Characterization of BTV-3 (SPA/2024) transmission in mammalian and insect cells and in the IFNAR(-/-) mouse model.

Bluetongue, caused by bluetongue virus (BTV), is a widespread arthropod-borne disease of livestock that entails a recurrent threat to the agricultural primary sector. In 2023, serotype BTV-3 emerged in the Netherlands and spread rapidly to neighboring countries including Spain. This new strain of BTV-3 courses with more severe clinical signs and faster spread of the virus than other serotypes that caused outbreaks in Europe. To better understand these aspects of the new BTV-3 strain, we have analyzed viral replication of BTV-3 (SPA/2024), BTV-4 (MOR/2009) and BTV-8 (BEL/2006) in mammalian (Vero and BSR) and insect (KC) cells. Growth kinetics showed significant differences in viral titers at 48 and 72 hpi in BSR and Vero cells, with higher viral titers in the supernatants of cells infected with BTV-3 than in those infected with BTV-4 or BTV-8. At 72 hpi, a greater cytopathic effect and a decrease in BTV-3 viral RNA in Vero cells were observed. In contrast, no significant differences were observed at other timepoints and in BSR cells. Interestingly, neither viral titers nor viral RNA showed significant differences among serotypes in KC cells.

In addition, BTV-3 virulence was analyzed in vivo in the IFNAR(-/-) mouse model. Mice infected with 10 PFU of virus developed clinical signs, RNAemia and viraemia between days 5 and 7 post-infection and all the animals died between days 7 and 10. For other BTV serotypes analyzed, lethal doses were established at 10² PFU for BTV-1 (ALG/2006) and BTV-8 (BEL/2006) and 5x10² PFU for BTV-4 (SPA/2005), indicating that this BTV-3 (SPA/2024) is also more virulent in the mouse model than other serotypes of BTV, as observed in the natural host.

These data suggest that BTV-3 presents greater transmissibility and virulence than other BTV serotypes in the mammalian host, but these dynamics are not altered in the vector cells.