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We report cases of mammalian infection with highly pathogenic avian influenza (HPAI) virus A(H5N1) clade in Northern Ireland. Two common gulls () and two red fox kits (), were found dead in close vicinity. Comparison of viral whole genome sequences obtained from the animals identified a novel mammalian adaptation, PB2-M535I. Analysis of genetic sequences from other recent mammalian infections shows that this mutation has arisen on at least five occasions in three European countries since April 2023.


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