Trends in Tick-Borne Virus Ecology: Vector Incrimination, Focal Distribution, and Reservoir Associations.

Ticks are prominent hosts for many viruses that are pathogenic to humans on almost every continent. Despite the documented historical association of ticks with viral disease in humans, clinical case reports are low, likely due to under-recognition of these pathogens. A key feature of understanding tick-borne virus transmission to humans lies in the natural invertebrate and vertebrate hosts supporting ecological maintenance. As humans are often infected as incidental hosts during the tick-virus life cycle, determining relevant ticks and mammals involved in maintenance can inform prevention strategies, surveillance priorities, and diagnostic assay development. This review aims to cover common trends in tick-borne virus ecology, including the wide breadth of implicated tick species, an unclear role for definitive vertebrate reservoirs, and the focal distributions of these viruses closely linked to these host ecological features. Here we also provide examples of prominent tick-borne viruses causing human disease between the Old (Africa, Asia, Europe) and New (Americas) Worlds, highlighting parallels in initial discoveries, vectors, and vertebrates including tick-borne encephalitis, Crimean-Congo hemorrhagic fever, severe fever with thrombocytopenia syndrome, Colorado Tick fever, and Powassan, Heartland, and Bourbon viruses.