No evidence of influenza A virus infection in marine mammals stranded in Basque Country coast (Northern Spain) in the context of H5N1 clade 2.3.4.4b expansion

Influenza A infection is frequently detected in marine mammals causing mortality events of different magnitude. In recent years, the highly pathogenic avian influenza (HPAI) H5N1 virus (clade 2.3.4.4b) has been rapidly expanding and infecting a new variety of avian and mammalian species, including marine mammals. Given this new epidemiological context, the objective of this work was to evaluate the influenza A virus infection of marine mammals stranded on the Basque Country coast. 38 marine mammals were studied (2012-2024), most of them (79%) being dolphins (striped dolphin, common dolphin, and common bottlenose dolphin). At necropsy, samples were collected from lung, intestine, and central nervous system when possible. During 2024, serum samples and oropharyngeal, nasal and faeces/rectum swabs were also collected. Real-time RT-qPCR was applied for influenza A virus detection in tissue and swab samples, and commercial ELISA was used to detect antibodies against influenza A viruses. All samples tested negative by PCR and ELISA. Despite the negative results obtained, we consider the continuation of the monitoring of influenza viruses in marine mammals essential by establishing collaboration networks and standardised protocols that allow us to gain insight into the epidemiology of influenza viruses in these species.