Spill-Over of Avian and Human Influenza a Viruses to Swine in Egypt and Lebanon

Pigs play a key role in the ecology of influenza A viruses (IAVs), particularly in avian influenza (AI)-endemic regions where co-circulation of viruses from different hosts increases reassortment risk. Between September 2023 and August 2024, we surveyed pigs from Lebanon and Egypt to study IAV ecology in AI-endemic countries. Nasal swabs and sera were collected and tested using real-time RT-PCR and hemagglutination inhibition assays against avian, swine, and human seasonal IAVs. Molecular analyses identified IAV-infections in both countries, including human H1 and avian H5 subtypes, which may reflect potential cross-species transmission from humans and birds. Serologic analyses revealed prior exposure to avian, swine, and human IAVs. Avian virus seropositivity reached 4.6% (H5N1) and 15.2% (H9N2) in Egypt and 8.6% (H5N1) and 4.3% (H9N2) in Lebanon. Antibodies against human H1N1 and H3N2 were prevalent in both countries. Serologic evidence exceeded molecular detection, indicating frequent past or transient infections not captured by PCR alone. Antibody responses were significantly associated with host-level factors such as housing type, age, shaping exposure risk. These findings demonstrate repeated multisource exposure of pigs to genetically distinct IAVs in AI–endemic countries, supporting the need for integrated virologic and serologic surveillance within a One Health framework.