Highly pathogenic avian influenza (HPAI) H5Nx viruses of the 2.3.4.4b lineage have been enzootic in both wild and domestic birds worldwide since 2020. These viruses have reassorted with low pathogenic avian influenza (LPAI) H5 viruses, resulting in several neuraminidase (NA) subtype combinations. Though HPAI H5Nx viruses primarily infect birds, infections are increasingly reported in carnivorous mammals, and recently also in cattle in the United States (US). Moreover, HPAI H5Nx infection and clinical disease have been reported in cats. Mostly, these were sporadic cases, although three outbreaks among cats in Poland (46 cats), South Korea (40 cats) and the US (12 cats) have been reported. In cats, infection with HPAI H5Nx is associated with respiratory or neurologic clinical signs and mortality, similar signs as described in other mammals. However, HPAI H5Nx virus and antibodies to HPAI H5 are also found in clinically healthy cats. Cat-to-cat transmission of HPAI H5Nx virus has been demonstrated in experimental settings. Furthermore, cat-to-cat transmission of influenza A viruses (IAV) under field conditions has been described for avian origin H3N2 and H7N2 IAV and for human origin H1N1pdm2009 influenza A virus. Although H1N1pdm2009 cat-to-cat-transmission was confirmed in experimental settings, human-to-cat-transmission, causing mild to moderate clinical signs, is considered more common.

Given that cats are susceptible to both human and avian IAV, simultaneous infection with these viruses may occur. Concurrent infection could generate IAV recombinants with an enhanced zoonotic potential. Moreover, replication of HPAI H5Nx in mammalian hosts may induce viral adaptations that facilitate replication and spread in mammalian hosts including humans. In view of the potential zoonotic risk involved with IAV infections, it is important to know the HPAI H5Nx and/or H1N1pdm2009 presence or seroprevalence in cats as an indicator of prior exposure.

Here, exposure to HPAI H5Nx and H1N1pdm2009 was investigated in domestic cats and rurally living stray cats, sampled in the Netherlands between 2020 and 2023. In addition, we analysed factors potentially associated with exposure to these viruses.