Conduct a Case-Control Study to Identify Specific Food 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
Rationale for Case-Control Study as the Immediate Next Step
The outbreak has already been established with 15 of 50 hospital staff ill with similar symptoms and common restaurant exposure, completing the initial investigation phase. 1 The critical next step is 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 broad interventions without epidemiologic direction. 1
Why Not the Other Options
Isolation (Option A) is not the immediate priority in a foodborne outbreak investigation where the source remains unidentified, and would be appropriate later only for specific pathogens with person-to-person transmission concerns. 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. 1
Collecting detailed restaurant data (Option D) should not precede the case-control study, as the WHO advises against delaying epidemiologic investigation to pursue environmental investigations first, which wastes time and resources without epidemiologic direction. 1
Implementation of the Case-Control Study
Define Cases and Controls
Define cases using clinical criteria such as acute gastroenteritis with specific symptom onset timing. 1
Identify controls from the same population—hospital staff who ate at the restaurant but remained well. 1
Conduct Structured Interviews
Conduct structured interviews about all specific food items consumed at the restaurant, not just general exposure. 1
Calculate odds ratios for each food exposure to identify the vehicle of infection. 1
Historical data demonstrates that this approach can identify associations with odds ratios as high as 84.0 for specific implicated foods, as shown in the Connecticut Department of Public Health investigation of potato salad. 1
Concurrent Laboratory Investigation
While conducting the case-control study, immediately collect stool specimens from at least 10 ill persons during the first 48 hours of illness. 1 Viral diagnostic yield drops precipitously after 2-3 days when viral shedding decreases below detectable levels. 2, 1
Store bulk diarrheal specimens at +4°C rather than frozen to preserve viral morphology for electron microscopy. 1
For norovirus outbreaks specifically, collect whole stool specimens from at least five persons during the acute phase (≤72 hours from onset) for diagnosis by RT-qPCR. 2
Subsequent Control Measures Based on Study Results
After the case-control study identifies the specific food vehicle and subsequent environmental investigation confirms the source:
Exclude infected food handlers until appropriate clearance criteria are met (typically 48-72 hours after symptom resolution for most pathogens). 2, 1
Provide food handler education on hygiene practices to prevent future outbreaks. 1
Do not overlook asymptomatic food handlers, as they can be colonized and transmit infection without symptoms. 1, 3
Critical Pitfall to Avoid
The most common error is assuming all restaurant exposures are equal and implementing broad control measures without first conducting the case-control study to identify specific high-risk items. 1 This approach is inefficient and may miss the actual source while wasting resources on unnecessary interventions.