Exceptional Bluetongue Epidemic Caused by Co-Circulation of Several Serotypes in Spain in 2024

Bluetongue (BT) is an infectious, non-contagious, arthropod-borne viral diseases of ruminants, producing severe impacts in livestock. It is caused by Bluetongue virus (BTV), a double-stranded (ds) RNA virus with segmented genome (10 segments), belonging to Seoreoviridae family, Orbivirus genus. Over the last 25 years, Europe has suffered multiple incursions of different BTV serotypes with serious consequences, which have been controlled mainly thanks to vaccination. In the case of Spain, since 2000 to 2023, BTV serotypes 1, 2, 4 and 8 have caused epidemics, and sporadically BTV-1 and -4 were detected in the same area and period. In 2024, BTV serotypes 1, 3 and 8 circulated simultaneously in the southwest of the country, causing a severe clinical impact especially on sheep but also in cattle and a multitude of outbreaks. Additionally, despite vaccination, also circulated serotype 4 that year, especially in areas where the other serotypes were already circulating. Whole Genome Sequencing and phylogenetic analyses allowed us to confirm that serotypes 1 and 4 were homologous to viruses circulating in the country since 2000s, while serotypes 3 and 8 were homologous to BTV viruses recently notified in neighboring countries. In this context, many BTV co-infections of two or more different serotypes were confirmed by serotype specific RT-PCRs both in farms and individual animals. An epidemic caused by four serotypes coinciding in space and time had never occurred before in Spain, being a challenge for the diagnosis and control of this disease. Moreover, it could have favored the appearance of reassortant viruses with an unknown virulence, posing an additional risk. The data presented here raises the question of whether the co-circulation of different BTV strains, an exceptional situation, could become the new normal in certain areas of Europe.