Use of pulsed-field gel electrophoresis for epidemiological study of Bordetella pertussis in a whooping cough outbreak

We used pulsed-field gel electrophoresis (PFGE) of chromosomal DNA digested with XbaI to determine the distribution of different Bordetella pertussis strains from clinical isolates obtained during a large whooping cough outbreak that occurred in Alberta, Canada, from December 1989 to May 1991. Our initial study analyzed 28 clinical isolates, 14 from the city of Edmonton and 1 from each of 14 northern Alberta towns. These clinical isolates were randomly chosen over the course of the 18-month outbreak. The DNA profiles were more heterogeneous than anticipated and caused concern that PFGE was too sensitive a technique to characterize strains. Further analysis showed that this was not the case, as clusters of similar PFGE patterns were observed in strains isolated from the same outlying town. Identical PFGE patterns were also seen in clinical strains obtained from different members of the same family. Two PFGE pattern types, a and b, predominated in the outbreak, accounting overall for 44 of 70 B. pertussis strains tested. Results from isolates from outlying towns, however, indicated involvement of local strains rather than a single, highly infectious strain in the whooping cough outbreak in Alberta.