Cetaceans are marine mammals that provide important ecosystem services. Conservation of these iconic species is threatened by factors such as climate change and accidental bycatch. Infectious diseases are another threat, as mass stranding events in cetaceans can be caused by viral infections, mostly by Morbillivirus. Influenza A virus (IAV) have a wide range of avian and mammalian host species, including humans. The virus circulates across different species, leading to disease outbreaks and increasing the risk of panzootics and pandemics. Since the 1970s, IAV has been diagnosed in wild animals belonging to the two living cetacean families, Delphinidae and Phocoenidae. A comprehensive review of IAV infection of cetacean species is necessary to understand the risk of HPAI outbreaks in cetaceans. Documented cases relate to IAV strains H1N3, H13N2, H13N9, and H5N1 and have been reported in cetaceans sampled in the Pacific, Atlantic, and Arcticoceans. The analysis of molecular markers of viral adaptation to mammals in H5N1 clade 2.3.4.4b cetacean isolates reveals mutations are present in three viral proteins: hemagglutinin (HA), polymerase basic protein 2 (PB2), and nucleoprotein (NP). No sustained transmission of the virus between cetaceans to date is suspected from the documented cases or from amino acid sequence phylogenetic analysis. IAV H5N1 appears to be reaching cetaceans after spillover from seabirds and other marine mammals. Increasing worldwide surveillance of IAV infection of cetaceans is crucial, as sentinel species for human pandemic preparedness and key species for marine biodiversity conservation and ecosystem health.

This study has been funded by the subproject ALMA in the framework of the complementary plan on biodiversity, financed by the NextGenerationEU Recovery and Resilience Mechanism, and coordinated under the agreement between Junta de Andalucía and Universidad Pablo de Olavide.