Next Step in Outbreak Investigation
The next step is to conduct a case-control study to systematically identify the specific food items or exposures associated with illness through structured comparison of affected and unaffected individuals. 1
Rationale for Case-Control Study
The outbreak investigation follows a sequential approach, and you have already completed the initial phase by establishing that an outbreak exists (15 of 50 staff ill with similar symptoms and common restaurant exposure). 1 The current priority is conducting a case-control study rather than jumping to environmental investigations or control measures without epidemiologic direction. 1
The CDC explicitly recommends conducting a formal case-control study to quantify the strength of association between suspected exposures and illness using odds ratios, which will guide all subsequent targeted interventions. 1 This approach prevents wasting time and resources on broad, unfocused interventions like quarantining all restaurant staff before identifying the actual food vehicle. 1
Why Not the Other Options
Option A (Isolation) - Premature
- Isolation of affected individuals is not the immediate priority when the source remains unidentified in a foodborne outbreak. 1
- This intervention becomes relevant later only for specific pathogens with significant person-to-person transmission concerns, which has not yet been determined. 1
Option C (Quarantine Restaurant Staff) - Inefficient
- Quarantining restaurant staff without epidemiologic evidence identifying the specific food vehicle wastes time and resources. 1
- This determination appropriately comes after the case-control study identifies the implicated food and subsequent environmental investigation confirms the source. 1
Option D (Collect Restaurant Data) - Out of Sequence
- While collecting detailed data about the restaurant's food processing is important, it represents the environmental investigation phase that follows the case-control study. 1
- Without epidemiologic direction from the case-control study, you won't know which specific foods, preparation areas, or handlers to focus on. 1
Implementation of Case-Control Study
Define cases using clinical criteria (acute gastroenteritis with specific symptom onset timing) and 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, then calculate odds ratios for each food exposure to identify the vehicle of infection. 1 Historical data shows this approach can identify associations with odds ratios as high as 84.0 for specific implicated foods. 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. 2 Bulk diarrheal specimens (enough to fill a large stool cup) should be stored at +4°C rather than frozen to preserve viral morphology for electron microscopy. 2
Subsequent Investigation Phases
After the case-control study identifies the implicated food, the environmental investigation phase will involve collecting samples of that specific food for microbiologic testing and screening relevant food handlers for infection. 1 Restaurant-associated outbreaks commonly involve norovirus (46% of confirmed outbreaks) and are frequently linked to food handler contamination, including asymptomatic carriers. 3, 4
Exclude infected food handlers until appropriate clearance (pathogen-specific criteria), and provide education on hygiene practices to prevent future outbreaks. 1