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

## TL;DR

House martins in Spain show signs of past exposure to flaviviruses and avian influenza, suggesting they may help spread these zoonotic viruses in urban areas.

## Contribution

This study reports the first AIV-seropositive house martins in Spain and confirms ongoing flavivirus exposure in this synanthropic bird species.

## Key findings

- 2.13% of house martins showed antibodies against avian influenza virus.
- 24.34% of house martins had antibodies against flaviviruses.
- No viral RNA was detected in the samples, but seroprevalence suggests prior exposure.

## Abstract

Emerging infectious diseases pose a growing threat to human health worldwide, particularly those caused by zoonotic pathogens that can spread from animals to people. Birds living in urban areas can carry these pathogens and serve as early indicators of outbreaks. In this study, we investigated a colony of house martins (Delichon urbicum) in southwestern Spain to assess exposure to avian influenza virus and flaviviruses. Although no viral RNA was detected, a small proportion of birds had antibodies against avian influenza, and nearly one-quarter had antibodies against flaviviruses. These findings suggest that house martins may contribute to the transmission of these viruses, highlighting their importance for monitoring emerging zoonotic diseases in urban environments.

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 (Delichon urbicum) 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 D. urbicum 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.