Schmallenberg virus non-structural proteins NSs and NSm are not essential for experimental infection of Culicoides sonorensis biting midges

The teratogenic orthobunyavirus Schmallenberg virus (SBV) is transmitted between its mammalian hosts by Culicoides biting midges. The genome of circulating SBV, i.e., variants present in viremic ruminants or insect vectors, is very stable, while variants found in malformed ruminant fetuses display a high genetic variability. It was suggested that fetal infection provides an environment that favors viral mutations that enable immune escape in the unborn but at the cost of limiting the ability of the virus to spread further. To investigate infection and dissemination rates of different SBV variants in the insect vectors, we fed laboratory-reared Culicoides sonorensis with blood containing the prototype strain BH80/11-4 from a viremic cow or strain D281/12, which was isolated from the brain of a sheep fetus and harbors multiple mutations in all three genome segments. Furthermore, virus variants lacking NSs, NSm, or both non-structural proteins were included. Six days after feeding, virus replication was found in about 2% of the midges exposed to wild-type strain BH80/11-4. The absence of the non-structural proteins had no obvious effect on the oral susceptibility to virus infection, as after 6 days, 2.78% of the midges fed with the NSs-deletion mutant displayed viral loads higher than the respective day-0 group, 1.92% of the midges exposed to the NSm-deletion mutant, and 1.55% of midges exposed to the NSs/NSm-deletion mutant. In contrast, strain D281/12 did not replicate at all in the midges, supporting the assumption that SBV variants arising in infected fetuses are unable to enter the normal insect-mammalian host cycle.