Molecular epidemiology of coxsackievirus A6 associated with outbreaks of hand, foot, and mouth disease in Tianjin, China, in 2013

Abstract

Since 2008, Mainland China has undergone widespread outbreaks of hand, foot, and mouth disease (HFMD). In order to determine the characteristics of epidemics and enteroviruses (EV) associated with HFMD in Tianjin, in northern China, epidemiological and virological data from routine surveillance were collected and analyzed. In Tianjin, a persistent epidemic of HFMD was demonstrated during 2008-2013, involving 102,705 mild, 179 severe, and 16 fatal cases. Overall, 8234 specimens were collected from 7829 HFMD patients for EV detection during 2008-2013. Enterovirus 71 (EV-A71) and coxsackievirus A16 (CV-A16) were the dominant serotypes during 2008-2012, and they were replaced by CV-A6 as the major causative agent in 2013. Phylogenetic analysis based on complete VP1 nucleotide sequences revealed that multiple CV-A6 lineages co-circulated in Tianjin, which grouped together with strains from China and other countries and split into two distinct clusters (clusters 1 and 2). Most Tianjin strains grouped in cluster 1 and were closely related to strains from several eastern and southern provinces of China during 2012 and 2013. Estimates from Bayesian MCMC analysis suggested that multiple lineages had been transmitted silently before the outbreaks at an estimated evolutionary rate of 4.10 × 10(-3) substitutions per site per year without a specific distribution of rate variances among lineages. The sudden outbreak of CV-A6 in Tianjin during 2013 is attributed to indigenous CV-A6 lineages, which were linked to the wide spread of endemic strains around eastern and southern China.

Fig. 1

Temporal distribution of reported cases of HFMD and cases of EV detected in Tianjin, 2008–2013. (A) Weekly distribution of reported cases and cases with specimens collected for EV detection, indicated on the left and right axis, respectively. (B) Monthly distribution of severe and fatal cases

Fig. 2

Incidence of HFMD in Tianjin in 2008–2013 according to age

Fig. 3

Maximum-likelihood tree based on complete VP1 nucleotide sequences (915 nucleotides) of CV-A6 strains from this study and references from GenBank. Tianjin sequences are indicated by arrows. The other sequences are indicated by GenBank accession number, country and year (where available). Chinese provinces: FJ, Fujian; GD, Guangdong; HeN, Henan; JS, Jiangsu; SD, Shandong

Fig. 4

MCC tree of complete VP1 nucleotide sequences (915 nucleotides) of CV-A6 strains representing D1 and D2 clusters. The scale is in units of evolutionary time in years, using the relaxed clock model [5], with an estimated mean rate of 4.49 × 10⁻³ substitutions/site/year. The time period from year 2012 to 2013 is indicated by a gray rectangle