A recent report by the World Health Organization indicated that during 1997 to 1999, of 1,672 nonpoliomyelitis enterovirus isolations in the United States, EV71 had the lowest incidence, only 2.1% (1
). EV71 was occasionally isolated in Taiwan in 1980 and 1986 (4
); however, 132 (44.4% of enterovirus isolates in 1998) and 195 (20.5% of enterovirus isolates in 2000) strains of EV71 were isolated in 1998 and 2000, respectively. This result clearly indicated that two outbreaks occurred in Taiwan within the 3-year period. In 1999, EV71 accounted for only 2% of the isolates; however, an outbreak of coxsackievirus A10 contributed to the large number of enteroviruses isolated that year (15
The apparent genetic distinction of EV71 isolates appeared not to correlate with the severity of the disease. EV71 from total cases were in both B and C genogroups in this study. Both genetic clusters consisted of fatal and mild HFMD cases. Similar results were obtained from studies of EV71 strains from fatal and nonfatal cases of a 1998 outbreak in Taiwan (12
). In addition, the Malaysian isolates obtained from patients with uncomplicated HFMD and fatal encephalitis in 1997 were virtually identical in their VP1 regions to those in a report by Brown et al. (3
). Thus, a final conclusion cannot be made from existing genetic analyses of the 5′-UTR and VP1 regions in relation to clinical manifestations.
Genetic analyses of the 5′-UTR and VP1 in this study showed consistent grouping of EV71 isolates from the 1998 outbreak into two clusters, genotypes B and C. The results suggest that recombination did not occur between these outbreak strains, an event which can lead to the evolution of highly virulent strains (12
). Several studies on EV71 strains from 1998 in Taiwan also support the existence of two cocirculating genotypes (12
). Furthermore, this study showed that EV71 isolates in the 2000 outbreak only grouped to one genogroup, genogroup B, a result of a genotype switch which occurred between two major HFMD epidemics within the 3-year study period.
The 5′-UTR of enteroviruses is generally highly conserved; in contrast, VP1 contributes to the most variable region on the surface of virion (7
). However, the nucleotide sequence diversities of the 5′-UTR and VP1 regions were similar in this study. Overall these two EV71 genotypes showed 79.3 to 81.5% nucleotide identity and 96.0 to 97.1% amino acid identity in the VP1 region and 74.1 to 82.8% nucleotide identity in the 5′-UTR in these two outbreaks. Despite the wide variation in clinical presentations, this study demonstrates the narrow range of EV71 genetic diversity between two genotypes of EV71 from two large outbreaks that occurred during a 3-year period in Taiwan.