8.3 Case Definitions, Case Interviews, & the 72-Hour Food History

8.3 Case Definitions, Case Interviews, & the 72-Hour Food History

Operational Standard: Investigating a foodborne outbreak requires a systematic, multi-step epidemiological approach. Public health investigators must establish precise case definitions, interview exposed individuals using standardized questionnaires, and move logically from descriptive observation to statistical hypothesis testing.

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The 10 Steps of a Foodborne Outbreak Investigation

The CDC framework outlines 10 standardized steps for conducting an outbreak investigation. While presented sequentially, steps often occur simultaneously in practice.

               THE 10-STEP OUTBREAK INVESTIGATION FLOWCHART
┌─────────────────────────────┐     ┌─────────────────────────────┐     ┌─────────────────────────────┐
│ 1. Detect Outbreak          │ ──> │ 2. Confirm Diagnosis        │ ──> │ 3. Establish Case Definition│
└─────────────────────────────┘     └─────────────────────────────┘     └─────────────────────────────┘
                                                                                       │
                                                                                       ▼
┌─────────────────────────────┐     ┌─────────────────────────────┐     ┌─────────────────────────────┐
│ 6. Develop Hypotheses       │ <── │ 5. Descriptive Epi (P,P,T)  │ <── │ 4. Count & Find Cases       │
└─────────────────────────────┘     └─────────────────────────────┘     └─────────────────────────────┘
              │
              ▼
┌─────────────────────────────┐     ┌─────────────────────────────┐     ┌─────────────────────────────┐
│ 7. Analytical Testing (RR/OR)│ ──>│ 8. Environmental Traceback  │ ──> │ 9. Implement Control        │
└─────────────────────────────┘     └─────────────────────────────┘     └─────────────────────────────┘
                                                                                       │
                                                                                       ▼
                                                                        ┌─────────────────────────────┐
                                                                        │ 10. Communicate Findings    │
                                                                        └─────────────────────────────┘

Step 1: Detect a Possible Outbreak

Outbreaks are detected through public health surveillance systems (PulseNet WGS cluster alerts, FoodNet signals), clinician reporting, or consumer illness complaints logged at local environmental health departments.

Step 2: Confirm the Diagnosis

Obtain clinical specimens (stool, blood, vomitus) from ill patients and confirm laboratory findings. Rule out laboratory error, misdiagnosis, or reporting artifacts.

Step 3: Establish a Case Definition

Formulate a standardized set of criteria to determine whether an individual should be classified as a case associated with the specific outbreak.

Step 4: Count Cases & Find Additional Ill Persons

Conduct active case finding by querying local hospitals, emergency departments, clinical labs, school nurses, and exposed event attendees to establish the true magnitude of the outbreak.

Step 5: Perform Descriptive Epidemiology

Organize and summarize case data according to Person (age, gender, underlying health conditions), Place (residence, event location, retail store locations), and Time (construct the epidemic curve).

Step 6: Develop Hypotheses

Integrate descriptive epidemiology, pathogen incubation characteristics, and known food vehicle vectors to formulate hypotheses regarding the suspect food source and point of contamination.

Step 7: Evaluate Hypotheses with Analytical Studies

Conduct formal statistical studies—such as Cohort Studies (calculating Relative Risk) or Case-Control Studies (calculating Odds Ratios)—to test the statistical association between suspect foods and illness.

Step 8: Conduct Environmental Health Assessments & Traceback

Inspect the implicated facility to identify root causes and trace suspect ingredients backward through distributors, processors, and farms to locate the point of contamination.

Step 9: Implement Control & Prevention Measures

Deploy interventions to halt pathogen transmission: execute product recalls, issue public health advisories, embargo contaminated inventory, exclude ill food handlers, and mandate facility deep cleaning.

Step 10: Communicate Findings

Document investigation findings in a written outbreak report, submit data to CDC NORS, and share lessons learned with public health partners and the food industry.

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Developing a Standardized Case Definition

A Case Definition is a restricted set of uniform criteria used to decide whether an individual's illness is part of the outbreak under investigation.

Core Components of a Case Definition

1. Clinical Criteria: Specific clinical signs and symptoms characteristic of the pathogen (e.g., acute watery diarrhea defined as $>= 3$ loose stools in 24 hours, fever $>= 100.4^°F$, or severe abdominal cramps).
2. Time Criteria: Specific onset date and time boundaries (e.g., symptom onset occurring between June 1 and June 15).
3. Place / Exposure Criteria: Geographic boundary or specific event attendance (e.g., patrons who dined at Restaurant X or attended the County Fair).
4. Laboratory Criteria: Laboratory confirmation of the pathogen via culture, PCR, or Whole Genome Sequencing (WGS).

Case Classification Hierarchy

                      CASE CLASSIFICATION MATRIX
┌─────────────────┬───────────────────────────────────────────────────────────────────┐
│ Classification   │ Criteria Required                                                 │
├─────────────────┼───────────────────────────────────────────────────────────────────┤
│ Confirmed Case  │ Meets clinical criteria AND has laboratory verification (WGS/PCR) │
│ Probable Case   │ Meets clinical criteria AND has epi-link, but lacks lab verification│
│ Suspect Case    │ Meets basic clinical symptoms compatible with illness only        │
└─────────────────┴───────────────────────────────────────────────────────────────────┘

• Confirmed Case: A case that meets clinical criteria and has laboratory isolation or molecular confirmation of the specific pathogen strain.
• Probable Case: A case that meets clinical criteria and has a clear epidemiological link (e.g., shared a meal with a confirmed case), but lacks laboratory specimen confirmation.
• Suspect Case: An ill individual who meets basic clinical criteria compatible with the illness, but lacks laboratory testing and a confirmed epidemiological link.

Balancing Sensitivity vs. Specificity

• Early Investigation (High Sensitivity): Investigators use a broad, sensitive case definition to capture as many potential cases as possible and establish outbreak scope.
• Analytical Phase (High Specificity): Investigators refine the case definition to be strictly specific (requiring lab confirmation) to prevent misclassifying healthy or unrelated ill individuals during hypothesis testing.

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Questionnaire Design & Exposure Histories

Administering standardized exposure questionnaires to ill cases and healthy controls is vital for gathering accurate data.

Exposure Time Windows

The questionnaire must cover a food exposure history matched to the pathogen's known incubation period:

• Short Incubation Pathogens (1 to 72 Hours): For Staphylococcus aureus, Clostridium perfringens, Salmonella, E. coli, or Norovirus, capture a 72-hour food history prior to symptom onset.
• Long Incubation Pathogens (5 Days to 8 Weeks): For Listeria monocytogenes (incubation up to about 70 days, average 21 days) or Hepatitis A virus (incubation 15 to 50 days), capture an extended food history covering the month before onset (up to 70 days) prior to onset.

Essential Questionnaire Sections

1. Demographic Profile: Name, age, sex, occupation, contact details.
2. Clinical History: Exact date and time of symptom onset, primary symptoms, illness duration, medical care sought, hospital admission, stool specimen submission.
3. Detailed Food History: Systematic review of all meals, snacks, beverages, ice, garnishes, and condiments consumed during the exposure window.
4. Non-Food Exposures: Contact with farm animals, pets, recreational water, travel history, and secondary household contacts.