Between 2020 and 2023, high pathogenicity avian influenza virus (HPAIV) H5Nx clade 2.3.4.4b caused devastating outbreaks across Europe and the United Kingdom among domestic poultry and wild bird populations. During winter 2022, unexpected mortality was observed in four of fourteen outdoor captive Humboldt penguins in a British zoological collection, without prior clinical signs. Swabs, one whole carcass and two heads were submitted for notifiable avian disease laboratory investigation. Clinical material was inspected by veterinary pathologists using an established post-mortem procedure and tissues were collected for official sampling according to a standard notifiable avian disease testing algorithm. A defined selection of tissues was tested using real-time reverse transcription PCR, whole-genome sequencing, histopathology and immunohistochemistry. Following molecular testing for H5N1 infection, positive results were detected in 5 of the 12 animals sampled. On gross examination of the carcass, generalised congestion was present. Histopathology revealed necrosis and acute inflammation, primarily in the spleen, liver and lungs. Immunohistochemistry demonstrated viral antigen in endothelial cells and lymphoid cells particularly in lung, spleen, liver, brain and heart muscle. This finding strongly suggests that endothelial cells and lymphoid cells are the primary targets for HPAIV infection in Humboldt penguins. Molecular characterisation identified the causative agent as HPAIV H5N1 clade 2.3.4.4b, genotyped as AIV223 according to the UK scheme. This was the first time that this genotype had been recorded in the UK. These findings provide novel insights into the pathobiology of this virus in naturally infected captive Humboldt penguins.Research Highlights Natural HPAIV H5N1 infection causes mortality and pathology in Humboldt Penguins.Molecular analysis identified the aetiology as a novel H5N1 clade 2.3.4.4b genotype.Immunohistochemical analysis demonstrated infection of endothelial cells and macrophages and reticular cells in lymphoid tissue.

Clade 2.3.4.4b H5N1 high pathogenicity avian influenza virus (HPAIV) continues to pose a significant global threat, affecting wild and domestic avian populations, and mammalian species. In early 2024, H5N1 HPAIV was detected in dairy cattle in the United States of America, where it has continued to circulate, with sporadic detections also reported in other ruminant species. The detection of high viral loads in milk from infected cattle, has resulted in several human infections, underscoring the zoonotic potential of these viruses. In response, several countries have intensified surveillance in non-avian species to evaluate the potential for undetected viral circulation in captive mammals. In Great Britain, bulk milk tank testing of cattle and targeted surveillance of captive mammalian species on an infected premises is undertaken in accordance with the outcome of a rapid risk assessment undertaken to determine the epidemiological links between the poultry and captive mammals. A result of this testing was the first recorded detection of clade 2.3.4.4b H5N1 HPAIV in a sheep in March 2025, identified on an infected poultry premises in Great Britain. An initial seropositive result in a single ewe triggered further investigation, confirming serological positivity across repeated sampling and the presence of viral RNA in milk samples. This detection was confined to a single animal and was likely attributable to proximity to infected poultry and a presumed heavily contaminated environment. The implications of this ruminant detection are discussed in the context of interspecies transmission and surveillance strategies.

The ongoing panzootic of H5N1 high pathogenicity avian influenza virus (HPAIV) has caused the deaths of over half a billion wild birds and poultry and has led to spillover events in both wild and domestic mammals, alongside sporadic human infections. A key driver of this panzootic is the apparent high viral fitness across diverse avian species, which facilitates an increased interface between wild and domestic species. Columbiformes (pigeons and doves) are commonly found on poultry premises, yet little is known about their potential role in contemporary HPAIV disease ecology. Here, we investigated the epidemiological role of pigeons (Columba livia) by determining their susceptibility using decreasing doses of clade 2.3.4.4b H5N1 HPAIV (genotype AB). We investigated infection outcomes and transmission potential between pigeons and chickens. Following direct inoculation, pigeons did not develop clinical signs, and only those inoculated with the highest dose shed viral RNA (vRNA) or seroconverted to H5N1-AB, revealing a 50% minimum infectious dose (MID) of 10⁵ 50% egg infectious dose. Even in the high-dose group, only low-level shedding and environmental contamination were observed, and low-level viral RNAs were present in the tissues of directly inoculated pigeons, with no distinct pathological lesions. Pigeons did not transmit the virus to pigeons or chickens placed in direct contact. We observed distinct differences in sialic acid receptor distribution in the pigeon respiratory tract compared to chickens and ducks. Together, these findings suggest that pigeons have low susceptibility to clade 2.3.4.4b H5N1 HPAIV and are unlikely to contribute significantly to virus maintenance, transmission to poultry or zoonotic infection.

Since 2020, the emergence and global dissemination of high pathogenicity avian influenza (HPAI) H5Nx clade 2.3.4.4b infection has had devastating effects in wild and domestic bird populations, and resulted in spillover events in many terrestrial and marine mammalian species, including susceptible captive big cat collections, and feral and domestic feline species. The recent outbreaks in dairy herds in the USA following proposed exposure to infection from wild birds, with subsequent spread to in-contact mammals, demonstrates successful interspecies transmission of this virus and warrants close monitoring. In this literature review, we provide an overview of HPAI H5N1 infection in feline species to date, focusing on domestic but also captive wild cats, looking at the epidemiology, clinical and pathological presentations and management, and a current perspective on risk, testing and strategies to prevent infection.

This study describes the first outbreaks with virulent avian paramyxovirus 1 (APMV‐1) since 2005 in Denmark. Both outbreaks were caused by pigeon specific variants, denoted pigeon paramyxovirus 1 (PPMV‐1). The first outbreak was in June 2022 and affected captive pigeons near Næstved, in southeast Denmark, where 1053 captive birds were housed, and hereof 851 pigeons. A second and separate outbreak occurred in June 2023 in Aalborg, northwest Denmark, involving 1851 captive birds, of which 40 were pigeons. In both outbreaks, pigeons were predominantly affected, characterised by high mortality, and presented with neurological signs, along with thin‐shelled eggs. Pathological and virological assessment revealed multi‐systemic infections in pigeons, including neuronal and vascular endothelial tropism. Chickens were affected only in the 2022 outbreak, with reported extended hatching periods and chicks dead at hatching, and with no apparent lesions detected at both macro‐ and microscopic investigations. Fusion protein (F) gene sequence classified the 2022 virus isolate as genotype VI.2.1.1.2.2 and the 2023 virus as genotype XXI.1.1, with polybasic cleavage sites ¹¹² RRQKRF ¹¹⁷ (2022) and ¹¹² KRQKRF ¹¹⁷ (2023). However, mean death time (MDT) tests categorised both virus isolates as mesogenic, and an intracerebral pathogenicity index (ICPI) test of the 2022 virus isolate showed an ICPI index of 0.65, categorising the virus as lentogenic. This is the first report of PPMV‐1 isolates with polybasic cleavage site and associated mortality in captive pigeons in Denmark. The sudden resurgence of outbreaks in Denmark after nearly two decades without similar incidents highlight the potential threat posed by circulating viruses in wild birds, such as feral pigeons, and emphasise the importance of surveillance in wild bird populations for improved risk recognition and early detection of emerging threats.

The ongoing panzootic of H5N1 high pathogenicity avian influenza virus (HPAIV) has caused the deaths of over half a billion wild birds and poultry, and has led to spillover events in both wild and domestic mammals, alongside sporadic human infections. A key driver of this panzootic is the apparent high viral fitness across diverse avian species, which facilitates an increased interface between wild and domestic species. Columbiformes (pigeons and doves) are commonly found on poultry premises, yet little is known about their potential role in contemporary HPAIV disease ecology. Here we investigated the epidemiological role of pigeons ( Columba livia ) by determining their susceptibility using decreasing doses of clade 2.3.4.4b H5N1 HPAIV (genotype AB). We investigated infection outcomes and transmission potential between pigeons and to chickens. Following direct inoculation, pigeons did not develop clinical signs, and only those inoculated with the highest dose shed viral RNA or seroconverted to H5N1-AB, revealing a MID ₅₀ of 10 ⁵ EID ₅₀ . Even in the high-dose group, only low-level shedding and environmental contamination was observed, and low-level viral RNA were present in the tissues of directly inoculated pigeons, with no distinct pathological lesions. Pigeons did not transmit the virus to pigeons or chickens placed in direct contact. We observed distinct differences in sialic acid receptor distribution in the pigeon respiratory tract compared to chickens and ducks. Together, these findings suggest that pigeons have low susceptibility to clade 2.3.4.4b H5N1 HPAIV and are unlikely to contribute significantly to virus maintenance, transmission to poultry, or zoonotic infection.