An outbreak investigation helps identify the mode of transmission of the disease, the etiologic agent, who may be at risk of infection, and ultimately prevent additional cases and reduce the overall morbidity and/or mortality rates. An outbreak investigation may also allow evaluation of the sensitivity and specificity of a surveillance system, evaluation or implementation of intervention strategies (i.e., vaccination, social distancing, or removal of a point source) and contribution to the epidemiology and scientific knowledge of the disease.

A systematic, step-by-step approach to conducting an outbreak investigation is imperative for identifying the source of the outbreak and for controlling and preventing additional cases. Any questions about the outbreak investigation process or need for assistance should be directed to the District Epidemiologists. The steps of an outbreak investigation are not rigid in their order and several often occur simultaneously.

1. Obtain the initial report.

• Collect available information, including:
  • Contact information for the person making the report
  • Suspected agent (laboratory findings or provider clinical diagnosis)
  • Signs and symptoms of illness
  • Person(s) or groups ill, number ill, number potentially exposed
  • Summary of demographic information (ages, sex, etc.)
  • Onset date(s) and duration of illness(es)
  • Suspected specific exposure or event
  • Location of ill persons (single county, multi-county, multi-state, etc.)
  • Suspected mode of transmission (food, water, animal, person-to-person, healthcare-associated)
  • Actions taken to date (control measures, environmental assessments/inspections, etc.)
• Notify the District Epidemiologists of any suspected or known outbreaks via email or phone call.

2. Establish the existence of an outbreak.

• Determine if there are other associated cases and if the number of cases exceeds the expected baseline by reviewing recent surveillance data (i.e., disease registries, syndromic data).

3. Verify the diagnosis.

• Collect additional clinical samples, if needed.
• Request confirmatory testing at a reference laboratory, if needed.

4. Notify public health partners.

• Ensure that key partners from public health laboratory, environmental health, regulatory agencies, and other partners are identified and aware of the outbreak.

5. Create an outbreak case definition.

• Combine clinical characteristics, laboratory test results, and epidemiological information into criteria for the categorization of cases to determine who is included in the outbreak. The outbreak case definition is almost always restricted by person (i.e., daycare attendees or staff), place (i.e., patients in a specific wing of a hospital, attendees of a wedding), and time (i.e., persons with illness onset between certain days). The initial case definition is often quite broad, to capture all possible cases; however, as the investigation proceeds it generally becomes more refined and divided into subcategories, such as suspect (i.e., fever only), probable (i.e., fever with cough and epidemiologically linked to a confirmed case), and confirmed (i.e., fever and cough with laboratory confirmation of influenza).

6. Find cases systematically and record information.

• Create a line list with case information, including variables such as onset date, patient demographics, clinical data, laboratory data, and risk factor or epidemiologic data.
• If appropriate, develop a focused questionnaire based on information from initial surveillance efforts (person, place, and time variables) and the hypothesized agent, source, and mode of transmission.
• If applicable, select an appropriate study design (e.g., cohort or case-control) based on the circumstances to test the hypothesis. The design should specify how a comparison group of non-ill persons will be selected and what statistical analyses will be performed.
• If appropriate, work with public health partners to conduct an environmental assessment and collect specimens. Work with the District Epidemiologists to coordinate analysis of clinical and environmental specimens with the Missouri State Public Health Laboratory.

7. Perform descriptive and analytical epidemiology.

• Characterize the data by person, place, and time.
• If appropriate, obtain denominator data to calculate attack rates and identify potential associations and risk factors.
• Create an epidemic curve that reflects case illness onsets over time. The epidemic curve can give clues about the mode of transmission (e.g., person-to-person transmission or point/common source exposure), size of the outbreak, and incubation period.
• Select the categories to be analyzed for risk factors and/or associations using Epi Info or other suitable statistical software such as SAS, SPSS, or R.
• Analyze the data to identify differences in exposure frequencies between the ill and well groups (if case/control study), or differences in illness rates between exposed and non-exposed (if cohort study), to confirm or refute the hypothesis.
• If appropriate, the data can also be displayed in map form, revealing potential information such as common source exposure or clustering of cases.

8. Develop and test hypotheses.

• Use the information from the statistical analysis, along with laboratory data, environmental findings, and any other relevant information, to evaluate the hypothesis and formulate conclusions. If the hypothesis does not appear to be confirmed, it may be necessary to modify the direction of the investigation or to formulate a new hypothesis.

9. Implement control and prevention measures.

• Implement control measures that are indicated by the statistical, environmental, laboratory, and other findings to prevent further spread of the agent. Examples include:
  • isolation and quarantine
  • providing vaccine or immune globulin to exposed contacts
  • recalling, embargoing, or destroying food
  • correcting a contaminated water source or supply system
  • making a public announcement of the outbreak
  • improving sanitation, food handling or infection control practices
  • closing a restaurant until corrections can be made
  • recommending antibiotic treatment and/or exclusion (from work, child care etc.)
  • using barrier precautions such as masks and gloves
• Evaluate the control measures for efficacy. Consider control measures successful if additional cases are prevented. Outbreaks are generally considered over after two incubation periods have passed since the last onset date occurred. If more than two incubation periods have passed and new cases are still occurring, then the outbreak is not under control. New cases may represent continuing exposure from a common source, a new common source, or person-to-person transmission from old cases.
• Continue to identify problem(s), develop new solution(s), implement control and prevention measures, and evaluate.

10. Continue or enhance surveillance.

• Continue or enhance surveillance to assure cases are not missed and that implemented control measures have been effective.

11. Communicate findings.

• The results of the investigation should be shared with stakeholders (i.e., hospital staff, local health authorities, scientific community) so that others can learn from the investigation and recommendations. Reliable, complete information about outbreaks contributes to understanding the trends and causal factors in disease incidence, and to detecting and evaluating new diseases and risks. An outbreak report may be used to justify resources that were expended and/or to identify a need for additional resources for future incidents. An outbreak report can be requested under Missouri’s Sunshine Law and may serve as evidence in legal proceedings. Please see the next section regarding outbreak documentation.