Serological Evidence of Flavivirus Exposure and Limited Avian Influenza Exposure in Urban House Martins from Southwestern Spain

Zoonotic diseases account for approximately one billion cases of illness and millions of deaths globally each year. Increasing contact between humans and competent wildlife hosts elevates the risk of zoonotic spillover. Synanthropic bird species are key players in the transmission of zoonotic pathogens, including flaviviruses such as West Nile virus (WNV) and influenza A viruses like Avian Influenza Virus (AIV). Active surveillance of sentinel birds inhabiting urban areas allows for early detection of emerging pathogens before they cause zoonotic outbreaks. Despite nesting in close proximity to humans, the role of the house martin (<i>Delichon urbicum</i>) in the circulation of flaviviruses and AIV remains poorly understood. Here, we analyzed the presence of antibodies against flaviviruses and AIV in a colony of house martins from southwestern Spain. In addition, we aimed to detect amplicons of the matrix and nucleoprotein genes of AIV using RT-qPCR. While none of the samples tested positive for AIV by RT-qPCR, we observed an AIV seroprevalence of 2.13% based on non-subtyped ELISA. Notably, this is the first report of AIV-seropositive <i>D. urbicum</i> individuals captured in Spain. Moreover, we detected a flavivirus-group seroprevalence of 24.34%, similar to rates reported in the same house martin population between 2018 and 2020, suggesting widespread circulation of flaviviruses within this synanthropic species. These results support the hypothesis that house martins may participate in the transmission of these viruses between wild bird populations and humans in urban environments.