Norovirus Outbreak among Attendees of a Hospital Teaching Conference

Background: Norovirus is a well-recognized cause of outbreaks of gastroenteritis in the hospital setting. 

Objective: We report an outbreak of norovirus gastroenteritis among attendees of a grand rounds conference at the Philadelphia Veterans Hospital.

Methods: The Department of Behavioral Health at the Philadelphia Veterans Hospital held its weekly grand rounds, and a local caterer provided lunch. Two days following the conference, the department noted a significant number of employees calling out sick, and Infection Control was contacted for further investigation and management. We developed a questionnaire and surveyed all employees who had attended the conference. We defined cases as individuals who developed vomiting following attendance at the conference, and controls were individuals who attended the conference but did not report the onset of an illness characterized by vomiting. A questionnaire was used to ask each attendee about the presence and timing of symptoms, as well as the food items they had eaten at the conference. We used a case-control design and calculated odds ratios for disease based on individual food items. Occupational health clinic analyzed stool samples from two employees for the presence of norovirus by nucleic acid testing as well as for bacterial pathogens.

Results: We obtained 63 surveys from conference attendees, including 23 individuals who reported vomiting and 35 individuals who reported no symptoms. There were 5 individuals who reported symptoms that did not include vomiting who were excluded from the analysis. The highest odds ratios were found with the following: ham sandwich (OR 18, 95% CI 3 to 179), ice (OR 7, 95% CI 2 to 29), and potato chips (OR 6, 95% CI 2 to 24). Both the ice and chips came from shared open containers. Stool samples from two symptomatic employees were both positive for norovirus by nucleic acid testing and negative for bacterial pathogens. Following our investigation, the city department of health was notified and the caterer was evaluated. The health department informed us that there were no significant health code violations, but could not report to us whether there were any ill employees. Following notification of Infection Control, all symptomatic employees were sent home, and instructed to remain at home until completely asymptomatic for 72 hours. No secondary cases were identified within the hospital, either among employees or patients.

Conclusions: This investigation illustrates the potential for multiple food source contamination by norovirus, which suggests contamination either at the site of preparation by a food handler or during distribution of food at the event. In summary, the investigation and containment of this outbreak highlights the value of an analytic outbreak investigation and the success that appropriate infection control measures can have in limiting the spread of norovirus in the hospital setting.