Strain variation among Bordetella pertussis isolates in Finland, where the whole-cell pertussis vaccine has been used for 50 years

Pertussis is an infectious disease of the respiratory tract caused by Bordetella pertussis. Despite the introduction of mass vaccination against pertussis in Finland in 1952, pertussis has remained an endemic disease with regular epidemics. To monitor changes in the Finnish B. pertussis population, 101 isolates selected from 1991 to 2003 and 21 isolates selected from 1953 to 1982 were studied together with two Finnish vaccine strains. The analyses included serotyping of fimbriae (Fim), genotyping of the pertussis toxin S1 subunit (ptxA) and pertactin (prn), and pulsed-field gel electrophoresis (PFGE) after digestion of B. pertussis genomic DNA with XbaI restriction enzyme. Strains isolated before 1977 were found to harbor the same ptxA as the strains used in the Finnish whole-cell pertussis vaccine, and strains isolated before 1982 harbored the same prn as the strains used in the Finnish whole-cell pertussis vaccine. All recent isolates, however, represented genotypes distinct from those of the two vaccine strains. A marked shift of predominant serotype from Fim serotype 2 (Fim2) to Fim3 has been observed since the late 1990s. Temporal changes were seen in the genome of B. pertussis by PFGE analysis. Three PFGE profiles (BpSR1, BpSR11, and BpSR147) were distinguished by their prevalence between 1991 and 2003. The yearly emergence of the three profiles was distributed periodically. Our study stresses the importance of the continuous monitoring of emerging strains of B. pertussis and the need to obtain a better understanding of the relationship of the evolution of B. pertussis in vaccinated populations.