What is the next step in managing an outbreak of acute gastroenteritis (inflammation of the stomach and intestines) among 15 out of 50 hospital staff who consumed food from the same restaurant before symptom onset?

Conduct a Case-Control Study to Identify Common Exposures (Option B)

The next step in this outbreak investigation is to conduct a formal case-control study to systematically identify which specific food items from the restaurant are associated with illness. 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 confirmed 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. 1
• This approach quantifies the strength of association between suspected exposures and illness using odds ratios, as demonstrated in the Connecticut outbreak investigation which identified an odds ratio of 84.0 for potato salad. 2, 1
• The case-control study provides epidemiologic direction for all subsequent control measures, preventing wasted time and resources on unfocused environmental investigations. 1

Why Other Options Are Premature or Inappropriate

Isolation of Affected Individuals (Option A)

• Isolation is not the immediate priority in a foodborne outbreak investigation where the source remains unidentified. 1
• This would be appropriate later for specific pathogens with person-to-person transmission concerns, but only after identifying the vehicle of infection. 1
• Most foodborne gastroenteritis outbreaks are self-limited and do not require isolation unless specific high-risk settings are involved. 2

Quarantining Restaurant Staff (Option C)

• Quarantining restaurant staff 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
• While ill food handlers are a common source in viral outbreaks, this must be confirmed through systematic investigation rather than assumed. 2

Collecting Restaurant Food Processing Data (Option D)

• The WHO advises against delaying the case-control study to pursue environmental investigations first, as this wastes time and resources without epidemiologic direction. 1
• Environmental and laboratory investigation, including detailed food processing data collection, is the subsequent phase that follows the case-control study. 1
• Without knowing which specific food item is implicated, investigating all food processing practices is inefficient and unfocused. 1

Practical Implementation of the Case-Control Study

• Define cases using clinical criteria: acute gastroenteritis with specific symptom onset timing after restaurant exposure. 1
• Identify controls: hospital staff who ate at the restaurant during the same time period but remained well. 1
• Conduct structured interviews: systematically ask about all specific food items consumed at the restaurant. 2, 1
• Calculate odds ratios: for each food exposure to identify the vehicle of infection with statistical confidence. 2, 1

Subsequent Steps After Case-Control Study

Once the case-control study identifies the implicated food item:

• Collect stool specimens from at least 10 ill persons during the first 48 hours of illness, as viral diagnostic yield drops precipitously after 2-3 days. 1
• Conduct environmental investigation of the restaurant, including inspection of food handling practices and screening of food handlers. 2, 1
• Implement targeted control measures based on identified risk factors, such as excluding infected food handlers until stool cultures are negative. 1
• Provide food handler education on hygiene practices to prevent future outbreaks. 1

Critical Pitfalls to Avoid

• Do not assume all restaurant exposures are equal without epidemiologic evidence—the case-control study identifies specific high-risk items. 1
• Do not overlook asymptomatic food handlers, as they can be colonized and transmit infection without symptoms, particularly with Salmonella. 2, 3
• Do not delay specimen collection, as diagnostic yield decreases significantly after 48-72 hours from symptom onset. 1