Regional climate variability and the spatial spread of the vector-borne virus vesicular stomatitis in the United States

For vector-borne viruses, such as vesicular stomatitis (VSV), outbreaks can be amplified or constrained by the influence of environmental variables on insect vectors. Indeed, climate variables either nearby or on individual ranch premises have been linked to occurrence of VSV, especially in years following initial incursion of the virus in the US from endemic areas in Mexico. In the incursion years of 2004 and 2014, there were similar spatial signatures of disease occurrence in the US, while in 2005 the outbreak then expanded west and north into western Colorado, Utah, Wyoming, Montana and Idaho, and in 2015, the outbreak expanded eastward into eastern Colorado, South Dakota and Nebraska. Here, we investigate the role of regional climate and its drivers on the difference in spatial signature between the 2005 and 2015 expansion years. VSV outbreak regions in 2005 were cooler in winter by up to 4°C followed by a wetter spring when compared with that same region in 2015. Cooler winter temperatures and wetter spring conditions can produce soil moisture and streamflow conditions ideal for VSV insect vectors. While both outbreak years occurred during El Niño events, in winter of 2015 there was a strong atmospheric ridging over the western US, making those 2005 outbreak regions much warmer and drier, and regions to the east cooler and wetter, leading to that eastward expansion of the 2015 outbreak. `Flavors' of ENSO and interactions with atmospheric variability can shift regional-scale climate and therefore the premises relevant for early warning notices of VSV, with implications for predictions during expansion years.