Co-Feeding Transmission of Tick-Borne Viruses

Tick-borne viruses (TBVs) are a diverse group of arthropod-borne pathogens maintained in complex transmission cycles involving both tick vectors and vertebrate hosts. Among the known TBV transmission routes, co-feeding transmission, in which virus is transferred from an infected tick to an uninfected tick feeding on the same vertebrate host even in the absence of a detectable viremia, represents an important route that contributes to viral maintenance in nature. Although co-feeding transmission has been demonstrated across multiple vector, host, and virus combinations, the mechanisms governing this transmission route remain poorly defined. This review synthesizes current understanding of co-feeding transmission and highlights the importance of ecological and immunological factors that shape this process in nature. Specifically, we emphasize the role of the cutaneous microenvironment at the tick co-feeding site, where localized viral replication and tick salivary factors create conditions favorable for virus transfer between co-feeding ticks. We also highlight the requirements for co-feeding transmission to occur in nature and across seasons. Together, these insights support a model in which localized skin infection is a central feature of co-feeding transmission while underscoring key gaps in our understanding of the cellular and molecular mechanisms that govern this process.