Within-host fitness of vesicular stomatitis virus and Culicoides sonorensis midges

Vesicular stomatitis (VS) is a viral disease affecting livestock. In the United States, VS produces devastating economic losses, particularly in the southwestern states where the outbreaks display five to ten-year intervals. During epizootics, Culicoides biting midges act as vectors having a significant role in the initial introduction of VS into animal herds and contributing to the viral spread in the absence of animal movement. We have shown that both Culicoides females and males are capable of efficient venereal transmission of VSV despite transferring relatively low amounts of virus. We hypothesize that the virus propagated in the midge may have increased fitness for subsequent insect infection, facilitating the efficiency of venereal transmission of VSV. To determine if insect-derived VSV is better adapted than mammalian-derived VSV for infection of midges, the infection and dissemination patterns in orally infected C. sonorensis midges were evaluated. Our results indicate that midges fed with mammalian-derived viruses had 50-93% infection rates and a maximum of 60% dissemination after ten days post-feeding. Conversely, midges fed with insect-derived viruses had infection rates of 100% at all sampled times, higher titer infections, and over 60% dissemination rates. Our research suggests that VSV replication in Culicoides cells results in an increased fitness for efficient midge-to-midge transmission and subsequent replication within the vector. This highlights the importance of cell line specificity and limitations in investigating VSV-vector interactions, and more importantly, demonstrates the importance of Culicoides midges in VSV maintenance and transmission dynamics.