EV71 and CA16 were two of the members of the Picornaviridae family, whose genomes were characterized by a single positive-stranded genomic RNA. Due to their poor fidelity replication and frequent recombination, the genomes of EV71 and CA16 mutated at a high rate. Different genotypes and sub-genotypes of these 2 viruses had alternated and co-circulated in the Asia-Pacific region, leading to repeated outbreaks of HFMD. The first reported large, severe and devastating HFMD epidemic occurred in Taiwan region in 1998 including about 130000 cases of HFMD, among whom 405 patients were severe and 78 died [3, 4, 31]. In 2000, there was another report of outbreak, with 80677 cases of HFMD and 41 deaths there . From February to August in 2006, Brunei with a population of about 370000 experienced its first reported major outbreak of EV71. More than 1681 children were affected, with 3 deaths resulting from severe neurologic complications . In Mainland China, HFMD broke out repeatedly in recent years. There were 83344, 488955 and 1155525 cases in the nationwide in 2007, 2008 and 2009, respectively, reported by the Ministry of Health, the People's Republic of China. The corresponding deaths for these years were 17, 126 and 353, respectively. It suggested that HFMD had been becoming a more and more serious public health problem in China.
In Beijing, no large HFMD epidemic has occurred so far, but sporadic infections are common. In 2007 and 2009, the predominant etiological agents of HFMD in Beijing were CA16 while the main etiological agent was EV71 in 2008. In general, comparison for nucleotides among vp1s or vp4s of EV71 indicated that the nucleotide identity of these sequences from strains isolated in the same year was higher than that of those sequences from strains isolated in the different years, and the nucleotide identity of these sequences isolated in this study was higher than that of those sequences reported in other parts of Mainland China and especially other countries of the world. However, it was not necessarily true. For example, the nucleotide identity of s374 vp4 isolated in 2009 and those isolated in 2008 in this research was higher than that of s374 vp4 and s366 vp4 isolated in the same year of 2009. This suggested that the transmission of EV71 was not strictly regional and temporal restriction. In addition, the nucleotide comparison also indicated that the severity of patients' illness caused by EV71 infection seemed not to be correlated with the sequence mutations in vp1 or vp4. The phylogenetic data in this study indicated that C4 of EV71 and lineage B2 (C) of CA16 had been circulating in Beijing in these 3 years and major mutations were not observed in these virus strains, which was similar to the results reported by other parts of Mainland China . In Mainland China there were no subgenotypes other than C4 of EV71 reported which seemed to be regionally and temporally restricted, but it was not true for the lineage of CA16. In Shenzhen, lineage B1 and B2 co-circulated in 1999 and 2000, but only lineage B2 was found from 2001 to 2004. In other parts of the world, the transmission of genotypes of EV71 and lineages of CA16 showed a different trend. For example, in Malaysia EV71 outbreaks occurred in 1997 and 2000, mainly associated with genotypes B3 and B4 alternating in the 2 years[32, 22], and lineage B1 and B2 of CA16 coexisted in 2000 and 2003. In Taiwan region, EV71 epidemics were associated with genotype C2 and B4.
The overall sero-positive rates of VP1 of EV71 and CA16 in this research were 64.55% and 75.13%, respectively, which were higher than those reported by Rabenau et al, whose data showed 42.8% for EV71 and 62.9% for CA16 for those individuals ≥ 1 years old . The difference of sero-positive rate in these two studies might be caused by the variety of the detection method used or age group of the participants. Nevertheless, both results from our study and Rabenau' suggested that the exposure rate of CA16 was higher than that of EV71 in the population.
EV71 other than CA16 was the cause of severe cases of HFMD in young children. Generally the severity of the patients infected by viruses was associated with 2 factors: host and virulence of the virus . When HFMD outbreaks were caused by EV71, there would be some severe cases and even deaths [3, 6]. CA16 was often associated with mild and benign clinical symptoms. Then the pathogenicity of EV71 should be stronger than that of CA16. EV71 and CA16 shared a lot in some characteristics. For example, both of them belonged to Enterovirus A and had a genome of about 7.4 k bp in length. The caspids of them consisted of 4 proteins: VP1, VP2, VP3 and VP4. Both of them could cause HFMD. However, there were also many differences between them. In this study, we designed experiments to compare EV71 and CA16 in some aspects and tried to find some of the differences. The nucleotide identities of VP1 between them were less than 66.80% and the identities of deduced amino acids were no more than 72.70%. Although VP4s from them were much conserved, there were still some differences in nucleotides and the deduced amino acids. The nucleotide identities of VP4s between them were 64.30%~73.90% and the deduced amino acids identities were 78.30%~79.70%. There were also some differences in inducing IgG in host's sera against VP1 and VP4 between EV71 and CA16. The sera-positive rate of EV71 VP1 in the population was lower than that of CA16 VP1 and similarly the sera-positive of EV71 VP4 was lower than that of CA16 VP4, for which there might be 2 reasons. One was that the exposure rate of EV71 might be lower than that of CA16. Another was that it was more difficult to induce IgG against EV71 than CA16 in hosts' sera, which might be associated with the different symptoms caused by EV71 and CA16. When the specific IgM against VP4s of these two viruses were tested in serum specimens collected from children currently infected with these two viruses, none of these sera showed positive reaction.