Surveillance Summaries / Vol. 61 / No. 9 December 14, 2012
U.S. Department of Health and Human Services
Centers for Disease Control and Prevention
Morbidity and Mortality Weekly Report
Outbreaks of Acute Gastroenteritis
Transmitted by Person-to-Person Contact —
United States, 2009–2010
Front cover photo: Two images, which include 1) norovirus and 2) hand washing.
The MMWR series of publications is published by the Office of Surveillance, Epidemiology, and Laboratory Services, Centers for Disease Control and Prevention (CDC),
U.S. Department of Health and Human Services, Atlanta, GA 30333.
Suggested Citation: Centers for Disease Control and Prevention. [Title]. MMWR 2012;61(No. SS-#):[inclusive page numbers].
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John W. Ward, MD, Atlanta, GA
MMWR / December 14, 2012 / Vol. 61 / No. 9 1
Outbreaks of Acute Gastroenteritis Transmitted
by Person-to-Person Contact — United States, 2009–2010
Mary E. Wikswo, MPH
Aron J. Hall, DVM
Division of Viral Diseases, National Center for Immunization and Respiratory Diseases, CDC
Problem/Condition: 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.
Reporting Period: 2009–2010.
Description of System: 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.
Results: 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
Interpretation: 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.
Public Health Action: 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.
Corresponding author: Mary Wikswo, Division of Viral Diseases,
National Center for Immunization and Respiratory Diseases, CDC,
1600 Clifton Rd., NE, MS A-34, Atlanta, GA 30333. Telephone:
404-639-0881; Fax: 404-235-7861; E-mail: firstname.lastname@example.org.
2 MMWR / December 14, 2012 / Vol. 61 / No. 9
Acute gastroenteritis (AGE) is a major cause of illness in
the United States, with approximately 179 million episodes
occurring each year (1,2). Outbreaks of AGE contribute to this
substantial public health problem. Data on waterborne and
foodborne AGE outbreaks have been collected nationally by
various methods since the 1920s (3,4). These methods were
formalized in 1971 and 1973 when the Waterborne Disease
and Outbreak Surveillance System (WBDOSS) and the
Foodborne Disease Outbreak Surveillance System (FDOSS),
respectively, were created as national surveillance systems
to provide complete and accurate data for waterborne and
foodborne disease outbreaks. However, no national system
existed for reporting AGE outbreaks caused by direct person-
to-person contact or other modes of transmission.
In 2006, the Council for State and Territorial Epidemiologists
passed a resolution recommending that all outbreaks of
AGE in the United States be nationally reported. The
National Outbreak Reporting System (NORS) addresses
this resolution by integrating WBDOSS and FDOSS with
the first national reporting system for person-to-person AGE
outbreaks, as well as AGE outbreaks caused by contact with
animals or contaminated environments or by unknown
modes of transmission.
The information collected through NORS can help guide
the development of appropriate strategies to prevent and
control AGE outbreaks resulting primarily from person-to-
person transmission. This information is vital to improving
understanding of these outbreaks, their frequency, and
population-level risk factors for severe illness and death. To
characterize the frequency and characteristics of person-to-
person AGE outbreaks, CDC analyzed 2009–2010 data (the
inaugural years that data were available) from NORS. This
report summarizes those findings and is intended to be used by
health departments and regulatory agencies to identify settings
and populations for interventions likely to yield the greatest
public health benefits.
NORS is a voluntary national surveillance system designed 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 United States and
its territories report these outbreaks to CDC via NORS using
a standard online data entry system. The NORS web-based
data entry system was launched in February 2009 to all 59
NORS reporting sites comprised of the 50 U.S. states, the
District of Columbia, five U.S. territories (American Samoa,
Guam, the Commonwealth of the Northern Mariana Islands,
Puerto Rico, and the U.S. Virgin Islands), and three Freely
Associated States (the Federated States of Micronesia, the
Republic of the Marshall Islands, and the Republic of Palau).
Sites were encouraged to report outbreaks occurring since
January 1, 2009, as well as those occurring prospectively.
Case Definition and Classification
All cases included in each NORS report were assumed to have
met the case definition used for that outbreak investigation.
For this analysis, the etiology of the outbreaks were categorized
as norovirus, Shigella, other/multiple, and unknown on the
basis of state reports. The term “no etiology reported” also
is sometimes referred to as an “unknown etiology.” The term
“any etiology” refers to all reported outbreaks, including those
with no etiology reported. NORS allows reporting sites to edit,
add, or delete reports at any time. To reduce this fluidity in
the data, only reports marked as “finalized” by the reporting
site administrators were included in the analyses. Data also
are subjected to basic logic checks at the conclusion of each
calendar year to improve data quality.
An outbreak from person-to-person transmission is defined
as ≥2 cases of a similar enteric illness associated with a common
exposure in which the primary mode of transmission is
reported as person-to-person contact, as determined by each
reporting site. The source or index case of each outbreak,
defined as the patient with the earliest illness onset, is included
among the outbreak-associated cases. Case definitions or
classification schemes might not be consistent across all sites.
The date of earliest illness onset is defined in NORS as the
date of outbreak occurrence. Data are reported on general
outbreak characteristics (e.g., dates, number of illnesses, and
locations), general demographic characteristics of cases (e.g.,
age and sex), symptoms, case outcomes, and laboratory testing
information and results. A reported etiology was considered
“confirmed” if ≥2 laboratory-confirmed cases were reported,
consistent with CDC guidelines for confirmation of etiologies
in foodborne disease outbreaks (5). If a reported etiology was
associated with <2 laboratory-confirmed cases, it was classified
as a suspected etiology. Symptoms were classified and reported
in NORS according to the definitions used during each
To calculate attack rates in certain settings in which exposure
occurred, states were asked to classify cases into two groups:
MMWR / December 14, 2012 / Vol. 61 / No. 9 3
residents and guests or staff. The former group is intended
to capture the number of persons who did not work in the
major setting of exposure, such as children attending childcare
centers, residents of a long-term–care facility (LTCF), or guests
of a hotel. The latter group is intended to capture the number
of persons who work in the major setting, such as health-care
providers, teachers, childcare center employees, and hotel staff.
The following outbreak reports were excluded from analysis:
all outbreak reports in which the total number of cases was not
entered, the total number of cases was <2, the reported etiology
does not cause AGE (e.g., measles, scabies, or Hepatitis A), and
outbreaks with a different primary mode of transmission (e.g.,
foodborne) with secondary person-to-person transmission.
Person-to-person AGE outbreaks with a first illness onset
during January 2009–December 2010, reported in NORS,
marked as finalized by a state administrator, and meeting
the inclusion criteria as stated in the methods are included
in this summary. Data were extracted from NORS on
January 24, 2012. Outbreak incidence in each reporting site
was calculated using national data from the U.S. Census Bureau
for 2009 for each state and expressed per 1 million population
per year (6). If a state reported outbreaks in both 2009 and
2010, the average number of outbreaks over the 2 years was
used to calculate the incidence per 1 million population per
year. If a state only reported outbreaks for 1 year, the incidence
per 1 million population for that single year was used.
During 2009–2010, NORS only allowed reporting of the
percentage of the number of cases in each age and sex category.
An age group category “unknown” is included in the analysis
because states were allowed to enter the percentage of cases that
were of unknown age as part of the total. Only outbreaks with
complete information were included in each analysis; reports
in which the total age or sex percentages were not entered or
did not add to 100% (+/- 2% to account for rounding errors)
were excluded from that analysis.
Comparisons of symptoms and case outcomes by etiology
were performed using Pearson’s chi square test. Comparisons of
the mean number of cases reported in outbreaks from different
etiologies were performed between each pair of etiologies
using the Tukey method in conjunction with one-way analysis
of variance. Comparisons of median attack rates among the
different etiologies were performed using the Kolmogorov-
Smirnov test, and comparisons of the staff and guest/resident
group were performed using the signed rank test. All analyses
were performed using SAS v 9.2 (SAS Institute, Inc.; Cary, North
Carolina). Significance was determined by p<0.05 for all analyses.
As of January 24, 2012, a total of 2,340 enteric person-to-
person outbreaks occurring during 2009–2010 were reported
to CDC through NORS. Of these, 2,259 outbreak reports
were marked finalized by a state administrator and met the
inclusion criteria (972 for 2009 and 1,287 for 2010). These
outbreaks were reported in 43 reporting sites representing
42 states and the District of Columbia. Eight states (Delaware,
Georgia, Illinois, Indiana, Montana, Nebraska, New York, and
Oklahoma), five U.S. territories (American Samoa, Guam, the
Commonwealth of the Northern Mariana Islands, Puerto Rico,
and the U.S. Virgin Islands), and three Freely Associated States
(the Federated States of Micronesia, the Republic of the
Marshall Islands, and the Republic of Palau) reported no
person-to-person AGE outbreaks occurring during 2009–2010
that met the inclusion criteria. Several of these sites, primarily
the territories (excluding Puerto Rico) and Freely Associated
States, have not reported any outbreaks in NORS or reported
only one or two outbreaks of any type during 2009–2010.
Puerto Rico and the eight U.S. states have regularly reported
foodborne and waterborne outbreaks in NORS but have not
reported any or have only reported very few outbreaks from
Of the 43 reporting sites with at least one finalized person-
to-person AGE outbreak report, 36 either began entering data
in early 2009 (during the launch period) or reported these
outbreaks retrospectively (Figure 1). Of the 36 states reporting
outbreaks occurring in 2009, five did not enter reports for
FIGURE 1. Date of first reported acute gastroenteritis outbreak
transmitted by person-to-person contact, by state — National
Outbreak Reporting System, United States, 2009–2010
4 MMWR / December 14, 2012 / Vol. 61 / No. 9
outbreaks occurring before July 2009. Forty-one states reported
at least one outbreak occurring in 2010. Of the 43 sites that
reported at least one outbreak occurring during 2009–2010,
a total of 16 sites reported <10 outbreaks each, 18 reported
10–99 outbreaks each, and nine reported >100 outbreaks each
(range: 1–276 outbreaks per site). Across all sites, the median
outbreak incidence was 4.7 per million population per year
(mean: 7.9; range: 0.03–41.9) (Figure 2).
Of the 2,259 person-to-person AGE outbreaks, 840 (37%)
had an unknown etiology, 1,410 (62%) had a single suspected
or confirmed etiology, and nine (<1%) had multiple etiologies
(Figure 3). Norovirus was the only suspected or confirmed
etiology reported in 1,270 (56%) outbreaks. Shigella was
the second most commonly reported etiology, although it
accounted for only 86 (4%) of all reported outbreaks. Other
single-etiology outbreaks were suspected or confirmed to be
caused by Salmonella (n = 16), rotavirus (n = 10), Shiga toxin-
producing E. coli (STEC) (n = 11), Giardia lamblia (n = 5),
Cryptosporidium spp. (n = 9), Clostridium difficile (n = 4), sapovirus
(n = 2), and Campylobacter jejuni (n = 1). Nine outbreaks had
multiple etiologies: norovirus and Clostridium spp. (n = 5),
norovirus and rotavirus (n = 1), Salmonella enterica and STEC
(n = 1), norovirus and Bacillus sp. (n = 1), and norovirus, rotavirus,
and Clostridium difficile (n = 1).
Of the 1,419 outbreaks with at least one reported etiology,
1,016 (72%) reported at least one laboratory confirmed etiology
and 403 (28%) outbreaks reported only a suspected etiology.
Of the 1,016 outbreaks with at least one laboratory confirmed
etiology, 886 (87%) reported norovirus as the only confirmed
etiology, and 80 (8%) listed Shigella spp. (Figure 4). Other
laboratory confirmed single-etiology outbreaks were caused by
Salmonella enterica (n = 16), STEC (n = 9), rotavirus (n = 7),
Giardia lamblia (n = 4), Cryptosporidium spp. (n = 4), Clostridium
difficile (n = 3), sapovirus (n = 2), and Campylobacter jejuni (n = 1).
Four outbreak reports listed ≥2 confirmed etiologies: norovirus
and Clostridium difficile (n = 2), norovirus and Bacillus sp. (n = 1),
and norovirus, rotavirus, and Clostridium difficile (n = 1).
Most (79%) of the 2,259 outbreaks occurred during winter
months (Figure 5). This pattern is attributed to the substantial
number of suspected and confirmed norovirus outbreaks,
which exhibit strong winter seasonality. An estimated 83%
of norovirus outbreaks occurred during October–March.
Approximately 80% of outbreaks of unknown etiology also
occurred during the winter season. In contrast, outbreaks
caused by suspected or confirmed Shigella were relatively more
No. of outbreaks
FIGURE 3. Number* and percentage of outbreaks of acute gastro-
enteritis transmitted by person-to-person contact, by etiology —
National Outbreak Reporting System, United States, 2009–2010
* N = 2,259.
† Includes Salmonella (n = 16), rotavirus (n = 10), Shiga toxin-producing E. coli
(STEC) (n = 11), Giardia lamblia (n = 5), Cryptosporidium spp. (n = 5), Clostridium
difficile. (n = 4), sapovirus (n = 2), Campylobacter jejuni (n = 1), norovirus and
Clostridium spp. (n = 5), norovirus and rotavirus (n = 1), Salmonella enterica and
STEC (n = 1), norovirus and Bacillus sp. (n = 1), and norovirus, rotavirus, and
Clostridium difficile (n = 1).
FIGURE 2. Rate* of outbreaks of acute gastroenteritis transmitted by
person-to-person contact, by state† — National Outbreak Reporting
System, United States, 2009–2010
* Incidence of outbreaks per state, per million population, on the basis of U.S.
Census Bureau population estimates.
† For states reporting outbreaks in both 2009 and 2010, the average number of
outbreaks per year was used to calculate the incidence. For states reporting
outbreaks for 1 year, the incidence per 1 million population for that single year
MMWR / December 14, 2012 / Vol. 61 / No. 9 5
frequent during summer months, although an insufficient
number of these outbreaks existed to determine a consistent
seasonal pattern. Only 38% of Shigella and 40% of other or
multiple etiology outbreaks occurred during October–March.
In the 2,259 person-to-person AGE outbreaks reported in
NORS, 81,491 cases were identified, with a mean of 36 cases
per outbreak (median: 26; range: 2–394) (Table 1). The
mean number of cases per outbreak was significantly higher
for suspected or confirmed norovirus outbreaks (44 cases)
than for outbreaks caused by Shigella (15 cases), multiple or
other etiology (22 cases), or an unknown etiology (27 cases).
Outbreaks suspected or confirmed to be caused by Shigella
had a significantly lower mean number of cases per outbreak
(15 cases) than that of outbreaks caused by an unknown
etiology (27 cases).
Of the 1,038 outbreak reports of any etiology with complete
information on distribution of cases by sex, 69% of cases
occurred in females (Figure 6). Of the 627 outbreaks with a
suspected or confirmed etiology of norovirus, 71% of cases
occurred among females. Cases reported in outbreaks suspected
or confirmed to be caused by Shigella were as likely to occur
among males as females. In outbreaks with another etiology
or unknown etiology, 66% of cases were in females, following
a similar pattern to those associated with norovirus outbreaks.
Of the 936 outbreak reports containing information on age
group distribution of cases, most cases (54%) occurred among
those aged >49 years. This pattern can be largely explained by
the substantial number of suspected and confirmed norovirus
outbreaks (N = 550), in which 58% of cases occurred among
those aged >49 years (Figure 7). Conversely, 76% of cases in
suspected or confirmed Shigella outbreaks occurred in children
FIGURE 4. Number* and percentage of outbreaks of acute gastro-
enteritis transmitted by person-to-person contact, by confirmed
etiology — National Outbreak Reporting System, United States,
* N = 1,016.
† Includes Salmonella enterica (n = 16), STEC (n = 9), rotavirus (n = 7), Giardia
lamblia (n = 4), Cryptosporidium spp. (n = 4), Clostridium difficile (n = 3),
sapovirus (n = 2), Campylobacter jejuni (n = 1), norovirus and Clostridium
difficile (n = 2), norovirus and Bacillus sp. (n = 1), and norovirus, rotavirus, and
Clostridium difficile (n = 1).
No. of outbreaks
FIGURE 5. Number* of outbreaks of acute gastroenteritis transmitted by person-to-person contact, by month of first illness onset and etiology
— National Outbreak Reporting System, United States, 2009–2010
* N = 2,259.
No. of outbreaks
Month of frst illness onset
Jul SepNov Jan MarMayJul SepNov
Shigella or Other/Multiple
6 MMWR / December 14, 2012 / Vol. 61 / No. 9
aged <10 years. In outbreaks caused by another or unknown
etiology, 43% and 52% of cases, respectively, occurred in
persons aged >49 years, similar to those associated with
norovirus outbreaks. However, these outbreaks also involved
relatively larger proportions (26% and 18%, respectively) of
children aged <10 years than norovirus outbreaks (5%).
The age distribution of cases by etiology largely relate
to the settings in which these outbreaks occurred. Overall,
of the 1,187 person-to-person AGE outbreak reports
containing information on setting, most were commonly
identified in LTCFs, schools, childcare centers, and hospitals
(Table 2). Outbreaks caused by norovirus or an unknown
etiology occurred most frequently in LTCFs (86% and
77%, respectively); thus, most cases occurred in older adults.
Outbreaks caused by Shigella occurred almost exclusively
(97%) in childcare centers; thus, the majority of cases were
reported in children aged <10 years. Other settings reported
included other health-care facilities (n = 26), camp (eight),
private settings (seven), prison or detention facilities (three),
restaurants (three), athletic facilities (three), youth centers
(two), hotels (two), adult day care facility (one), a harbor (one),
and communitywide (e.g., not limited to a single setting) (one).
Symptoms and Clinical Outcomes
Information on the proportion of cases reported with at
least one symptom was available for 1,156 (51%) outbreaks,
although not all cases or all outbreaks provided information on
each of the four symptoms analyzed in this report (Figure 8).
Among cases for which information was available, diarrhea and
vomiting were the most common symptoms reported (80% of
39,055 cases and 65% of 37,464 cases, respectively). Outbreaks
caused by norovirus or unknown etiology involved vomiting
in a significantly higher percentage of cases (65% and 68%,
respectively) than in outbreaks caused by any other suspected
or confirmed etiology (43% of cases). Fever and bloody stools
were each reported significantly more often in outbreaks caused
by Shigella (32%), multiple, or other etiologies (15%) than
TABLE 1. Number of cases in outbreaks of acute gastroenteritis transmitted by person-to-person contact, by etiology — National Outbreak
Reporting System, United States, 2009–2010
Etiology Total cases No. of outbreaks Mean casesMedian casesRange
FIGURE 6. Percentage* of cases in outbreaks of acute gastroenteritis
transmitted by person-to-person contact,† by sex and etiology —
National Outbreak Reporting System , United States, 2009–2010
* Percentages might not total 100% because of rounding.
† N = 1,038.
FIGURE 7. Percentage* of cases in outbreaks of acute gastroenteritis
transmitted by person-to-person contact,† by age group distribution
and etiology — National Outbreak Reporting System, United States,
* Percentages might not total 100% because of rounding.
† N = 936.
§ N = 550.
MMWR / December 14, 2012 / Vol. 61 / No. 9 7
with outbreaks caused by norovirus or an unknown etiology
(1% and 2%, respectively).
During 2009–2010, a total of 136 deaths were reported
among 1,670 outbreaks with information on death, and 1,339
hospitalizations were reported among 1,576 outbreaks with
information on hospitalizations. Of the 94 outbreak reports
with an associated death, 61 outbreaks occurred in an LTCF,
three in a hospital or other health-care setting, and one in a
childcare center; setting was not reported for 29 outbreaks. In
80 (85%) of these 94 outbreak reports, which accounted for 118
(87%) deaths, norovirus was identified as the only suspected or
confirmed etiology. Cryptosporidium sp. and E. coli O157:H7
were the confirmed etiology in one outbreak each, and each was
associated with one reported death. A total of 16 reported deaths
occurred in 12 outbreaks for which an etiology was not reported.
Patients reported in outbreaks caused by Shigella were
significantly more likely to seek health care (i.e., outpatient
health-care provider visits, emergency department visits, and
hospitalizations combined) than cases associated with norovirus
outbreaks (odds ratio: 10.2; 95% confidence interval = 9.3–11.2);
however, norovirus outbreaks contributed to the largest
number of deaths and health-care visits (Table 3).
Unique to person-to-person AGE outbreak surveillance,
data are collected on the total number of persons exposed,
categorized as either guests and residents or staff, thereby
allowing for calculation of attack rates. The median attack rate
for guests and residents was significantly higher in suspected
or confirmed norovirus outbreaks (35%) than in Shigella
outbreaks (12%) (Table 4). Likewise, the median attack rate
for guests and residents in outbreaks of unknown etiology
(21%) also was significantly higher than in Shigella outbreaks.
The median staff attack rate in norovirus outbreaks (19%) was
significantly higher than the median staff attack rate in Shigella
outbreaks (8%). For each outbreak etiology, the median attack
rates were significantly lower among staff than among guests
NORS is a novel reporting system that provides the first
national data on AGE outbreaks spread through person-
to-person transmission, highlighting the frequency of these
outbreaks. During 2009–2010, a total of 2,259 person-to-
person AGE outbreaks and 81,491 outbreak-related illnesses
were reported to CDC via NORS from 43 reporting sites. The
actual frequency of these outbreaks is likely underestimated, as
evidenced by the inconsistent levels of reporting in NORS of
outbreaks from different modes of transmission. In 2009, a total
TABLE 2. Number and percentage of outbreaks of acute gastroenteritis transmitted by person-to-person contact, by setting and etiology —
National Outbreak Reporting System, United States, 2009–2010
LTCFChildcare center SchoolHospitalOther*
No. (%)No. (%)No. (%)No. (%) No. (%)
Abbreviation: LTCF = long-term–care facilities.
* Other settings reported included other health-care facilities (N=26), camp (8), private settings (7), prison or detention facilities (3), athletic facilities (3), restaurants (3),
youth centers (2), hotels (2), an adult day care facility (1), a harbor (1), and communitywide (1).
FIGURE 8. Percentage*of outbreaks of acute gastroenteritis
transmitted by person-to-person contact, by symptom and etiology
— National Outbreak Reporting System, United States, 2009–2010
* Percentage calculations do not include missing data; different denominators
are used for each category (e.g., etiology by reported system).
8 MMWR / December 14, 2012 / Vol. 61 / No. 9
of 972 person-to-person AGE outbreaks and 33,085 illnesses
were reported in NORS by only 36 sites. In comparison,
during the same year, 668 foodborne outbreaks and 13,497
illnesses were reported by 45 sites (7). This further illustrates
that person-to-person transmission is an important cause of
AGE outbreaks in the United States, although surveillance for
these outbreaks has only recently been prioritized and has not
yet been fully implemented in all states.
Norovirus was the most frequently reported cause of person-
to-person AGE outbreaks. It was reported as the only suspected
or confirmed etiology in 56% of all person-to-person AGE
outbreaks and in 89% of outbreaks with an etiology reported.
These data are consistent with other studies indicating that
norovirus is the leading cause of AGE outbreaks and that
person-to-person transmission is the most common mode of
transmission (8–13). Consistent with previous reports (9–16),
LTCFs were the most frequent setting of person-to-person
norovirus outbreaks (86%) reported in NORS, which might
explain, in part, the predominance of cases among females
and older adults. The 2004 National Nursing Home Survey
indicated that 71% of nursing home patients were female (17),
which is a similar proportion of males and females in norovirus
outbreaks (86%) reported in NORS. However, among the
936 outbreak reports that include complete information
by age, only 58% of cases in norovirus outbreaks occurred
among persons aged >50 years. This might be explained, in
part, by outbreaks in which not all ages of persons with cases
were known; age was unknown in 19% of cases in norovirus
outbreaks for which at least some age information was provided
for the outbreak report. In addition, information on patient
age was reported in fewer than half of all outbreaks reported,
possibly introducing some bias. Nursing home outbreaks
also included cases among staff members, which might have
shifted the age of cases downward relative to the age of the
The median attack rates reported for suspected and
confirmed person-to-person norovirus outbreaks were
significantly higher than those reported for suspected and
confirmed Shigella outbreaks. This high norovirus attack rate
is consistent with other attack rates calculated for outbreaks
in LTCFs and previous reports in the literature (10,13). The
lower attack rates among staff might result from several factors
including better hand hygiene practices, immunity acquired
from more frequent exposures to these pathogens from
working in a high-risk setting (18,19), reluctance to report
illness (20,21) or, in settings such as LTCFs, staff members
who are younger than their residents and therefore less likely
to experience symptomatic or severe disease (12,13,16,22–24).
TABLE 3. Number and percentage* of cases in outbreaks of acute gastroenteritis transmitted by person-to-person contact, by clinical outcome
and etiology — National Outbreak Reporting System, United States, 2009–2010
Patients who visited
Patients who sought
No. (%)No. (%)No. (%)No. (%)
Abbreviation: ED = emergency department.
* Percentages might not round to 100% because each percentage was calculated using a different denominator.
† Excludes ED visits and hospitalizations.
§ Confirmed Cryptosporidium sp. and E. coli O157:H7 outbreaks were responsible for one death each.
TABLE 4. Attack rates among guests and residents or staff in outbreaks of acute gastroenteritis transmitted by person-to-person contact, by
etiology — National Outbreak Reporting System, United States, 2009–2010
* N = 976 outbreaks.
† N = 768 outbreaks.
§ As calculated using the signed rank test.
MMWR / December 14, 2012 / Vol. 61 / No. 9 9
The overall case-hospitalization and case-fatality rates for
person-to-person norovirus outbreaks also were consistent with
other studies conducted during norovirus outbreaks in LTCFs
(10,12–15). Although hospitalizations and deaths represent
a relatively small fraction of all person-to-person norovirus
outbreak-associated cases (2% and 0.3%, respectively), the
high frequency of these outbreaks resulted in 118 reported
deaths and 919 hospitalizations during 2009–2010. Norovirus
infection often results in a mild, self-limiting illness;
however, consequences of norovirus disease in elderly and
immunosuppressed populations could be especially severe and
have been previously reported to include hospitalization and
Furthermore, other studies have demonstrated that all-cause
gastroenteritis-associated hospitalizations and deaths appear
to be increasing, which might be attributable at least in part
to norovirus. In the United States, sporadic and outbreak-
associated norovirus causes an estimated 800 deaths and
70,000 hospitalizations each year. These numbers increase by
up to 50% during epidemic years associated with emergent
strains (25,26). The highest rates of norovirus-associated
hospitalizations occurred among adults aged ≥65 years; within
this group, rates increased with advancing age (25). The
elderly also accounted for 83% of AGE-associated deaths,
of which norovirus was the second leading infectious cause
after Clostridium difficile (26). A literature review of norovirus
outbreaks during 1993–2011 indicated that hospitalizations
and deaths were significantly more likely when outbreaks
occurred in health-care settings, including LTCFs (27). The
findings in this report affirm that norovirus can account for
substantial morbidity and mortality among persons with acute
gastrointestinal illness in the United States, particularly among
Person-to-person AGE outbreaks of unknown etiology
reported through NORS had epidemiologic characteristics
consistent with norovirus outbreaks. Norovirus illnesses tend to
peak during winter months, and many enteric bacterial illnesses
tend to predominate in summer (9,11,25,26,28–30). Both the
confirmed and suspected norovirus outbreaks and outbreaks of
unknown etiology reported through NORS exhibited a strong
winter seasonality, whereas outbreaks caused by other etiologies
demonstrated slight peaks during spring and summer months.
Norovirus and unknown etiology outbreaks also exhibited
similarly high frequencies of diarrhea and vomiting and low
frequencies of fever and bloody stools. A high proportion of
patients with vomiting (≥50%) and a relatively low proportion
with fever are characteristics that have been previously
demonstrated as helpful in differentiating norovirus outbreaks
from AGE outbreaks caused by other etiologies (31,32).
However, bloody stools are more commonly associated with
bacterial infections than with norovirus infections (8,33–36).
Other characteristics common among norovirus and unknown
etiology outbreaks reported through NORS included a higher
frequency of cases among older adults and females, a high
number of cases in LTCFs, and high attack rates. These findings
suggest that many of the outbreaks of unknown etiology might
have been caused by norovirus.
Although surveillance for person-to-person AGE outbreaks
is predominated by norovirus, other etiologies were identified
as important contributors. Shigella was suspected or confirmed
to have caused 86 person-to-person AGE outbreaks and 1,305
outbreak-related illnesses reported through NORS during the
first 2 years of the system. Similar to norovirus, Shigella has a
low infectious dose and is commonly transmitted person-to-
person (37–39). However, the profile of Shigella outbreaks was
distinct from norovirus and unknown etiology outbreaks, likely
in part because of the difference in setting. Almost all (97%)
Shigella outbreaks for which setting was reported occurred
in a childcare center, thus explaining why approximately
75% of reported outbreak-associated cases occurred among
children aged <10 years. Most cases of shigellosis are identified
in children aged <5 years, and Shigella is a well-recognized
cause of AGE outbreaks in childcare facilities (29,37,40–45).
Shigella outbreaks tend to affect more females than males, but
outbreaks in young children tend to have an equal distribution
of male and female patients, as is reflected in the findings of
this report (29,41).
Cases involved in Shigella outbreaks were more likely
to include fever and bloody stools than cases in norovirus
outbreaks, and substantially more patients in Shigella
outbreaks sought health care than those involved in norovirus
outbreaks (69% and 10%, respectively). These outbreaks
might have even broader implications because transmission
to household contacts is common during Shigella outbreaks
(44–46), and adults, particularly those aged ≥65, have much
higher hospitalization rates for shigellosis than children (37).
Furthermore, antibiotics are widely used during shigellosis
outbreaks in the United States, yet multidrug resistance is
common among Shigella bacteria, limiting antibiotic treatment
Some reported etiologies in person-to-person AGE outbreaks,
including STEC and Salmonella, are primarily considered
foodborne pathogens, and before 2009, national surveillance
systematically captured only foodborne outbreaks of these
pathogens. In 2009, a total of 119 foodborne outbreaks of
Salmonella were reported via NORS; five additional Salmonella
outbreaks were reported through person-to-person AGE
outbreak surveillance. An estimated 40 foodborne outbreaks
of STEC were reported in 2009, and five additional STEC
outbreaks were reported as transmitted by person-to-person
10 MMWR / December 14, 2012 / Vol. 61 / No. 9
contact (7). In 2010, six additional STEC person-to-
person outbreaks were reported, including one outbreak of
STEC O157:H7 in a childcare facility that resulted in a death.
Studying these outbreaks is especially important because
person-to-person transmission of STEC is a recognized cause of
outbreaks in childcare settings, and young children are most at
risk for STEC infection and the complication hemolytic uremic
syndrome (HUS) (47). Surveillance for person-to-person AGE
outbreaks via NORS also might be important in clarifying the
epidemiology and role of non-O157 STEC, which has been
underrecognized because of limitations in surveillance and
The findings in this report are subject to at least two
limitations. First, similar to many other passive reporting
systems, NORS is subject to underreporting. NORS ultimately
relies on health-care providers and facilities reporting potential
outbreaks to state and local health departments, which often
depends on the general public seeking medical care. However,
only 15%–20% of persons with an acute diarrheal illness seek
medical care and only 17%–19% of those submit a stool sample
for testing (51,52). This type of underreporting is influenced by
behavioral patterns and nuances of the health-care system in the
United States. In addition, underreporting to NORS probably
occurs because state and local health departments, which often
have limited resources and competing responsibilities, might
not have the resources available to investigate each potential
outbreak or submit a completed report in NORS. Furthermore,
NORS is a new reporting system with variable adoption and
use, which might vary by modes of illness transmission. For
example, Puerto Rico and the eight U.S. states that did not
have any NORS reports that met the inclusion criteria for
this summary have been regularly reporting foodborne and
waterborne outbreaks in NORS but have not reported any or
have only reported very few outbreaks from person-to-person
transmission. Only 36 sites reported person-to-person AGE
outbreak data in 2009, compared with 45 sites that reported
foodborne outbreaks during the same year (7). The primary
mode of illness transmission also is determined by each
reporting site and case definitions or classification schemes
might not be consistent across all sites. Notably, the most
populous states reported no or disproportionately low numbers
of person-to-person AGE outbreaks, suggesting that the
number of outbreaks in this report is an underestimate, and the
findings reported might not be generalizable. System coverage
could improve as sites become more familiar with NORS and
as features are enhanced or added to facilitate reporting.
Second, only four variables (outbreak identification, date of
first illness, reporting site, and total ill) are required to submit
an outbreak report via NORS. Although this affords flexibility
to sites that would like to report outbreaks for which they have
only limited information available, it results in variable levels
of completeness of other variables collected in the system (e.g.,
age, sex, etiology, and setting).
The findings in this report enable a better understanding of
the frequency, causes, and patient outcomes of AGE outbreaks
in the United States, especially those caused by person-to-
person transmission. During 2009–2010, norovirus was the
most frequently reported cause of person-to-person AGE
outbreaks. On the basis of epidemiologic characteristics,
norovirus also might be responsible for a substantial portion
of the 840 reported outbreaks of unknown etiology. An
additional 86 outbreaks were reported to be caused by Shigella.
No vaccines exist for either norovirus or Shigella in the United
States, and recommendations for prevention and control
of person-to-person AGE outbreaks depend primarily on
appropriate hand hygiene and isolation of ill persons.
Although norovirus and Shigella were associated with the
majority of AGE outbreaks, they were not the only reported
cause of person-to-person AGE outbreaks. Approximately 3%
of outbreaks were caused by other or multiple etiologies, many
of which are considered primarily foodborne pathogens but can
be transmitted through multiple routes, such as Salmonella and
STEC. Further study of these person-to-person AGE outbreaks
should provide a better understanding of these pathogens and
how they can be spread. 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.
This report is based on contributions by state, territorial, and local
health departments reporting this information to CDC. Constructive
comments on this report were contributed by Umesh Parashar, Ben
Lopman, Anna Bowen, Benjamin Nygren, Katherine Heiman,
Barbara Mahon, Benjamin Silk, Rebecca Hall, Hannah Gould,
and Jonathan Yoder.
MMWR / December 14, 2012 / Vol. 61 / No. 9 11
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