevated risk of disease.

  • 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."

    Another useful way to generate hypotheses is to talk to a few of the case-patients, as discussed in Step 3. The conversations about possible exposures should be open-ended and wide-ranging, not necessarily confined to the known sources and vehicles. In some challenging investigations that yielded few clues, investigators have convened a meeting of several case-patients to search for common exposures. In addition, investigators have sometimes found it useful to visit the homes of case-patients and look through their refrigerators and shelves for clues to an apparent foodborne outbreak.

    Just as case-patients may have important insights into causes, so too may the local health department staff. The local staff know the people in the community and their practices, and often have hypotheses based on their knowledge.

    The descriptive epidemiology may provide useful clues that can be turned into hypotheses. If the epidemic curve points to a narrow period of exposure, what events occurred around that time? Why do the people living in one particular area have the highest attack rate? Why are some groups with particular age, sex, or other person characteristics at greater risk than other groups with different person characteristics? Such questions about the data may lead to hypotheses that can be tested by appropriate analytic techniques.

7. Evaluate hypotheses/perform additional studies as necessary

Additional Studies

• Types

Cohort

Case-control

• Designed to assess exposures equally among ill and non-ill Cohort Studies

• Include EVERYONE who could have been exposed

– Only use if a complete list is available

– Meeting attendees, students, LTCF residents, etc

. • Measure of association = Relative Risk Relative Risk (RR)

• RR = 1.0

Risk same among exposed and unexposed

• RR > 1.0

Risk is HIGHER among exposed

• RR < 1.0

Risk is LOWER among exposed

Case-Control Studies

• Compare exposures among ill persons (case-patients) and non-ill persons (controls)

• Used when a complete list is not available or too large

– Restaurant outbreaks, national outbreaks, etc

. • Measure of association = Odds Ratio Interpretation of Odds Ratio

• OR = 1.0

Same odds of exposure among ill and non-ill

• OR > 1.0

HIGHER odds of exposure among ill

• OR < 1.0

LOWER odds of exposure among ill

8. Implement control measures

Control Measures

• Can occur at any point during outbreak

• Isolation, cohorting, product recall

• Balance between preventing further disease and protecting credibility and reputation of institution

• Should be guided by epidemiologic results in conjunction with environmental investigation

9. Communicate findings

Inform Public and Media

• Public & press are not aware of most outbreak investigations

• Media attention desirable if public action needed

• Response to media attention important to address public concerns about outbreak

– Single overriding communication objective (SOCO)

• Results of investigations public information

10. Maintain surveillance

Maintain Surveillance include

 Deciding if outbreak is over

 Documenting effectiveness of control measures

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. Is 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?

Second, you need to know whether the outbreak has spread outside its original area or the area where the interventions were targeted. If so, effective disease control and prevention measures must be implemented in these new areas.