Conduct a Case-Control Study to Identify Common Exposures
The next step is to conduct a formal case-control study (Option B) to systematically identify which specific food items from the restaurant are associated with illness through structured interviews and statistical analysis. 1
Why Case-Control Study is the Immediate Priority
The outbreak investigation has already completed the initial phase—establishing that an outbreak exists with 15 of 50 staff ill with similar symptoms and common restaurant exposure. 1 The current phase requires conducting a case-control study to define cases, identify controls from the same exposed population, and conduct structured interviews about specific food items consumed. 1
The CDC explicitly recommends conducting a formal case-control study to identify specific food items or exposures associated with illness through systematic comparison of cases and controls, rather than pursuing environmental investigations first. 1 This approach is demonstrated by the Connecticut Department of Public Health investigation, which identified an odds ratio of 84.0 for potato salad through case-control methodology, providing clear epidemiologic direction for targeted interventions. 2, 1
Practical Implementation Steps
Define cases using clinical criteria: acute gastroenteritis with specific symptom onset timing after restaurant exposure. 1
Identify controls from the same population: hospital staff who ate at the restaurant but remained well. 1
Conduct structured interviews about all specific food items consumed at the restaurant, including preparation methods and timing. 2, 1
Calculate odds ratios for each food exposure to identify the vehicle of infection with statistical precision. 1
Collect stool specimens immediately from at least 10 ill persons during the first 48 hours of illness, as viral diagnostic yield drops precipitously after 2-3 days when viral shedding decreases below detectable levels. 1
Why Other Options Are Premature or Inefficient
Isolation of affected individuals (Option A) is not the immediate priority in a foodborne outbreak investigation where the source remains unidentified. 1 Isolation would be appropriate later for specific pathogens with person-to-person transmission concerns, but the primary goal now is identifying the contaminated food vehicle. 1
Quarantining restaurant staff (Option C) without epidemiologic evidence identifying the specific food vehicle wastes time and resources. 1 This determination comes after the case-control study identifies the implicated food and subsequent environmental investigation confirms the source. 1
Collecting detailed data about food processing (Option D) should occur after the case-control study provides epidemiologic direction about which specific foods to investigate. 1 The WHO advises against delaying the case-control study to pursue environmental investigations first, as this wastes time and resources without epidemiologic direction. 1
Subsequent Investigation Phases
After completing the case-control study, the investigation will proceed to environmental and laboratory investigation, including:
- Collecting samples of implicated foods for microbiologic testing. 1
- Screening food handlers for infection, particularly those who handled the implicated food items. 1
- Excluding infected food handlers until stool cultures are negative. 1
- Providing food handler education on hygiene practices to prevent future outbreaks. 1
Critical Pitfall to Avoid
Do not assume all restaurant exposures are equal—the case-control study identifies specific high-risk items with quantifiable associations (odds ratios), which is essential for targeted control measures rather than broad, inefficient interventions. 1 The Connecticut outbreak investigation demonstrates how this approach can identify associations as strong as OR 84.0 for specific foods, providing clear direction for intervention. 2, 1