Investigating the role of stable flies (Stomoxys calcitrans) and biting midges of the genus Culicoides as potential mechanical vectors of African swine fever virus in Lithuania, Poland and Romania.

BACKGROUND: Since its emergence in Georgia in 2007, the seasonal pattern of African swine fever virus (ASFV) genotype II outbreaks in European pig populations has been evident. It is hypothesized that summer-related farming practices, along with the increased activity and abundance of arthropod vectors during warmer months, contribute to the increased incidence of these outbreaks during this period. This study investigated the potential role of stable flies (Stomoxys calcitrans) and biting midges of the genus Culicoides as mechanical vectors of ASFV. In addition, the potential distribution and abundance of different species of Culicoides biting midges on pig farms was investigated. METHODS: From August 2021 to August 2023, vector surveillance was conducted as part of a case-control study on 42 outbreak farms and 70 control farms across Romania, Lithuania, and Poland. Collected insect specimens were pooled and tested for ASFV DNA using real-time polymerase chain reaction (PCR). RESULTS: A total of 8604 biting midges of the genus Culicoides specimens and 742 S. calcitrans flies were collected, with ASF DNA detected in 27 out of 1219 insect pools. Positive pools were predominantly observed in Culicoides punctatus, C. newsteadi, and the Obsoletus complex, with most detections occurring in August. However, ASFV isolation was unsuccessful. Statistical analyses revealed no significant association between farm status (outbreak versus control) and ASFV DNA detection in pools of biting midges of the genus Culicoides, likely due to limited sample size. Additionally, two ASFV-positive S. calcitrans pools were identified, supporting their potential role as mechanical vehicles for ASFV. The findings highlight the affinity of C. punctatus, C. newsteadi, and the Obsoletus complex and S. calcitrans for pig farms - an aspect that was previously undocumented. This association may increase the likelihood of ASFV acquisition and dissemination by these species. Environmental factors, such as pig farm density and proximity to other hosts, likely influence this risk. CONCLUSIONS: These results emphasize the importance of vector control strategies, including insect netting, to mitigate ASFV transmission risks. Further research is needed to understand the dynamics of ASFV infection in arthropods. Notably, this study also reports the first identification of Culicoides riethi and Culicoides salinarius in Lithuania.