Louise H. Moncla

In November 2025, the first global H5N5 infection was reported in a person in Washington State. Previously, these viruses had only been detected in seabirds and shorebirds near the Atlantic Ocean, suggesting potentially distinct ecology from dominant H5N1 strains. Using 354 global H5N5 genomes, we show that H5N5 viruses have been carried by high latitude seabirds and shorebirds across the Atlantic at least 9 times, with introductions more frequent in the fall and spring. We reconstruct a plausible path of cross-continental movement via Arctic or sub-Arctic regions, followed by a year of cryptic circulation prior to human spillover. These data show that H5N5 viruses have undergone far more intercontinental movements than previously recognized, and exhibit host and geographic patterns that are fundamentally distinct from dominant H5N1 lineages. The distinct host ecology, geography, and movement, paired with the capacity to infect a person, suggest that enhanced surveillance may be warranted.

Despite its high propensity for host switching, the evolutionary mechanisms underlying influenza host adaptation remain unclear. H3Nx influenza viruses are uniquely generalist, with long-term lineages that circulate in avian, human, swine, equine, and canine hosts. Using 13,295 H3Nx sequences, we quantified host-specific adaptive evolution, and developed a pipeline to map reassortment events onto trees with measures of statistical uncertainty. We find that while H3Nx viruses in mammals undergo adaptive evolution in HA and NA, viruses in birds experience very little directional selection. Instead, avian lineages exhibit high rates of reassortment, frequently generating novel reassortant lineages that persist transiently and turn over rapidly. 29.8-47.4% of all avian reassortant lineages are purged within the first year of circulation, and reassortment shows no fitness benefit in birds. In contrast, reassorted lineages in swine are more likely to persist long-term, suggesting that reassortment in swine may be broadly beneficial. Segment-specific reassortment patterns were also distinct between avian and mammalian viruses, with NA reassorting more frequently than expected in birds, but less frequently than expected in swine. Reassortment events are enriched between mammalian, but not avian, host switches, suggesting that reassortment may be most beneficial for mediating host switches among mammalian species. Together, our data suggest that host differences drive fundamentally different evolutionary outcomes for influenza viruses, transitioning from reassortment-dominant evolution in their avian reservoir, to varying degrees of adaptation upon establishment in mammals.

Since late 2021, highly pathogenic H5 avian influenza viruses have spread across the Americas, resulting in hundreds of outbreaks in agriculture and 71 human spillovers. In the United States, human infections overwhelmingly derive from exposure to infected agricultural animals. However, little is known about how these viruses evolve during outbreaks in agriculture, or how they spread locally, key information for understanding human infection risk. We produced high-quality deep sequence data from commercial and live bird market (LBM) outbreaks in Pennsylvania from 2023-2025. We uncover repeated spillovers from wild to domestic birds, and evidence of high circulating diversity in LBMs that likely moved across state lines. Samples collected from the same outbreak shared a significant fraction of viral variation, suggesting high circulating diversity and a strong signal of transmission linkage. Samples from poultry collectively harbored 9 known adaptive variants, including PB2 D701N, suggesting that rare, adaptive variants may circulate in settings where human exposure risk is high. Our data suggest that outbreaks in poultry pose opportunities for subsequent viral evolution, supporting continued efforts to stamp out transmission. We also find that LBMs experience outbreak risk from domestic and wild sources and promote viral mixing between species, suggesting that enhanced tracking and biosecurity may be useful in these settings. Finally, our data suggest the potential to use within-sample variation for enhanced outbreak linkage within these outbreak settings.

Clade 2.3.4.4b H5NX influenza viruses have spread widely in birds since 2020. In addition to causing disease in birds, these viruses have infected a variety of mammals, including humans. Clade 2.3.4.4b H5N1 viruses are currently causing an outbreak among dairy cattle in the United States, and it is important to determine if other mammals have been exposed to H5NX viruses. Cats, specifically outdoor and feral cats, frequently predate wild birds. Recent studies have shown that cats living on dairy cattle farms can be infected with H5N1. Here, we completed serological studies to determine if owned and feral cats living in an urban environment in the United States have evidence of past H5N1 exposures. We used multianalyte bead-based assays to measure clade 2.3.4.4b hemagglutinin (HA) antibody levels in serum samples collected in July 2023 to June 2025 from 417 feral and 228 owned cats from the greater Philadelphia area. We also measured antibody levels against a panel of HAs from other human and non-human influenza viruses, and the receptor binding domain (RBD) of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We completed additional H5N1 and SARS-CoV-2 neutralization assays using samples that had detectable antibodies in the multianalyte bead-based assays. One cat (0.16%) was positive for H5 antibodies and twenty cats (3.1%) were positive for SARS-CoV-2 antibodies in both binding and neutralization assays. These data suggest that cats in the Philadelphia area have not been routinely exposed to clade 2.3.4.4b H5N1 viruses but have been more commonly exposed to SARS-CoV-2. Importance: Highly pathogenic H5NX avian influenza viruses have caused widespread disease and mortality in both wild and domestic birds, and have infected marine mammals, cattle, and humans. Recent studies have demonstrated that cats can also be infected with H5NX viruses, but it is unknown if these infections are common. In this report, we completed a seroprevalence study and found that H5NX exposure in feral and owned cats in the Philadelphia area appears to be rare. We also completed SARS-CoV-2 serological experiments and found that 3.1% of cats possessed antibodies reactive to this virus.

Since 2021, highly pathogenic (HPAI) H5N1 viruses have spread across the Americas, diversifying via reassortment into new genotypes that have spilled into humans and livestock, raising fears of a new influenza pandemic. Pandemic lineages are typically associated with reassortment, but we currently have limited understanding of where and when reassortment is expected to occur, which limits our ability to assess pandemic risks. Using a dataset of 9,052 full-genome sequences, we show that reassortment and novel genotype formation are associated with seasonal variation in low pathogenicity avian influenza (LPAI) cases and with the spatial and host distributions of viral transmission. We pinpoint ducks, geese, and the Central flyway as frequent sources of new genotypes, and show that reassortment rates vary seasonally, driven by mixing between high- and low-pathogenicity viruses. Cattle spillover genotypes (B3.13 and D1.1) evolved during periods of high reassortment, implicating reassortment as a common occurrence in lineages evolving during particular time periods. Together, these findings reframe reassortment as a predictable ecological process, with direct implications for how surveillance and pandemic risk assessment should be designed.

Motivation: Genome sequencing provides an exceptional window into the evolutionary and epidemiological dynamics of endemic and emerging pathogens, and thus allows for better, more targeted, public health interventions. Online genomic surveillance platforms can provide near real-time insight into these dynamics. Results: Nextstrain provides continually updated real-time genomic surveillance for 21 viruses and the bacterial pathogen Mycobacterium tuberculosis, with most analyses relying solely on open sequence data. Each pathogen includes steps to fetch and curate open data, classify sequences using established nomenclature systems, perform phylodynamic analyses, and share the results publicly. These analyses are automated, with most running daily to provide continually updated snapshots of pathogen evolution. Availability and Implementation: All source code is available at https://github.com/nextstrain. Phylodynamic results can be visualized and downloaded at https://nextstrain.org/pathogens, and open sequence data and curated metadata are available at https://nextstrain.org/pathogens/files.

Since late 2021, a panzootic of highly pathogenic H5N1 has devastated wild birds, agriculture and mammals. Here an analysis of 1,818 haemagglutinin sequences from wild birds, domestic birds and mammals reveals that the North American panzootic was driven by around nine introductions into the Atlantic and Pacific flyways, followed by rapid dissemination through wild, migratory birds. Transmission was primarily driven by Anseriformes, while non-canonical species acted as dead-end hosts. In contrast to the epizootic of 2015 (refs. ^1,2 ), outbreaks in domestic birds were driven by around 46–113 independent introductions from wild birds that persisted for up to 6 months. Backyard birds were infected around 9 days earlier on average than commercial poultry, suggesting potential as early-warning signals for transmission upticks. We pinpoint wild birds as critical drivers of the epizootic, implying that enhanced surveillance in wild birds and strategies that reduce transmission at the wild–agriculture interface will be key for future tracking and outbreak prevention.

The ongoing panzootic of highly pathogenic avian influenza (HPAI) A(H5) viruses is the largest in history, with unprecedented transmission to multiple mammalian species. Avian influenza A viruses of the H5 subtype circulate globally among birds and are classified into distinct clades based on their hemagglutinin (HA) genetic sequences. Thus, the ability to accurately and rapidly assign clades to newly sequenced isolates is key to surveillance and outbreak response. Co-circulation of endemic, low pathogenic avian influenza (LPAI) A(H5) lineages in North American and European wild birds necessitates the ability to rapidly and accurately distinguish between infections arising from these lineages and epizootic HPAI A(H5) viruses. However, currently available clade assignment tools are limited and often require command line expertise, hindering their utility for public health surveillance labs. To address this gap, we have developed datasets to enable A(H5) clade assignments with Nextclade, a drag-and-drop tool originally developed for SARS-CoV-2 genetic clade classification. Using annotated reference datasets for all historical A(H5) clades, clade 2.3.2.1 descendants, and clade 2.3.4.4 descendants provided by the Food and Agriculture Organization/World Health Organization/World Organisation for Animal Health (FAO/WHO/WOAH) H5 Working Group, we identified clade-defining mutations for every established clade to enable tree-based clade assignment. We then created three Nextclade datasets which can be used to assign clades to A(H5) HA sequences and call mutations relative to reference strains through a drag-and-drop interface. Nextclade assignments were benchmarked with 19,834 unique sequences not in the reference set using a pre-released version of LABEL, a well-validated and widely used command line software. Prospective assignment of new sequences with Nextclade and LABEL produced very well-matched assignments (match rates of 97.8% and 99.1% for the 2.3.2.1 and 2.3.4.4 datasets, respectively). The all-clades dataset also performed well (94.8% match rate) and correctly distinguished between all HPAI and LPAI strains. This tool additionally allows for the identification of polybasic cleavage site sequences and potential N-linked glycosylation sites. These datasets therefore provide an alternative, rapid method to accurately assign clades to new A(H5) HA sequences, with the benefit of an easy-to-use browser interface.

The 2022 panzootic of highly pathogenic avian influenza (HPAI) A(H5) viruses has led to unprecedented transmission to multiple mammalian species. Avian influenza A viruses of the H5 subtype circulate globally among birds and are classified into distinct clades based on their haemagglutinin (HA) genetic sequences. Thus, the ability to accurately and rapidly assign clades to newly sequenced isolates is key to surveillance and outbreak response. Cocirculation of endemic, low-pathogenicity avian influenza (LPAI) A(H5) lineages in North American and European wild birds necessitates the ability to rapidly and accurately distinguish between infections arising from these lineages and epizootic HPAI A(H5) viruses. However, currently available clade assignment tools are limited and often require command-line expertise, hindering their utility for public health surveillance labs. To address this gap, we have developed datasets to enable A(H5) clade assignments with Nextclade, a drag-and-drop tool originally developed for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) genetic clade classification. Using annotated reference datasets for all historical A(H5) clades, clade 2.3.2.1 descendants, and clade 2.3.4.4 descendants provided by the Food and Agriculture Organization/World Health Organization/World Organisation for Animal Health H5 Working Group, we identified clade-defining mutations for every established clade to enable tree-based clade assignment. We then created three Nextclade datasets which can be used to assign clades to A(H5) HA sequences and call mutations relative to reference strains through a drag-and-drop interface. Nextclade assignments were benchmarked with 19 834 unique sequences not in the reference set using a prereleased version of LABEL, a well-validated and widely used command-line software. Prospective assignment of new sequences with Nextclade and LABEL produced very well-matched assignments (match rates of 97.8% and 99.1% for the 2.3.2.1 and 2.3.4.4 datasets, respectively). The all-clades dataset also performed well (94.8% match rate) and correctly distinguished between all HPAI and LPAI strains. This tool additionally allows for the identification of polybasic cleavage site sequences and potential N-linked glycosylation sites. These datasets therefore provide an alternative, rapid method to accurately assign clades to new A(H5) HA sequences, with the benefit of an easy-to-use browser interface.

Since late 2021, a panzootic of highly pathogenic H5N1 has devastated wild birds, agriculture, and mammals. Analysis of 1,818 Hemagglutinin sequences from wild birds, domestic birds and mammals revealed that the North American panzootic was driven by ∼9 introductions into the Atlantic and Pacific Flyways, followed by rapid dissemination via wild, migratory birds. Transmission was primarily driven by Anseriformes, while non-canonical species acted as dead-end hosts. Unlike the epizootic of 2015, outbreaks in domestic birds were driven by ∼46-113 independent introductions from wild birds that persisted for up to 6 months. Backyard birds were infected ∼9 days earlier on average than commercial poultry, suggesting potential as “early warning signals” for transmission upticks. We find that wild birds are an emerging source of H5N1 in North America, necessitating enhanced, continuous surveillance. Prevention of agricultural outbreaks may now require novel strategies that reduce transmission at the wild bird/agriculture interface.