During 2010, we conducted a one-year surveillance of viral etiology of ILI and found that of 337,272 outpatients who sought attention in the Outpatient Department of Zhuhai Municipal People’s Hospital, 3,747 (1.11%) presented with ILI. Of 3,747 outpatients with ILI, 924 (24.66%) had available samples and were enrolled in this study and ranged in age from one month to 78 years. Only two patients were older than 60 years of age, while 536 (58.01%) were under five years old and 308 (33.33%) were between the ages of 5–14. The age distribution of the 924 study participants was representative of the 3,747 patients presenting with ILI (Figure 1). This susceptibility of ILI in the younger age categories may be caused by several reasons. First, the immune system of children under five years old is naive and more susceptible of viral infections. Second, juveniles between the ages of 5–14 are generally students who are densely-gathered in schools promoting the spread of respiratory infectious. In addition, their immune systems are also still in development. Third, although aging impacts immunity to viral infection , patients older than 60 years of age usually have some comorbidities, such as hypertension, diabetes, or chronic bronchitis, and seek medical attention only when these primary diseases worsen when combined with ARI. Moreover, elderly people may self-administer according to their own experience. Of 924 samples, about 44.48% were positive for at least one virus, which is consistent with previous studies where viruses were detected in a range between 36% and 58% [31–34]. Among the tested viral pathogens, influenza virus, RSV, and HMPV exhibited characteristic seasonal patterns. In addition, there were correlations between the incidence of sFluA (H3N2), ADV, and RSV with air temperature, as well as between incidence of sFluB and air humidity. Therefore, the knowledge from our study is important for planning and implementation of effective interventions, including vaccinations.
During the study period, the introduction and emergence of H1N1pdm09 virus occurred, while sFluA (H1N1) was not detected in Zhuhai city. It is possible that the epidemic of H1N1pdm09 interfered with the appearance of sFluA (H1N1). The incidence of H1N1pdm09 was highest in adults between 25–59 years of age in 2010 (10.91%), with significant differences compared to other age groups (P = 0.033). Of the total cases of ILI, 10.39% were infected with sFluB, which was the predominant viral pathogen and occurred mainly from February–May in 2010, corresponding with a previous study of seasonal patterns . Among 308 patients between 5–14 years of age, 66 (21.43%) were infected with sFluB and there was a significant difference in age distribution (P < 0.001). Infection with sFluA (H3N2) occurred predominantly in children less than five years old, which was significantly higher than other age groups (P = 0.009). In addition, the rate of sFluA (H3N2) + RSV was the most common viral co-infection in the respiratory tract of patients with ILI. In accordance with a previous study , the seasonal variability of sFluA (H3N2) was clear, presenting mainly from July–October, perhaps influenced by the epidemic of H1N1pdm09. We found that influenza strains were the most commonly detected viruses in agreement with other studies . In addition, two significant correlations were observed; first, the higher the air temperature, the higher the incidence of sFluA (H3N2) (r = 0.592, P = 0.043), and second, the incidence of sFluB positively correlated with air relative humidity (r = 0.866, P < 0.001).
RSV, reported to be almost as common as influenza viruses, had the greatest impact on the youngest age groups [18, 37] and was a major viral pathogen of ILI during the study period. Its incidence was 9.42% (87/924), which is consistent with the result reported by Fry et al. . Our study showed that RSV infection occurred predominantly from January and March and October and December, with the majority of cases in children under five years old (P < 0.001). This is also in accordance with the study by Fry et al., and RSV was the most commonly detected respiratory viral pathogens in co-infected patients. Additionally, RSV infection was negatively correlated with air temperature (r = 0.699, P = 0.011).
The detection rate for HMPV was 5.84% (54/924), which is consistent with the results of several other studies [20–23, 25, 27, 39]. Although the study by van den Hoogen et al. showed that most HMPV infections occur in children less than five years of age, our study did not show a significant difference in age distribution of HMPV infections. Previous studies indicate that the activity of HMPV in temperate climates peaks between December and February [24, 41], where as in subtropics it peaks in the spring and summer months  and the peak of HMPV activity often coincides with or follows that of RSV [21, 23]. In this study from Zhuhai city, located in subtropics, the incidence of HMPV peaked in January and did not follow that of RSV. HMPV infection did not correlate with air temperature unlike RSV, which is similar to another study  and was likely impacted by the circulation of H1N1pbm09 virus in Zhuhai city 2010.
Between two and seventeen percent of all cases of ARI are caused by PIV, making it second only to RSV as a cause of ARI among children under five years of age [44–46]. We detected PIV serotypes 1, 2, and 3 in 924 samples and the results showed that the incidence of all PIV serotypes combined was 5.74% (53/924). The lack of seasonality of PIV infection observed in this study is different from previous reports [45, 47]. In addition, no significant differences were observed among the different age groups, possibly affected by the epidemic of H1N1pdm09 virus.
ADV is responsible for approximately 7%–8% of childhood viral respiratory infections worldwide [7, 8], which is consistent with the result from the present study where the incidence of ADV infections was 7.79% (72/924) and ADV was mainly identified in patients under 15 years of age. Although our study determined that the activity of ADV was not seasonal, the infection rate of ADV appears to positively correlate with air temperature (r = 0.699, P = 0.011).
While the information provided by this study is valuable, there may be some limitations. First, recruitment bias could have affected the results of our study. However, Figure 1 shows that the age distribution of the 924 study participants with available samples was representative of the 3,747 patients presenting with ILI and sought attention at the Outpatient Department of Zhuhai Municipal People’s Hospital in Zhuhai city in 2010. Second, some viruses, such as rhinoviruses and coronaviruses, as well as bacteria, such as M. pneumoniae or C. pneumoniae, were not studied. The inclusion of these additional viral pathogens might greatly increase virus detection frequency, as indicated by a previous study  that reported a higher proportion of these viruses compared to influenza virus and might explain the relatively high proportion (55.52%, 513/924) of unidentified etiologies in our study.
In summary, the etiologic and epidemiologic data from the present study provide useful information to clinicians when treating patients presenting with ILI and to government officers when designing and implementing effective intervention plans.