A total of 572 specimens were included in the study. Routine bacterial culture and viral PCR were performed on all specimens, and 375 (65.6%) were also tested for ova and parasites, 161 (28.1%) for C. difficile, and 28 (4.9%) for rotavirus by using EIA. Specimens not tested for ova and parasites or C. difficile were considered not positive for those pathogens. Adult patients 26–65 years of age provided 325 (56.8%) of the specimens tested and 316 (55.2%) were from female patients. Clinical diagnostic testing at the KP laboratory identified a pathogen in 42 (7.3%) specimens. Subsequent PCR testing at CDC identified >1 virus in 53 (9.3%) specimens, for a total of 88 (15.4%) specimens with a detected pathogen (includes 5 specimens previously positive for rotavirus by EIA and 2 specimens positive for bacteria and virus). Among these 88 specimens, viruses were detected in 53 (60.2%), bacteria in 30 (34.1%), and parasites in 7 (8.0%). Multiple pathogens were identified in only 4 specimens (Table 2). Norovirus was the most frequently identified pathogen overall, detected in 25 (4.4%) of all specimens and 28.4% of the 88 specimens with any pathogen identified. Detection of any pathogen in feces was most likely in children <5 years of age (32.1%) and decreased significantly with age to 3.6% for persons >65 years of age (p<0.001).

Most (62.4%) specimens were received for laboratory testing within 3 days of the patient’s medical visit. However, 90 (15.7%) were received >1 week later. Time lag did not differ by age group. Pathogens were detected most frequently among specimens received by the laboratory within 3 days of outpatient visit (18.2%) and were significantly less likely to be detected with increasing delay, decreasing to 6.7% among specimens received after >1 week (p<0.001). This time lag effect was more pronounced for viruses than bacteria and was likely caused by the decrease in viral shedding and test sensitivity after the acute phase of illness. For example, norovirus was identified in 5.0% of specimens received <7 days of outpatient visit, but in only 1.1% of those received >1 week after outpatient visit; overall detection of bacteria decreased from 5.6% to 3.3% during the same time frame. Most (81.4%) specimens were unformed (i.e., took on the shape of the collection cup), indicating that the patients were still symptomatic at the time of specimen collection. No differences in rates of pathogen detection were found between formed and unformed specimens.

Overall rate of pathogen detection and the relative distribution of pathogens showed apparent seasonal variation, although insufficient sample size precluded identification of significant temporal trends. Overall, viruses predominated during winter and spring, and bacteria predominated during summer and fall (Figure 2). Relatively low prevalence of parasites was observed year round with no discernible seasonal pattern. Pronounced winter peaks in prevalence were observed for norovirus (10.5%) and astrovirus (4.2%). All rotavirus-positive specimens were received during winter and spring, during which prevalence was 1.4% and 3.5%, respectively. Similarly, peak prevalence of sapovirus (2.8%) and adenovirus (2.1%) was during spring.

Overall estimated community incidence of AGE of any etiology was 41,000 cases/100,000 person-years (90% CI 38,000–44,000 cases), of which 13,000 (32%, 90% CI 10,000–20,000 cases) were caused by identified agents (Table 3). Estimated incidence of AGE in outpatients was 5,400 cases/100,000 person-years (90% CI 4,400–6,700 cases), of which 1,600 (30%, 90% CI 1,300–2,400 cases) could be attributed to a specific pathogen. Norovirus was the leading identifiable cause of illness in community and outpatient settings and was associated with 16% of all community AGE (6,500 cases/100,000 person-years, 90% CI 3,700–12,000 cases) and 12% of all outpatient AGE (640/100,000 person-years, 90% CI 360–1,200 cases).

In comparison with estimated community incidence of AGE caused by bacteria (1,700 cases/100,000 person-years, 90% CI 1,100–2,300) and parasites (420 cases/100,000 person-years, 90% CI 200–790), the community incidence of norovirus was ≈4-fold and >15-fold greater, respectively. Likewise, the incidence of norovirus infection prompting medical care was ≈3-fold greater than that of bacterial pathogens (240 cases/100,000 person-years, 90% CI 160–320 cases) and >10-fold greater than that of parasites (60 cases/100,000 person-years, 90% CI 29–110 cases). Astrovirus was the second leading identifiable pathogen associated with AGE in community and outpatient settings; estimated incidences were 1,800 cases (90% CI 880–3,400 cases) and 270 (90% CI 130–590 cases) per 100,000 person-years, respectively.

Sequence analysis of the 25 norovirus-positive samples identified 24 (96%) as GII strains and 1 (4%) as a GI strain (Figure 3). Of all strains, approximately one third could be typed as GII.4, specifically the Farmington Hills and Hunter subclusters. GII.3 viruses were equally prevalent in this population. Other norovirus genotypes identified included GI3b, GII.2, GII.14, and GII.17.

This study provides US laboratory–based incidence rates for norovirus disease determined by using direct outpatient surveillance of routinely collected fecal specimens. Despite examination of only fecal specimens submitted for bacterial culture, our data demonstrate that viruses, and noroviruses specifically, were the leading cause of AGE among persons of all ages seeking medical care. We estimate that 24% of persons seeking medical care for AGE do so because of viral infection, including 12% because of norovirus infection. Overall, viruses accounted for 27% of AGE episodes in the community and noroviruses accounted for 16%.

In contrast to bacterial and parasitic pathogens, viral AGE pathogens were primarily detected during winter and spring months, which is consistent with previous descriptions of norovirus and rotavirus seasonality in temperate climates (16). Norovirus strains identified in this study were similar to those identified in studies of sporadic AGE (17) and those implicated in norovirus outbreaks confirmed by CDC during the same period (18). The predominance of GII.4 strains in epidemic and sporadic norovirus disease demonstrates the need for including this genotype in vaccine development efforts.

Our findings are consistent with those of studies conducted in other industrialized countries, which demonstrated the relative role of viral pathogens in causing AGE (19–23). A recent systematic literature review that included 13 etiologic studies of community and outpatient cases of sporadic diarrhea, most of which focused on children, determined that norovirus was responsible for 12% (range 5%–36%) of AGE cases (17). In a community-based study in Germany, the incidence of AGE requiring medical consultation attributable to norovirus was 626 cases/100,000 person-years (19). In England, the incidence of general practice consultations for norovirus-associated infectious intestinal disease was 540 cases/100,000 person-years (24). The estimated incidence of norovirus among outpatients in our study (640 cases/100,000 person-years) supports the findings of the studies in England and Germany. A similar study in the Netherlands reported a considerably lower incidence of gastroenteritis in general practices (797 cases/100,000 person-years), in which 5% (40 cases/100,000 person-years) were caused by norovirus (20). However, standardized gastroenteritis incidence reported in the Dutch community of 28,300 cases/100,000 person-years is more similar to that estimated in our study (41,000 cases/100,000 person-years) (25). Dutch guidelines for general practitioners, which recommend that cases of uncomplicated AGE be handled by telephone consultation (http://nhg.artsennet.nl), may explain the apparent difference in health care utilization practices.

Prior US estimates of the incidence of norovirus have been based on a bottom-up approach of using overall incidence of AGE from population surveys and estimating the proportion attributable to norovirus on the basis of data from other countries, given the lack of laboratory-based data available at the time (3). This approach yielded an estimate of ≈21 million cases of norovirus illness annually, corresponding to a community incidence similar to that estimated in our study (6,974 cases/100,000 person-years vs. 6,500 cases/100,000 person-years). Similarly generated bottom-up estimates for astrovirus, rotavirus, and sapovirus (each 1,024 cases/100,000 person-years) were likewise within 90% CIs of our community incidence estimates (3).

Although markedly different approaches were used, each subject to considerable uncertainty, concordance of the respective incidence estimates generated is reassuring. However, given the inclusion of only specimens submitted for routine bacterial diagnostics, our study likely underestimated the true incidence of norovirus disease in the community. Furthermore, use of health care utilization multipliers based primarily on diarrhea may underestimate the incidence of pathogens that can cause other AGE signs (e.g., vomiting) in the absence of diarrhea, as has been reported for norovirus (26). Previous studies have demonstrated that seeking medical care is influenced by disease severity and social factors (27,28), which leads to a smaller proportion of viral AGE patients in the community who seek medical care, compared with bacterial and parasitic AGE (22). Identifying laboratory-confirmed cases of norovirus infection and extrapolating community incidence by using pathogen-specific health care utilization multipliers in a top-down approach may be preferable when such data are available and would likely yield more reliable estimates.

The primary limitation of this study results from use of routinely collected specimens, as opposed to systematic, active patient recruitment. The study specimens may have therefore been obtained from patients with noninfectious or chronic diseases, which may have contributed to the low detection rate for pathogens in this study. Delays in collection of specimens may have also contributed to the low rates of pathogen detection. Routinely collected samples also overrepresent some age groups, as demonstrated by differential fecal submission rates (Table 1), which may not reflect age groups at greatest risk for infection by specific pathogens or risk for the overall population.

We have adjusted for this sampling artifact by development of pathogen-specific, age-adjusted health care utilization multipliers. These multipliers resulted in the observed increase from pathogen prevalence in specimens (e.g., 4.4% for norovirus) to pathogen prevalence among estimated community AGE cases (e.g., 16% for norovirus). In addition to age, pathogen-specific health care utilization multipliers also accounted for duration of illness and yielded different pathogen distributions in community and outpatient settings. These results are consistent with those of previous studies (20,22,25). In contrast to a previous study that estimated US incidence of AGE (3), our study did not adjust for differential health care utilization rates based on illness severity (i.e., bloody vs. nonbloody diarrhea) to maintain adequate power to enable the adjustment for age group. Nonetheless, there was insufficient sample size within each age group to generate age group–specific disease incidence estimates.

Attributing causality of AGE to norovirus infection is complicated by the fact that asymptomatic infections may occur; healthy persons used in previous studies as controls have shown background rates of infection of 1%–16% (17). However, low prevalence (1.1%) of norovirus in specimens received >1 week after outpatient visit and low number of mixed infections identified in our study suggest that norovirus was likely the etiologic agent when detected. Incidence estimates were not adjusted for test sensitivity because the molecular methods used for viral diagnostics generally show extremely high sensitivities if appropriate and timely specimens are tested. Prevalence and incidence of parasites and C. difficile may be underestimated given that not all specimens were tested for these pathogens and indications for clinicians requesting specific tests are unknown and likely varied. Finally, given that the study population included only Georgia residents and health maintenance organization members, which tends toward younger age and higher socioeconomic status, the results may not be generalizable to the overall US population. Restriction of FoodNet data to only respondents in Georgia who had health insurance was evaluated for comparison but did not differ. Therefore, all respondents were ultimately included to enable adequate power for age stratification.

Development of sensitive clinical assays for identification of viral agents of AGE, such as norovirus, and more widespread use of such assays may help close the diagnostic gap on sporadic AGE cases and guide more appropriate case management. The demonstrated predominance of viruses among medically attended AGE cases should help prevent the unnecessary use of antimicrobial drugs and spur development of novel interventions specific for the unique transmission pathways of viruses. Compared with bacterial AGE etiologies, many of which result from foodborne transmission from infected animal sources, viral AGE pathogens originate in human reservoirs and usually involve direct or indirect person-to-person spread. Although occasionally implicated in outbreaks (29,30), the role of foods contaminated during processing in the overall norovirus disease incidence remains largely unknown. Further assessment of the incidence of enteric viruses, including hospital-based studies of risk factors for severe disease and attribution to specific transmission pathways, are needed to improve control measures and assess future potential of vaccines.