A Year of Boom or Bust for Equine Viruses with Different Insect Vectors

2012 was a year of warmer temperatures across the continental US and included a summer of rapid drought intensification- flash drought- that extended across the central US and drove large crop losses. Here we turn the focus of 2012 climate anomalies to impacts on geohealth and the predictive disease ecology of two vector-borne equine viruses with different insect vectors. We compared climate variables with county level disease occurrence data of equine West Nile Virus (spread by mosquitoes) from the Center for Disease Controls ArboNET surveillance program, and equine Vesicular Stomatitis Virus (spread by black flies and biting midges) from the Animal and Plant Health Inspection Service (APHIS). We found divergent impacts of temperature and drought on the spread of WNV and VSV. WNV experienced a relative peak in occurrence across conUS, including the central US counties impacted by drought. VSV, on the other hand, exhibited a full stop in its spread across the central US with the June flash drought. These divergent responses are likely driven by the reliance of black flies on flowing water, while mosquitoes have demonstrated increased vector competency under drought conditions. As the research around flash drought evolves, including potential rate of return, intensity, and spatial extent, and we observe temperature thresholds being met, we can be better equipped to identify threats to the health of horses across the US.