Phylogenetic evidence from the recent resurgence of high-pathogenicity avian influenza (HPAI) virus subtype H5N1, clade 2.3.4.4b, observed in European wild birds and poultry since October 2021, suggests at least two different and distinct reservoirs. We propose contrasting hypotheses for this emergence: (i) resident viruses have been maintained, presumably in wild birds, in northern Europe throughout the summer of 2021 to cause some of the outbreaks that are part of the most recent autumn/winter 2021 epizootic, or (ii) further virus variants were reintroduced by migratory birds, and these two sources of reintroduction have driven the HPAI resurgence.

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A n episode of unusual disease resulting in deaths in different species at a wildlife rehabilitation center in the United Kingdom during late 2020 led to the retrospective detection of infl uenza A virus subtype H5N8 of avian origin in 5 mute swans, a fox, and 3 seals. The wildlife rehabilitation center admits >6,000 animals each year. New arrivals are initially housed in a quarantine facility upon admission. Four juvenile common seals (Phoca vitulina), 1 juvenile gray seal (Halichoerus grypus), and 1 juvenile red fox (Vulpes vulpes) died or were euthanized over a 2-day period. The fox died suddenly after a short period of nonspecifi c malaise and inappetence. The seals exhibited sudden-onset neurologic signs, including seizures before death or euthanasia (Figure 1). This mortality event occurred ≈1 week after the deaths or euthanasia of 5 mute swans (Cygnus olor) held in isolation at the center because of acute-onset malaise and terminal seizures. The 5 swans were submitted for examination and testing under the Avian Infl uenza Wild Bird Surveillance Scheme (undertaken by the United Kingdom's Animal and Plant Health Agency) (1), and they tested positive for highly pathogenic avian infl uenza A(H5N8) virus.The unusual spatiotemporal cluster of unexplained death and neurologic disease in multiple avian and nonavian species warranted further investigation. Infl uenza of avian origin was not suspected in the fox and seals, and none of the other captive birds at the center showed any clinical signs of disease. The linkage between the mortality event in the swans and that observed in the fox and seals was not

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