Outbreaks of Acute Gastroenteritis Transmitted by Person-to-Person Contact, Environmental Contamination, and Unknown Modes of Transmission--United States, 2009-2013.

PROBLEM/CONDITION: Acute gastroenteritis (AGE) is a major cause of illness in the United States, with an estimated 179 million episodes annually. AGE outbreaks propagated through direct person-to-person contact, contaminated environmental surfaces, and unknown modes of transmission were not systematically captured at the national level before 2009 and thus were not well characterized.

REPORTING PERIOD: 2009-2013.

DESCRIPTION OF SYSTEM: The National Outbreak Reporting System (NORS) is a voluntary national reporting system that supports reporting of all waterborne and foodborne disease outbreaks and all AGE outbreaks resulting from transmission by contact with contaminated environmental sources, infected persons or animals, or unknown modes. Local, state, and territorial public health agencies within the 50 U.S. states, the District of Columbia (DC), five U.S. territories, and three Freely Associated States report outbreaks to CDC via NORS using a standard online data entry system.

RESULTS: A total of 10,756 AGE outbreaks occurred during 2009-2013, for which the primary mode of transmission occurred through person-to-person contact, environmental contamination, and unknown modes of transmission. NORS received reports from public health agencies in 50 U.S. states, DC, and Puerto Rico. These outbreaks resulted in 356,532 reported illnesses, 5,394 hospitalizations, and 459 deaths. The median outbreak reporting rate for all sites in a given year increased from 2.7 outbreaks per million population in 2009 to 11.8 outbreaks in 2013. The etiology was unknown in 31% (N = 3,326) of outbreaks. Of the 7,430 outbreaks with a suspected or confirmed etiology reported, norovirus was the most common, reported in 6,223 (84%) of these outbreaks. Other reported suspected or confirmed etiologies included Shigella (n = 332) and Salmonella (n = 320). Outbreaks were more frequent during the winter, with 5,716 (53%) outbreaks occurring during December-February, and 70% of the 7,001 outbreaks with a reported setting of exposure occurred in long-term-care facilities (n = 4,894). In contrast, 59% (n = 143) of shigellosis outbreaks, 36% (n = 30) of salmonellosis outbreaks, and 32% (n = 84) of other or multiple etiology outbreaks were identified in child care facilities.

INTERPRETATION: NORS is the first U.S. surveillance system that provides national data on AGE outbreaks spread through person-to-person contact, environmental contamination, and unknown modes of transmission. The increase in reporting rates during 2009-2013 indicates that reporting to NORS improved notably in the 5 years since its inception. Norovirus is the most commonly reported cause of these outbreaks and, on the basis of epidemiologic data, might account for a substantial proportion of outbreaks without a reported etiology. During 2009-2013, norovirus accounted for most deaths and health care visits in AGE outbreaks spread through person-to-person contact, environmental contamination, and unknown modes of transmission.

PUBLIC HEALTH ACTION: Recommendations for prevention and control of AGE outbreaks transmitted through person-to-person contact, environmental contamination, and unknown modes of transmission depend primarily on appropriate hand hygiene, environmental disinfection, and isolation of ill persons. NORS surveillance data can help identify priority targets for the development of future control strategies, including hygiene interventions and vaccines, and help monitor the frequency and severity of AGE outbreaks in the United States. Ongoing study of these AGE outbreaks can provide a better understanding of certain pathogens and their modes of transmission. For example, certain reported outbreak etiologies (e.g., Salmonella) are considered primarily foodborne pathogens but can be transmitted through multiple routes. Similarly, further examination of outbreaks of unknown etiology could help identify barriers to making an etiologic determination, to analyze clinical and epidemiologic clues suggestive of a probable etiology, and to discover new and emerging etiologic agents. Outbreak reporting to NORS has improved substantially since its inception, and further outreach efforts and system improvements might facilitate additional increases in the number and completeness of reports to NORS.