Highly pathogenic avian influenza H5 clade 2.3.4.4 and human new pandemic H1N1 virus exposure in domestic cats with outdoor access in the Netherlands in 2024.

ObjectivesCats are susceptible to highly pathogenic avian influenza H5 clade 2.3.4.4 (HPAI H5) and human new pandemic H1N1 (H1N1pdm09) influenza A viruses. A simultaneous infection with multiple influenza A virus subtypes could potentially result in the generation of reassortant viruses with enhanced zoonotic potential. Previously, high seropositivity (11.8%) to HPAI H5 virus has been detected in rural stray cats in the Netherlands, presumably through contact with or feeding on infected birds. Seropositivity was much lower (0.46%) in concurrently sampled domestic cats with unknown outdoor access, which were frequently (4.6%) seropositive to H1N1pdm09 virus. As outdoor access is expected to increase the risk of HPAI H5 exposure, in this study we determined seropositivity to HPAI H5 as well as H1N1pdm09 in domestic cats with known outdoor access.MethodsIn 2024, sera from 254 outdoor cats were collected and screened for antibodies to HPAI H5 and H1N1pdm09 virus using in-house developed ELISAs and haemagglutination inhibition assays (HAIs).ResultsAntibodies to HPAI H5 virus were detected in seven (2.8%) cat sera (95% confidence interval [CI] 1.1-5.6) by ELISA, but not by HAI. Antibodies to H1N1pdm09 were detected in 14 (5.5%) sera (95% CI 3.1-9.1), of which seven (2.8%) were positive by HAI (95% CI 1.1-5.6). Two sera (0.79%) reacted in ELISA to both HPAI H5 and H1N1pdm09 (95% CI 0.1-2.8).Conclusions and relevanceAntibodies to both HPAI H5 and H1N1pdm09 viruses were detected in outdoor domestic cats, with a higher seroprevalence for H1N1pdm09. Seropositivity for HPAI H5 was higher than was previously detected in domestic cats with unknown outdoor access, but lower than in stray cats. This warrants further investigation into the potential role of outdoor domestic cats as mixing vessels and as a source of (novel) zoonotic viruses.