Mixed outbreak ofBordetella pertussis andBordetella parapertussis infection in Finland

The epidemiology of whooping cough in a vaccinated population was studied during an outbreak of paroxysmal cough in an elementary school with 258 pupils in Turku, Finland. Nasopharyngeal specimens for isolation ofBordetella pertussis and/or sera for ELISA detection of antipertussis immunoglobulin M, A and G antibodies were taken from 94 % of children who were prospectively followed for two months.Bordetella pertussis was isolated in six patients, and 17 culture-positive cases withBordetella parapertussis were identified. Patients withBordetella pertussis orBordetella parapertussis were found simultaneously in the same classrooms. Comparison of immunoglobulin M responses toBordetella pertussis andBordetella parapertussis was used for differential diagnosis of these two infections. Twenty-six cases with pertussis and 27 cases with parapertussis were diagnosed. The results of this prospective study suggest thatBordetella parapertussis is a more common etiologic agent of mild respiratory tract infection among vaccinated school-aged children than is generally recognised. The possibility thatBordetella pertussis was converted toBordetella parapertussis during this outbreak is discussed.