Seasonal Drivers of the Epidemiology of Arthropod-Borne Viruses in Australia.

Arthropod-borne viruses are a major cause of emerging disease with significant public health and economic impacts. However, the factors that determine their activity and seasonality are not well understood. In Australia, a network of sentinel cattle herds is used to monitor the distribution of several such viruses and to define virus-free regions. Herein, we utilize these serological data to describe the seasonality, and its drivers, of three economically important animal arboviruses: bluetongue virus, Akabane virus and bovine ephemeral fever virus. Through epidemiological time-series analyses of sero-surveillance data of 180 sentinel herds between 2004–2012, we compared seasonal parameters across latitudes, ranging from the tropical north (−10°S) to the more temperate south (−40°S). This analysis revealed marked differences in seasonality between distinct geographic regions and climates: seasonality was most pronounced in southern regions and gradually decreased as latitude decreased toward the Equator. Further, we show that both the timing of epidemics and the average number of seroconversions have a strong geographical component, which likely reflect patterns of vector abundance through co-varying climatic factors, especially temperature and rainfall. Notably, despite their differences in biology, including insect vector species, all three viruses exhibited very similar seasonality. By revealing the factors that shape spatial and temporal distributions, our study provides a more complete understanding of arbovirus seasonality that will enable better risk predictions. Author Summary: Arthropod-borne viruses (arboviruses) are a group of viruses that can have major impacts on public health, animal health and agricultural trade, and appear to be increasing in both number and prevalence worldwide. Despite their importance as emerging pathogens, the spatial patterns, long-term seasonal characteristics and drivers of seasonality in many arboviruses are poorly understood. The island continent of Australia provides an ideal case study for the spatial analysis of emerging arboviruses, harboring diverse climatic conditions across a wide range of latitudes. Herein we utilize long-term serological data from a nationwide network of sentinel herds in Australia to describe the seasonality of three economically important animal arboviruses: bluetongue virus, Akabane virus and bovine ephemeral fever virus. Using epidemiological time series analysis, we demonstrate that these viruses exhibit a distinct spatial pattern in both the peak timing and intensity of annual epidemic cycles, with the strongest seasonality observed in southerly geographic regions. In addition, we reveal the climatic factors that drive patterns of arbovirus distribution and, by doing so, provide a more complete understanding of arbovirus seasonality, which in turn will improve the risk assessment of these viruses. [ABSTRACT FROM AUTHOR]