Influenza virus imposes a significant burden of disease on humans and other domesticated animals. Transmission and viral diversification are modulated by complex interactions between climate and the structure of host populations and communities, including host population size, connectivity and sociodemographic and immunological factors. A global latitudinal gradient in epidemic periodicity (more strongly seasonal epidemics at higher latitudes) is hypothesized to result from a context-dependent mixture of humidity and temperature regimes, while, at the spatial scale of cities, poverty is associated with increased epidemic intensity. The specific mechanisms that underlie these patterns are not well understood. Forecasting and controlling influenza thus requires integrating ecological and evolutionary dynamics over space, time, and across scales of observation—from circadian immune dynamics within a single host to seasonal- and multiyear- patterns in the intercontinental co-circulation of viral strains. I will describe recent work that elucidates the dominant modes of interaction between climatic and host factors in the circulation of influenza. We use high-volume, fine-scale, data on influenza incidence across multiple host species and viral strains, in combination with process-based transmission models, to leverage urbanization and climate change as natural experiments in the viral ecology and evolution.
Results/Conclusions
Global patterns in epidemic periodicity and viral diversity are best explained by interactions between climatic conditions and the demographic, population and community structure of hosts. Adding urbanization data to standard climate-based transmission models simplifies the relationship between transmission and temperature/humidity, providing a better explanation for endemic epidemic dynamics at low latitudes, where differential climatic conditions interact with higher average population densities. Metropolises emerge as both viral ‘diversity pumps’ and ‘museums’ for influenza diversity, through interactions between host mobility patterns and immune dynamics. These interactions in turn depend on socioeconomic conditions. Urbanization and climate are key features of global change. Understanding their interacting roles in the ecological and evolutionary dynamics of influenza will promote the development of more reliable epidemic and evolutionary forecasts, better vaccination strategies, and an increased capacity to manage complex social-ecological systems.