Overall selective pressures acting on VP1 antigenic region were estimated by using the CODEML program in the PAML package [40] . We used site models M7 (with a discrete distribution of 10 categories and accounting for sites not allowed to be positively selected) and M8 (estimates dN/dS for an extra class (p11) of sites, accounting for positively selected sites with dN/dS>1). Models were compared by means of likelihood ratio test and statistical support was taken from the Bayes-Empirical-Bayes output (BEB, see additional file 6: Log-likelihood and parameter estimates for PAML analysis) [40] . To detect adaptative molecular evolution, we used the complete dataset 2.