Vector competence is strongly affected by a small deletion or point mutations in bluetongue virus

Authors: VAN GENNIP, RENE - Wageningen University; Drolet, Barbara; ROZO LOPEZ, PAULA - Kansas State University; ROOST, ASHLEY - Wageningen University; BOONSTRA, JAN - Kansas State University; VAN RIJN, PIET - Wageningen University

Submitted to: Parasites & Vectors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/3/2019
Publication Date: 10/11/2019
Citation: Van Gennip, R., Drolet, B.S., Rozo Lopez, P., Roost, A., Boonstra, J., Van Rijn, P. 2019. Vector competence is strongly affected by a small deletion or point mutations in bluetongue virus. Parasites & Vectors. 12:470. https://doi.org/10.1186/s13071-019-3722-2.
DOI: https://doi.org/10.1186/s13071-019-3722-2

Interpretive Summary: Transmission of viruses by insects is a complex mechanism. Insects must obtain virus from an infected animal during blood-feeding. The virus then must multiply and spread within the insect so that it reaches the salivary glands to be excreted into another animal when the insect blood-feeds again. Bluetongue virus (BTV) is transmitted by biting midges (Culicoides) and causes disease in domestic and wild ruminants. In this study, we used genetic approaches to show that small changes in specific proteins of the virus significantly affect its ability to multiply in these insects and therefore prevent virus transmission. This suggests that different types of BTV that are slightly different genetically, may have significantly different abilities to be transmitted by midges.

Technical Abstract: Background: Transmission of vector-borne virus by insects is a complex mechanism consisting of many different processes; viremia in the host, uptake, infection and dissemination in the vector, and delivery of virus during blood feeding leading to infection of the susceptible host. Bluetongue virus (BTV) is the prototype vector-borne orbivirus (Reoviridae family). BTV serotypes 1-24 (typical BTVs) are transmitted by competent biting Culicoides midges and replicate in mammalian (BSR) and midge (KC) cells. Previously, we showed that genome segment 10 (S10) encoding NS3/NS3a protein is required for virus propagation in midges. BTV serotypes 25-27 (atypical BTVs) do not replicate in KC cells. Several distinct BTV26 genome segments cause this so-called ‘differential virus replication’ in vitro. Methods: Virus strains were generated using reverse genetics and their growth was examined in vitro. The midge feeding model has been developed to study infection, replication and disseminations of virus in vivo. A laboratory colony of C. sonorensis, a known competent BTV vector, was fed or injected with BTV variants and propagation in the midge was examined using PCR testing. Crossing of the midgut infection barrier was examined by separate testing of midge heads and bodies. Results: A 100 nl blood meal containing +105.3 TCID50/ml of BTV11 which corresponds to +20 TCID50 infected +50% of fully engorged midges and is named one Midge Alimentary Infective Dose (MAID50). BTV11 with a small in-frame deletion in S10 infected blood-fed midge midguts but virus release from the midgut into the haemolymph was blocked. BTV11 with S1[VP1] of BTV26 could be adapted to virus growth in KC cells, and contained mutations subdivided into ‘corrections’ of the chimeric genome constellation and mutations associated with adaptation to KC cells. In particular, one amino acid mutation in outer shell protein VP2 overcomes differential virus replication in vitro and in vivo. Conclusion: Small changes in NS3/NS3a or in the outer shell protein VP2 strongly affect virus propagation in midges and thus vector competence. Therefore, spread of disease by competent Culicoides midges can strongly differ for very closely related viruses.