Virus detection in Culicoides biting midges during the BTV-3 outbreak in north–west Germany 2024

BACKGROUND: Bluetongue virus (BTV), Schmallenberg virus (SBV) and epizootic hemorrhagic disease virus (EHDV) are arthropod-borne pathogens that are transmitted by biting midges of the genus Culicoides. Following the initial introduction of BTV in central Europe in 2006, the emergence of SBV in 2011, and the first detection of EHDV in southern Europe in 2022, it became evident that Culicoides-borne viruses pose a significant threat to European livestock. In 2023, BTV serotype 3 was detected for the first time in the Netherlands and subsequently caused a severe outbreak in Germany in 2024. The aim of this study was to compare the prevalences of the viruses in the Culicoides population during the BTV-3 outbreak in north-west Germany in 2024. METHODS: Culicoides were caught by UV-light traps on 14 farms from April to October 2024 in north-west Germany. 337,359 of those were selected for virus screening and sorted into 7,549 pools based on their morphology. The pools were tested by real-time RT-PCRs for the genomes of SBV, BTV and EHDV. Virus-positive pools were further analyzed in species-specific PCRs to identify the Culicoides species present. RESULTS: A total of 1,359 pools (18.0%) tested positive for BTV-RNA, while 402 pools (5.3%) showed SBV-RNA. None of the pools were positive for EHDV. The majority of the positive pools contained specimens of the Culicoides subgenus Avaritia Fox. A smaller part belonged to the subgenus Culicoides Latreille. The taxa most commonly found in the virus-positive pools of the subgenus Avaritia were C. obsoletus clade O1, C. scoticus and C. dewulfi. The most prevalent taxa in the virus-positive pools of the subgenus Culicoides were C. punctatus, C. pulicaris and C. lupicaris haplotype L1. CONCLUSIONS: We confirmed the ongoing circulation of SBV as well as an extensive BTV circulation in the Culicoides vectors of the subgenera Avaritia and Culicoides in north-west Germany in 2024. Despite belonging to different virus families, the circulation patterns of BTV and SBV observed over the months were very similar, showing peaks in August.