Bordetella pertussis, Finland and France

We used pulsed-field gel electrophoresis analysis and genotyping to compare clinical isolates of Bordetella pertussis recovered since the early 1990s in Finland and France, 2 countries with similar histories of long-term mass vaccination with whole-cell pertussis vaccines. Isolates from both countries were similar genetically but varied temporally.

same wP vaccine, composed of 2 strains, has been used for >40 years. In both Finland and France, incidence of pertussis has increased and the disease has shifted to older age groups, especially adolescents and adults. The cycles of pertussis disease are observed every 3-5 years (5,13). The aim of this study was to analyze and compare the isolates circulating in the 2 European countries with long-term and intensive vaccination.
B. pertussis isolates were selected from collections of the Finnish Pertussis Reference Laboratory of the National Public Health Institute (Turku, Finland) and the French Pertussis National Center of Reference (Paris, France). Of the 503 Finnish isolates recovered from 1991 to 2004, 64 were selected either because they represent all available isolates from 1 community (Paimio) or they were recovered from a geographic area as wide as possible. In addition, 6 isolates from a school outbreak that occurred in Heinavesi (Finland) in 1982 were included to study the changes over time. Of the 1,049 French isolates recovered from 1991 to 2004, we selected 61 because they are representative of the French collection from a temporal and geographic viewpoint and they correlate with the different cycles of pertussis observed in France and Finland.
All Finnish and French isolates showed a high similarity with a minimum of 80.3% overall relatedness by using pulsed-field gel electrophoresis (PFGE) analysis after digestion of B. pertussis genomic DNA with XbaI restriction enzyme (14) (Appendix Figure, Figure), might represent an emerging group. The new profile was further confirmed with PFGE by using the second restriction endonuclease SpeI (data not shown), as previously recommended (5,14). We cannot say whether PFGE VII isolates are actually emerging or whether evidence for their emergence is anecdotal, as is the case with French PFGE VI isolates, which represent only 0.6% of French isolates (data not shown).
We previously showed that the major PFGE group circulating in Europe from 1999 to 2001 is group IV (14). In France and in Finland, the PFGE group IV was overrepresented from 1992 to 2004 with 85.3% level of relatedness, confirming the limited polymorphism of B. pertussis. PFGE group IV was subdivided into 3 subgroups, α, β, and γ, whose frequencies varied between countries (14). In our study, we show that, as in France (5) Figure).
The analysis also included genotyping of the genes encoding pertussis toxin S1 subunit (ptxA) and pertactin (prn) and serotyping of FIM, performed as described previously (5,13). The sequence of ptxA is the same (ptxA allele type 1) for all Finnish and French isolates. The same types of prn alleles are also harbored by Finnish and French isolates (Appendix Figure). The emergence of isolates harboring ptxA1 and prn2 or prn3 alleles in both countries might be explained by the fact that the wP vaccine strains used in both countries harbor ptxA2 or A4 and prn1 alleles (13,15). A similar hypothesis might be proposed for the expression of FIM. In fact, most of the Finnish isolates collected from 1991 to 2004 express FIM2, whereas in France most of the isolates express FIM3 (Figure, Panel B). The differences in the expression of FIM between Finnish and French isolates might reflect the difference in strains used for Finnish and French wP vaccines. The Finnish wP vaccine contains 2 strains expressing FIM2,3 and FIM3, whereas the French wP vaccine includes 2 strains expressing FIM2,3 and FIM2 (13,15). A marked shift of predominant serotype from FIM2 to FIM3 has been observed in Finland since 1999, although the wP vaccine remained the same. The emergence of isolates with FIM3 and PFGE subgroup IVβ, a new subgroup found in Finland, might be due to the increase in the frequency of this subgroup in the neighboring countries and the increased mobility of people within the European Union in the last decades.
We show that the B. pertussis isolates circulating in 2 countries with a long history of wP vaccination are genetically close. In the 2 countries, similar PFGE groups and subgroups are present, but their frequencies were different in the 1990s. Further, the subgroup emerging according to the cycles of pertussis in each country varies. The difference observed in frequency of subgroups could be due to herd immunity or human density of the populations concerned. Does this herd immunity vary depending on the human genetic population concerned or are the vaccine strains used expressing similar factors but not at the same level (e.g., FIM2 vs. FIM3)? This question needs further investigation.