Clinical and Epidemiologic Profiles for Identifying Norovirus in Acute Gastroenteritis Outbreak Investigations

Abstract

Background: Noroviruses are the leading cause of acute gastroenteritis (AGE) outbreaks in the United States. However, outbreaks attributed to norovirus often lack confirmation by diagnostic testing. Clinical and epidemiologic profiles, such as the Kaplan criteria (vomiting in >50% cases, mean incubation period of 24-48 hours, mean duration of illness for 12-60 hours, and negative bacterial stool culture), have been used to distinguish norovirus outbreaks from those caused by bacteria.

Methods: Kaplan criteria were evaluated among 10 023 outbreaks reported to the National Outbreak Reporting System (NORS) during 2009-2012. An alternate profile for distinguishing norovirus outbreaks from outbreaks caused by nonviral etiologies was identified using classification and regression tree (CART) modeling. Performance of the Kaplan criteria and alternate profile were compared among laboratory-confirmed outbreaks.

Results: The Kaplan criteria were 63.9% sensitive and 100% specific in discriminating norovirus from nonviral outbreaks, but only 3.3% of norovirus and 1.2% of nonviral outbreaks reported all criteria. Clinical and epidemiologic characteristics identified with CART modeling (ratio of proportion of cases with fever to the proportion of cases with vomiting <1, proportion of cases with bloody stool <0.1, proportion of cases with vomiting ≥0.26) were 85.7% sensitive and 92.4% specific for distinguishing norovirus from nonviral outbreaks and were applicable to more than 8 times as many outbreaks compared with the Kaplan criteria.

Conclusions: Compared with the Kaplan criteria, the CART-derived profile had higher sensitivity and broader application in reported AGE outbreaks. Thus, this alternate profile may provide a more useful tool for identifying norovirus during outbreak investigations.

Keywords: CART modeling; Kaplan criteria; acute gastroenteritis; clinical and epidemiologic profiles; norovirus; outbreaks.

Figure 1.

Classification of outbreaks reported to the National Outbreak Reporting System (NORS) based on classification and regression tree (CART) model-derived characteristics. Characteristics derived from CART modeling distinguish confirmed norovirus outbreaks from confirmed nonviral outbreaks with NORS 2009–2012 data. Starting with 2939 confirmed norovirus and 1573 confirmed nonviral outbreaks, clinical and epidemiologic predictors were assessed. Of the outbreaks with all reported characteristics, 706 confirmed norovirus and 607 confirmed nonviral outbreaks were categorized. Each rectangular partition represents a node, with the number of outbreaks and the outbreak percentage by category (total norovirus and nonviral outbreaks) displayed. Text in boldface represents the category with the most common class of outbreaks in each node. The “fever-to-vomiting ratio” represents the proportion of cases with fever divided by the proportion of cases with vomiting. The “proportion with bloody stool” represents the proportion of cases with bloody stool. The “proportion with diarrhea” represents the proportion of cases with diarrhea. The “proportion with vomiting” represents the proportion of cases with vomiting.