Bluetongue virus serotype 27: detection and characterization of two novel variants in Corsica, France

During the compulsory vaccination program against bluetongue virus serotype 1 (BTV-1) in Corsica (France) in 2014, a BTV strain belonging to a previously uncharacterised serotype (BTV-27) was isolated from asymptomatic goats. The present study describes the detection and molecular characterisation of two additional distinct BTV-27 variants found in goats in Corsica in 2014 and 2015. The full coding genome of these two novel BTV-27 variants show high homology (90-93% nt/93-95% aa) with the originally described BTV-27 isolate from Corsican goats in 2014. These three variants constitute the novel serotype BTV-27 ("BTV-27/FRA2014/v01 to v03"). Phylogenetic analyses with the 26 other established BTV serotypes revealed the closest relationship to BTV-25 (SWI2008/01) (80% nt/86% aa) and to BTV-26 (KUW2010/02) (73-74% nt/80-81% aa). However, highest sequence homologies between individual segments of BTV-27/FRA2014/v01-v03 with BTV-25 and BTV-26 vary. All three variants share the same segment 2 nucleotype with BTV-25. Neutralisation assays of anti-BTV27/FRA2014/v01-v03 sera with a reassortant virus containing the outer capsid proteins of BTV-25 (BTV1VP2/VP5 BTV25) further confirmed that BTV-27 represents a distinct BTV serotype. Relationships between the variants and with BTV-25 and BTV-26, hypotheses about their origin, reassortment events and evolution are discussed.