Approximately 179 million cases of acute gastroenteritis (AGE) occur in the United States each year, and outbreaks of AGE are a substantial public health problem. Although CDC has conducted national surveillance for waterborne and foodborne AGE outbreaks since 1971 and 1973, respectively, no national surveillance existed for AGE outbreaks resulting primarily from person-to-person transmission before implementation of the National Outbreak Reporting System (NORS) in 2009.
NORS is a national surveillance system launched in 2009 to support the reporting of all waterborne outbreaks and enteric disease outbreaks from foodborne, person-to-person, animal contact, environmental, and unknown modes of transmission. State and local public health agencies in the 50 U.S. states, the District of Columbia, five U.S. territories, and three Freely Associated States report these outbreaks to CDC via NORS using a standardized online data entry system. Data are collected on general outbreak characteristics (e.g., dates, number of illnesses, and locations), demographic characteristics of cases (e.g., age and sex), symptoms, case outcomes, and laboratory testing information and results. Only outbreaks reported in NORS with a primary mode of transmission of person-to-person contact are included in this report.
During 2009-2010, a total of 2,259 person-to-person AGE outbreaks were reported in NORS from 42 states and the District of Columbia. These outbreaks resulted in 81,491 reported illnesses, 1,339 hospitalizations, and 136 deaths. No etiology was reported in approximately 40% (n = 840) of outbreaks. Of the remaining 1,419 outbreaks with a reported etiology, 1,270 (89%) were either suspected or confirmed to be caused solely by norovirus. Other reported etiologies included Shigella (n = 86), Salmonella (n = 16), Shiga toxin-producing Escherichia coli (STEC) (n = 11), and rotavirus (n = 10). Most (82%) of the 1,723 outbreaks caused by norovirus or an unknown etiology occurred during the winter months, and outbreaks caused by Shigella or another suspected or confirmed etiology most often occurred during the spring or summer months (62%, N = 53 and 60%, N = 38, respectively). A setting was reported for 1,187 (53%) of total outbreaks. Among these reported settings, nursing homes and other long-term-care facilities were most common (80%), followed by childcare centers (6%), hospitals (5%), and schools (5%).
NORS provides the first national data on AGE outbreaks spread primarily through person-to-person transmission and describes the frequency of this mode of transmission. Norovirus is the most commonly reported cause of these outbreaks and, on the basis of epidemiologic characteristics, likely accounts for a substantial portion of the reported outbreaks of unknown etiology. In the United States, sporadic and outbreak-associated norovirus causes an estimated 800 deaths and 70,000 hospitalizations annually, which could increase by an additional 50% during epidemic years. During 2009-2010, norovirus outbreaks accounted for the majority of deaths and health-care visits in person-to-person AGE outbreaks reported to NORS.
Prevention and control of person-to-person AGE outbreaks depend primarily on appropriate hand hygiene and isolation of ill persons. NORS surveillance data can help identify the etiologic agents, settings, and populations most often involved in AGE outbreaks resulting primarily from person-to-person transmission and guide development of targeted interventions to avert these outbreaks or mitigate the spread of infection. Surveillance for person-to-person AGE outbreaks via NORS also might be important in clarifying the epidemiology and role of certain pathogens (e.g., STEC) that have been traditionally considered foodborne but can also be transmitted person-to-person. As ongoing improvements and enhancements to NORS are introduced, participation in NORS has the potential to increase, allowing for improved estimation of epidemic person-to-person AGE and its relative importance among other modes of transmission.