Bordetella pertussis and B. parapertussis are the causative agents of whooping cough. From an ecological perspective their co-circulation in human populations is an interesting case study in pathogen competition and coexistence, because they inhabit the same ecological niche and immunological studies indicate that there should be strong competition between them through cross-immunity (though results differ as to the perfection of this). From the inception of the pertussis-specific vaccine following World War II until the 1980s there was a steady decline in whooping cough incidence. Since the 1980s, however, there has been an increase in case reports, especially among adolescents. One possible explanation for this phenomenon is that as the population is becoming more fully vaccinated against B. pertussis, B. parapertussis incidence is increasing. We investigate two broad questions: (1) does temporal and/or age-specific niche segregation allow coexistence of these two strains and (2) does the pertussis-specific vaccine provide a competitive release for B. parapertussis in vaccinated children whose immunity has not yet waned. To address these questions we analyzed B. pertussis and B. parapertussis data from Denmark from 1946-1969 and Massachusetts from 1992-2004.

Results/Conclusions

The Danish data reveal coexistence through temporal segregation of B. pertussis and B. parapertussis with each pathogen exhibiting four-year cycles offset by exactly two years. In this data the distributions of age-of-infection are almost identical. In the more recent Massachusetts data, there is no evidence of temporal segregation. Here, coexistence appears to involve vaccine-induced niche segregation by host age, with B. parapertussis infecting age classes for which vaccine-induced B. pertussis immunity is strongest.