Next Step in Outbreak Investigation
The next step is to conduct a case-control study to identify specific food items or exposures associated with illness (Option B). 1
Rationale for Case-Control Study
The outbreak investigation has already completed its initial phase—establishing that an outbreak exists with 15 of 50 staff ill with similar symptoms and common restaurant exposure. 1 The current priority is conducting a formal case-control study to systematically compare cases and controls, which will:
- 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
- Quantify the strength of association between suspected exposures and illness using odds ratios, which can be as high as 84.0 for specific implicated foods, as demonstrated in similar investigations 1
Why Other Options Are Premature
Isolation of affected individuals (Option A) is not the immediate priority when the source remains unidentified, and would be appropriate later only for specific pathogens with person-to-person transmission concerns. 1 While viral gastroenteritis can spread person-to-person among hospital staff 2, identifying the food vehicle takes precedence in a suspected foodborne outbreak.
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 data about food processing (Option D) represents the environmental investigation phase, which should follow—not precede—the case-control study. 1 The CDC and WHO advise 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 and screening food handlers for infection 1
- Collecting 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 1
- Excluding infected food handlers until stool cultures are negative 1
Critical Pitfalls to Avoid
Do not assume all restaurant exposures are equal—the case-control study identifies specific high-risk items rather than broad interventions. 1 Historical outbreak data shows that specific foods (like potato salad in one investigation) can have dramatically higher odds ratios than others. 1
Do not overlook asymptomatic food handlers, as they can be colonized and transmit infection without symptoms. 1, 3 In documented outbreaks, pre-symptomatic food handlers have contaminated food before developing symptoms themselves. 4