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BACKGROUND: In the United States, outbreaks of acute gastroenteritis (AGE) spread by person-to-person contact, contaminated environmental surfaces, and unknown modes of transmission have not been well characterized compared to foodborne and waterborne outbreaks. Herein, we report the frequency and characteristics of AGE outbreaks spread through these three modes of transmission that are reported to CDC through the National Outbreak Reporting System (NORS), a web-based national surveillance system.
METHODS: Local, state, and territorial public health agencies report enteric disease outbreaks, which are defined as ≥2 illnesses associated with a common exposure, to NORS. We performed a descriptive analysis of all finalized 2009–2013 NORS reports of outbreaks resulting from person-to-person, environmental, and unknown transmission routes based on case counts, etiology, seasonality, and setting.
RESULTS: A total of 9,194 person-to-person, environmental, or unknown transmission mode outbreaks of AGE were reported by 49 states, the District of Columbia, and Puerto Rico during 2009–2013. Of these, 7,635 (83%) outbreaks were spread through person-to-person contact, resulting in 270,428 illnesses, 472 deaths, and 4,367 hospitalizations. Forty-three (0.5%) outbreaks were reportedly due to environmental contamination, resulting in 1,467 illnesses, 2 deaths, and 37 hospitalizations. Additionally, 1,516 (16%) outbreaks were spread by unknown transmission routes, resulting in 26,942 illnesses, 35 deaths, and 872 hospitalizations. Among the outbreaks with at least one suspected or confirmed etiology (n=5,892), norovirus was the most common, reported in 4,842 (82%) of all outbreaks and in 4,350 (89%) person-to-person, 25 (66%) environmental contamination, and 467 (49%) unknown transmission mode outbreaks. Most of the outbreaks spread through these three modes of transmission occurred during the months of October through March (n=6,978, 76%). Among the outbreaks with a reported major setting of exposure (n=6,694), long-term care facilities were the most common, reported in 4,601 (72%) of 6,371 person-to-person outbreaks, 3 (21%) of 14 environmental contamination outbreaks, and 97 (31%) of 309 unknown transmission mode outbreaks.
CONCLUSIONS: NORS provides the first national data on AGE outbreaks spread through person-to-person contact, environmental contamination, and unknown modes of transmission and highlights the significant public health burden that these outbreaks represent. Norovirus accounts for the majority of these outbreaks, which occur most frequently in long-term care facilities and during the winter months. NORS surveillance data can help identify priority targets for the development of control strategies and assess trends to evaluate their future impact towards decreasing the frequency and severity of enteric disease outbreaks in the United States.