Caractérisation génétique des virus Tilligerry et Mitchell River

Résumé

Viruses that are normally safely contained within their host spe­cies can emerge due to intense livestock farming, trade, travel, climate change and encroachment of human activities into new environments. The unexpected emergence of bluetongue virus (BTV), the prototype species of the genus Orbivirus, in economi­cally important livestock species (sheep and cattle) across the whole of Europe (since 1998), indicates that other orbiviruses represent a potential further threat to animal and human popula­tions in Europe and elsewhere. The genus Orbivirus is the largest within the family Reoviridae, containing 22 virus species, as well as 14 unclassified orbiviruses, some of which may repre­sent additional or novel species. The orbiviruses are transmitted primarily by arthropod vectors (e.g. Culicoides, mosquitoes or ticks).  Viral genome sequence data provide a basis for virus taxonomy and diagnostic test development, and make it possible to address fundamental questions concerning virus biology, pathogenesis, virulence and evolution, that can be further explored in mutation and reverse genetics studies. Genome sequences also provide criteria for the classification of novel isolates within individual Orbivirus species, as well as the identification of different sero­types, topotypes, reassortants and even closely related but dis­tinct virus lineages.  Full-length genome characterization of Tilligerry virus (TILV), a member of the Eubenangee virus species, and Mitchell River virus (MRV), a member of the Warrego virus species, have revealed highly conserved 5’ and 3’ terminal hexanucleotide sequences. Phylogenetic analyses of orbivirus T2 ‘sub-core-shell’ protein sequences reinforce the hypothesis that this protein is an important evolutionary marker for these viruses. The T2 protein shows high levels of amino acid (AA) sequence identity (> 91%) within a single Orbivirus species / serogroup, which can be used for species identification. The T2-protein gene has therefore been given priority in sequencing studies. The T2 protein of TILV is closely related to that of Eubenangee virus (~91% identity), con­firming that they are both members of the same Eubenangee virus species. Although TILV is reported to be related to BTV in serological assays, the TILV T2 protein shows only 68-70% AA identity to BTV. This supports its current classification within a different serogroup (Eubenangee).  Warrego virus and MRV are currently classified as two distinct members (different serotypes) within the Warrego virus species. However, they show only about 79% AA identity in their T2 pro­tein (based on partial sequences). It is therefore considered likely that they could be reclassified as members of distinct Orbivirus species. The taxonomic classification of MRV will be reviewed after generating full length sequences for the entire genomes of both viruses. The taxonomic status of each of these viruses will also be tested further by co-infections and attempts to create reassortants between them (only viruses belonging to the same species can reassort their genome segments). TILV and MRV are the first viruses from their respective serogroups / virus species to be genetically fully characterized, and will provide a basis for the further characterization / identification of additional viruses within each group / species. These data will assist in the devel­opment of specific diagnostic assays and potentially in control of emerging diseases. The sequences generated will also help to evaluate current diagnostic [reverse transcriptase - polymerase chain reaction (RT-PCR)] tests for BTV, African horse sickness virus, epizootic haemorrhagic disease virus, etc., in silico, by identifying any possibility of cross reactivity.