Culicoides-specific fitness increase of vesicular stomatitis virus in insect-to-insect infections

Vesicular stomatitis (VS) is an arthropod-borne viral disease affecting livestock. In the U.S., sporadic, one- or two-year outbreaks result in significant disease and economic losses every 5-10 years. Culicoides biting midges are competent vectors and contribute significantly to the geographic spread of outbreaks in the absence of animal movement. Recently, surprisingly high rates of venereal VSV transmission was demonstrated between Culicoides females and males indicating a possible overwintering role in outbreaks that re-emerge a second year. Considering this efficiency, we hypothesized that compared to VSV from mammalian cells, VSV replicated in midge cells had increased fitness for midges, facilitating midge-to-midge transmission. In this study, VSV propagated in either pig or midge cell lines were used to evaluate infection efficiency in C. sonorensis midges. Viruses from both sources replicated well in midges when high titers were inoculated via intrathoracic injection. However, when exposed to only a few virus particles via ingestion, midges had significantly higher rates of replication and dissemination with midge-derived viruses, compared to pig-derived viruses. Genome sequencing showed amino acid differences between viruses, affecting polarity and/or hydrophobicity in four of the five VSV proteins. Our research suggests VSV replication in Culicoides cells may result in genetic changes that increase the viral fitness for efficient replication in midge vectors, which may help explain the highly efficient venereal transmission seen in midges. This study highlights the importance of cell line specificity and limitations in investigating VSV-vector interactions and emphasizes the importance of Culicoides midges in VSV maintenance and transmission dynamics.