Ecological interference: A mechanism of pathogen interaction may explain erratic periodicity of whooping cough dynamics

Background/Question/Methods

Incidence of whooping cough unlike many other childhood infections exhibits variable dynamics across time and space. The periodicity of this disease varies from 2 to 5 years in different geographic regions from developing to developed countries. Many hypotheses have been put forward to explain this variability such as nonlinearity and seasonality, stochasticity, variable recruitment of susceptibles via birth, immunization and immune boosting. In this piece of work, we propose an alternative hypothesis to describe the variability in periodicity –the intricate dynamical variability of whooping cough incidence may arise from interactions of two strains of Bordetella, namely B. pertussis and B. parapertussis. We develop a two-strain age-structured model, where these two pathogens interact via age-dependent convalescence of individuals with severe illness from infections. We numerically analyze the model and characterize the outbreak dynamics under different strength of interactions. We also explore the dynamical response of the model to different perturbations like case importations and noise in transmission.  

Results/Conclusions

With moderate strength of interactions, the model exhibits multi-annual coexisting attractors depending on the basic reproduction ratio of the two pathogens. For instance in higher R₀, four different stable attractors coexist in a finely structured intertwined basin of attraction. In two attractors, both strains exhibit in-phase dynamics; while in the other two, they oscillate out-of-phase with relatively high fluctuations. It is also seen that perturbation due to movement or case importation and noise in transmission pushes the system from one dynamical regime to another. The coexistence of multi-annual cycles and the behavior of switching between attractors suggest that variable dynamics of whopping cough could be an emergent property of the interacting system, which is not observed earlier.