Enteric illness outbreaks reported through the National Outbreak Reporting System, United States, 2009-19.

BACKGROUND The National Outbreak Reporting System (NORS) captures data on foodborne, waterborne, and enteric illness outbreaks in the United States. The aim of this study is to describe enteric illness outbreaks reported during 11 years of surveillance. METHODS We extracted finalized reports from NORS for outbreaks occurring during 2009-2019. Outbreaks were included if they were caused by an enteric etiology or if any patients reported diarrhea, vomiting, bloody stools, or unspecified acute gastroenteritis. RESULTS A total of 38,395 outbreaks met inclusion criteria, increasing from 1,932 in 2009 to 3,889 in 2019. Outbreaks were most commonly transmitted through person-to-person contact (n=23,812, 62%) and contaminated food (n=9,234, 24%). Norovirus was the most commonly reported etiology, reported in 22,820 (59%) outbreaks, followed by Salmonella (n=2,449, 6%) and Shigella (n=1,171, 3%). Norovirus outbreaks were significantly larger, with a median of 22 illnesses per outbreak, than outbreaks caused by the other most common outbreak etiologies (p<0.0001, all comparisons). Hospitalization rates were higher in outbreaks caused by Salmonella and E. coli outbreaks (20.9% and 22.8%, respectively) than those caused by norovirus (2%). The case fatality rate was highest in E. coli outbreaks (0.5%) and lowest in Shigella and Campylobacter outbreaks (0.02%). CONCLUSIONS Norovirus caused the most outbreaks and outbreak-associated illness, hospitalizations, and deaths. However, persons in E. coli and Salmonella outbreaks were more likely to be hospitalized or die. Outbreak surveillance through NORS provides the relative contributions of each mode of transmission and etiology for reported enteric illness outbreaks, which can guide targeted interventions.