We have presented longitudinal data reflecting changing patterns of enterovirus prevalence over a 13-year period in the South Korea while explicitly noting the changing laboratory methodology over the period. The results of prevalence and distribution of genotypes of EV in this study reflected nationwide outbreaks and detection and subtyping methods for EV surveillance.

We described 4 EV outbreaks in South Korea during 1999–2011: aseptic meningitis caused by EV71 in 2000, by E13 in 2002, by CB5 in 2005, and by E30 in 2008 (25–27). EV detection rates in 2002, 2005, and 2008 were relatively higher than those in other years, and the dominant genotypes found during these years were outbreak-associated genotypes.

The prevalence increased as detection technology changed from cell culture (phase I), to RT-PCR (phase II), to real-time RT-PCR (phase III). Overall increases were probably caused by a combination of outbreaks of EV infection and enhanced sensitivity of detection methods. Because of a greatly advanced molecular detection method, molecular-based methods enable detection of uncultivable EV by use of small sample quantities and specific primer sets. Consistent with our findings, Roth et al. reported higher sensitivity by using the RT-PCR method rather than cell culture for fecal and CSF samples (11). In our study, although there are no data from parallel testing to address the issue of relative sensitivity of the 3 detection methods used over this period in this study, it is possible that increased prevalence during phases II and III could have been accounted for by the enhanced sensitivity of detection methods.

We described the prevalence, seasonal trend, and epidemiologic data for human EV infection collected by the national enterovirus surveillance system during 1999–2011 in South Korea. Our laboratory identified EV from fecal, CSF, nasopharyngeal, blood, and other sample types such as urine from persons who had an array of symptoms. Although feces is the most convenient specimen type for detecting EVs for surveillance purposes, detecting EV in fecal samples is not the most specific way to confirm the cause of an individual patient's symptoms (4). In this study, a higher frequency of EV detection from fecal samples, in contrast to CSF, was observed; this finding is in agreement with the finding of a previous study that used both cell culture and RT-PCR (11). However, Antona et al. showed that when compared with other specimens, the highest percentage of positive detection was found in CSF specimens (1). This finding could be influenced by the fact that different detection methods were used for each sample type: Antona et al. used cell culture for fecal samples and RT-PCR for CSF.

Genotyping has been shown to greatly improve epidemiologic investigation of common EV types when compared with seroneutralization testing (28). During phase I of this study, CA types and some E and EV types did not propagate well in cell culture and, therefore, were underdiagnosed. Confirmation of EV genotype by sequencing was systematically conducted after 2005 in South Korea; untypeable EV decreased but reemerged as real-time RT-PCR methods were introduced for detection during 2008 (data not shown). These findings could be related to a low quantity of EV in the sample, which can be detected by using real-time PCR but not by PCR-based VP1 sequencing.

As far as clinical aspects are concerned, the prevalent ages and clinical manifestation are consistent with results from previous studies that showed that the majority of cases occurred in children <10 years of age (1,29,30). In addition, a link between clinical syndromes and genotypes was in accordance with previous studies (1, 2,4–6,31); aseptic meningitis by E30, E6, and CB5; HFMD by CA16; and HFMD with neurologic complications by EV71. In this study, EV71 was the most frequent type of EV detected during 1999–2011 in South Korea. This finding is probably because our expanded surveillance detected more patients with neurologic disease. Since 1997, multiple cases of EV71 infection have been associated with severe aseptic meningitis and pulmonary edema in the Asia–Pacific region, including Taiwan, Malaysia, Singapore, and Japan (26,32–37). In addition, E30 was the second most common genotype detected during this period in this study and in previous studies from other countries; Asia and European countries reported that E30 was the predominant genotype (1,4,11).

EV infections have been known to increase in summer and early autumn in countries in temperate climates (1,2,13). As expected in a temperate climate, our surveillance data revealed a seasonal pattern of distribution, with transmission peaking in the summer and decreasing in the period from autumn to spring.

Our study has limitations that may affect the interpretation of its findings. First, although the patients were from almost all regions of South Korea, the number of patients and strains obtained from each is unequal. This variability is related to the level of cooperation and workload related to surveillance among different hospitals and local public health institutes. Second, some EV types that cannot propagate well in cell culture were underdiagnosed during 1999–2005, when ≈23.1% of isolates were recorded as untypeable (data not shown). Third, EV71 has been the most frequently detected type since 2009. It is likely that because HFMD with neurologic complications was actively monitored by our surveillance, EV serotypes associated with this clinical manifestation may have been overdetected.

This study focused on EV epidemiology in South Korea over a 13-year period by using a nationwide EV surveillance system. This surveillance provides valuable data on the epidemiologic pattern and clinical manifestations associated with specific genotypes and provides vital information that can be used to control annual EV epidemics. The public health impacts of EVs vary: some of the viruses are benign and some cause serious illness. Although it is appropriate in some instances to use cell cultures, we recommend the use of real-time RT-PCR for samples from patients who have typical symptoms of infection with the more virulent genotypes described here. Evaluation of findings from surveillance of enterovirus infections will contribute to development of prevention and treatment plans.