RESEARCH lular receptors (41). Recently, studies of the EV11 receptor (31) indicate that differences of few amino acid residues of the viral surface affecting the canyon structure may change the infectivity of the virus dramatically. The molecular epidemiology of EV13 presents similar- ities and differences with other HEVs. The studies to date have demonstrated an epidemic pattern in viruses related with outbreaks such as ENV70 and EV30. This pattern suggests a unique strain of virus causing outbreaks all over the world similar the epidemics attributable to influenza A and B viruses. However, since the human population is believed to be the only ENV reservoir, this pattern should be completed with the continuous isolation of the agent over nonepidem- ic years. This type of circulation has been observed in sev- eral HEVs, such as EV30 or EV9 but has not been observed in the case of EV13 or other rare HEVs. However, EV13 may have been circulating in the popula- tion at undetectable levels or in a nonpathogenic way. Since HEVs are usually typed only when they are related to unexpected outbreaks, a nonpathogenic virus could cir- culate in the population for a number of years before caus- ing outbreaks. The circulation of EV13 in Spain seem to have sudden- ly stopped. Since October 2000, over 700 ENV isolates have been typed in the Spanish Enterovirus Reference Laboratory, and only 1 imported EV13 isolate (data not shown) was detected in 2001 in a tourist from the Czech Republic who was visiting the Canary Islands. Acknowledgments We thank Almudena Otero, Hortensia Pozo, and Isidoro Bustillo for their technical help and W. Ian Lipkin for his com- ments, corrections, and revisions. The work was supported by the MPY1197/02 grant from the Instituto de Salud Carlos III (Spanish Ministry of Health) and in part by NIH grants NS29425 and AI51202. 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