Rapid evolution leads to extensive genetic diversification of cattle flu Influenza D virus

Influenza D virus (IDV), the cattle flu virus, is a novel multi-host RNA virus, circulating silently worldwide, with widespread seropositivity among US cattle, reaching up to 80% in some areas raising a potential threat of cattle-to-human transmission. Currently, five genetic lineages of IDV have been described, but their evolutionary dynamics have not been studied. Although IDV was first identified in 2011, our comprehensive analysis of all known IDV genomes suggests that the earliest ancestors of IDV likely to have evolved towards the end of the 20^th century and D/OK lineage appears to have emerged in 2005. We confirmed a significantly higher substitution rate in IDV than in Influenza C virus, which is consistent with their global distribution and multi-host tropism. We identified multiple sub-populations within the D/OK lineage, highlighting extensive diversification and dissemination. Other findings are evidence for potential reassortment among IDV strains in the USA and transboundary circulation of IDV in Europe with introductions into Danish cattle, some of which potentially originated from France. IDV, an emerging virus with a higher rate of evolution and uncontrolled circulation, could facilitate its adaptation to humans. Our findings underscore the importance of targeted surveillance for IDV in humans and at-risk animal populations.