Fig. 1.

Frequencies of the prn alleles in the period 1998–2012. Isolates were aggregated in three periods, 1998–2001, 2002–2006 and 2007–2012. a Allele frequencies in the 12 pooled countries: Belgium, the Czech Republic, Denmark, Finland, France, Germany, Ireland, Norway, Spain, Sweden, the Netherlands and the United Kingdom. b Allele frequencies in Poland. Due to the limited availability of Polish isolates in the period 2007–2012 (n = 2), no data are included for this period. The percentages and number of strains analysed in the different periods are indicated