Highly pathogenic avian influenza virus H5N6 (clade 2.3.4.4b) has a preferable host tropism for waterfowl reflected in its inefficient transmission to terrestrial poultry

Comparison of the Clinical Manifestation of HPAI H5Nx in Different Poultry Types in the Netherlands, 2014-2022

This study describes clinical manifestations of highly pathogenic avian influenza (HPAI) H5N1, H5N8 and H5N6 outbreaks between 2014 and 2018 and 2020 and 2022 in the Netherlands for different poultry types and age groups. Adult duck (breeder) farms and juvenile chicken (broiler and laying pullet) farms were not diagnosed before 2020. Outbreaks in ducks decreased in 2020-2022 vs. 2014-2018, but increased for meat-type poultry. Neurological, locomotor and reproductive tract signs were often observed in ducks, whereas laying- and meat-type poultry more often showed mucosal membrane and skin signs, including cyanosis and hemorrhagic conjunctiva. Juveniles (chickens and ducks) showed neurological and locomotor signs more often than adults. Diarrhea occurred more often in adult chickens and juvenile ducks. Mortality increased exponentially within four days before notification in chickens and ducks, with a more fluctuating trend in ducks and meat-type poultry than in layers. For ducks, a mortality ratio (MR) > 3, compared to the average mortality of the previous week, was reached less often than in chickens. A lower percentage of laying flocks with MR > 3 was found for 2020-2022 vs. 2014-2018, but without significant differences in clinical signs. This study provides a basis for improvements in mortality- and clinical-sign-based early warning criteria, especially for juvenile chickens and ducks.

Genomic epidemiology of highly pathogenic avian influenza A (H5N1) virus in wild birds in South Korea during 2021-2022: Changes in viral epidemic patterns

Clade 2.3.4.4b highly pathogenic avian influenza A (HPAI) viruses have been detected in wild birds worldwide, causing recurrent outbreaks since 2016. During the winter of 2021-2022, we detected one H5N8 and forty-three H5N1 clade 2.3.4.4b HPAI viruses from wild birds in South Korea. Phylogenetic analysis revealed that HA gene of H5N1 viruses was divided into two genetically distinct groups (N1.G1 and N1.G2). Bayesian phylodynamic analysis demonstrated that wild birds play a vital role in viral transmission and long-term maintenance. We identified five genotypes (N1.G1.1, N1.G2, N1.G2.1, N1.G2.2, and N1.G2.2.1) having distinct gene segment constellations most probably produced by reassortments with low-pathogenic avian influenza viruses. Our results suggest that clade 2.3.4.4b persists in wild birds for a long time, causing continuous outbreaks, compared with previous clades of H5 HPAI viruses. Our study emphasizes the need for enhancing control measures in response to the changing viral epidemiology.

Transmission dynamics and pathogenesis differ between pheasants and partridges infected with clade 2.3.4.4b H5N8 and H5N1 high-pathogenicity avian influenza viruses

During the UK 2020-2021 epizootic of H5Nx clade 2.3.4.4b high-pathogenicity avian influenza viruses (HPAIVs), high mortality occurred during incursions in commercially farmed common pheasants (Phasianus colchicus). Two pheasant farms, affected separately by H5N8 and H5N1 subtypes, included adjacently housed red-legged partridges (Alectoris rufa), which appeared to be unaffected. Despite extensive ongoing epizootics, H5Nx HPAIV partridge outbreaks were not reported during 2020-2021 and 2021-2022 in the UK, so it is postulated that partridges are more resistant to HPAIV infection than other gamebirds. To assess this, pathogenesis and both intra- and inter-species transmission of UK pheasant-origin H5N8-2021 and H5N1-2021 HPAIVs were investigated. Onward transmission to chickens was also assessed to better understand the risk of spread from gamebirds to other commercial poultry sectors. A lower infectious dose was required to infect pheasants with H5N8-2021 compared to H5N1-2021. However, HPAIV systemic dissemination to multiple organs within pheasants was more rapid following infection with H5N1-2021 than H5N8-2021, with the former attaining generally higher viral RNA levels in tissues. Intraspecies transmission to contact pheasants was successful for both viruses and associated with viral environmental contamination, while interspecies transmission to a first chicken-contact group was also efficient. However, further onward transmission to additional chicken contacts was only achieved with H5N1-2021. Intra-partridge transmission was only successful when high-dose H5N1-2021 was administered, while partridges inoculated with H5N8-2021 failed to shed and transmit, although extensive tissue tropism was observed for both viruses. Mortalities among infected partridges featured a longer incubation period compared to that in pheasants, for both viruses. Therefore, the susceptibility of different gamebird species and pathogenicity outcomes to the ongoing H5Nx clade 2.3.4.4b HPAIVs varies, but pheasants represent a greater likelihood of H5Nx HPAIV introduction into galliforme poultry settings. Consequently, viral maintenance within gamebird populations and risks to poultry species warrant enhanced investigation.

Virulence and transmission characteristics of clade 2.3.4.4b H5N6 subtype avian influenza viruses possessing different internal gene constellations

Clade 2.3.4.4 H5N6 avian influenza virus (AIV) has been predominant in poultry in China, and the circulating haemagglutinin (HA) gene has changed from clade 2.3.4.4h to clade 2.3.4.4b in recent years. In 2021, we isolated four H5N6 viruses from ducks during the routine surveillance of AIV in China. The whole-genome sequencing results demonstrated that the four isolates all belonged to the currently prevalent clade 2.3.4.4b but had different internal gene constellations, which could be divided into G1 and G2 genotypes. Specifically, G1 possessed H9-like PB2 and PB1 genes on the H5-like genetic backbone while G2 owned an H3-like PB1 gene and the H5-like remaining internal genes. By determining the characteristics of H5N6 viruses, including growth performance on different cells, plaque-formation ability, virus attachment ability, and pathogenicity and transmission in different animal models, we found that G1 strains were more conducive to replication in mammalian cells (MDCK and A549) and BALB/c mice than G2 strains. However, G2 strains were more advantageously replicated in avian cells (CEF and DF-1) and slightly more transmissible in waterfowls (mallards) than G1 strains. This study enriched the epidemiological data of H5 subtype AIV to further understand its dynamic evolution, and laid the foundation for further research on the mechanism of low pathogenic AIV internal genes in generating novel H5 subtype reassortants.

Different Outcomes of Chicken Infection with UK-Origin H5N1-2020 and H5N8-2020 High-Pathogenicity Avian Influenza Viruses (Clade 2.3.4.4b)

Clade 2.3.4.4 H5Nx highly pathogenic avian influenza viruses (HPAIVs) of the "goose/Guangdong" lineage have caused a series of European epizootics since 2014. During autumn/winter 2020-2021, several H5Nx subtypes were detected in the UK, with H5N8 being the dominant subtype in wild birds and poultry. Despite the greater subtype diversity (due to viral neuraminidase gene reassortment) reported in wild birds, only H5N8 and H5N1 subtypes caused clade 2.3.4.4 UK HPAIV poultry outbreaks during this period. The direct inoculation of layer chickens showed that H5N8-2020 was more infectious than H5N1-2020, which supported the European H5N8 dominance during that season. However, the mean death time was longer for H5N8-2020 (3.42 days) than for H5N1-2020 (2.17 days). Transmission from directly infected to naive in-contact chickens was inefficient for both subtypes. Histological lesions, the tissue dissemination of viral antigen, and nucleic acid were more extensive and abundant and accumulated more rapidly for H5N1-2020 compared with H5N8-2020. Although inefficient, H5N1-2020 transmission was faster, with its greater virulence indicating that this subtype posed a major concern, as subsequently shown during H5N1 dominance of the clade 2.3.4.4 epizootic since autumn 2021. An evaluation of these in vivo viral characteristics is key to understanding the continuing poultry threats posed by clade 2.3.4.4 H5Nx HPAIVs.

Efficient and Informative Laboratory Testing for Rapid Confirmation of H5N1 (Clade 2.3.4.4) High-Pathogenicity Avian Influenza Outbreaks in the United Kingdom

During the early stages of the UK 2021-2022 H5N1 high-pathogenicity avian influenza virus (HPAIV) epizootic in commercial poultry, 12 infected premises (IPs) were confirmed by four real-time reverse-transcription-polymerase chain reaction (RRT)-PCRs, which identified the viral subtype and pathotype. An assessment was undertaken to evaluate whether a large sample throughput would challenge laboratory capacity during an exceptionally large epizootic; hence, assay performance across our test portfolio was investigated. Statistical analysis of RRT-PCR swab testing supported it to be focused on a three-test approach, featuring the matrix (M)-gene, H5 HPAIV-specific (H5-HP) and N1 RRT-PCRs, which was successfully assessed at 29 subsequent commercial IPs. The absence of nucleotide mismatches in the primer/probe binding regions for the M-gene and limited mismatches for the H5-HP RRT-PCR underlined their high sensitivity. Although less sensitive, the N1 RRT-PCR remained effective at flock level. The analyses also guided successful surveillance testing of apparently healthy commercial ducks from at-risk premises, with pools of five oropharyngeal swabs tested by the H5-HP RRT-PCR to exclude evidence of infection. Serological testing at anseriform H5N1 HPAIV outbreaks, together with quantitative comparisons of oropharyngeal and cloacal shedding, provided epidemiological information concerning the chronology of initial H5N1 HPAIV incursion and onward spread within an IP.

Clade 2.3.4.4b H5N1 high pathogenicity avian influenza virus (HPAIV) from the 2021/22 epizootic is highly duck adapted and poorly adapted to chickens

The 2021/2022 epizootic of high pathogenicity avian influenza (HPAIV) remains one of the largest ever in the UK, being caused by a clade 2.3.4.4b H5N1 HPAIV. This epizootic affected more than 145 poultry premises, most likely through independent incursion from infected wild birds, supported by more than 1700 individual detections of H5N1 from wild bird mortalities. Here an H5N1 HPAIV, representative of this epizootic (H5N1-21), was used to investigate its virulence, pathogenesis and transmission in layer chickens and Pekin ducks, two species of epidemiological importance. We inoculated both avian species with decreasing H5N1-21 doses. The virus was highly infectious in ducks, with high infection levels and accompanying shedding of viral RNA, even in ducks inoculated with the lowest dose, reflecting the strong waterfowl adaptation of the clade 2.3.4.4 HPAIVs. Duck-to-duck transmission was very efficient, coupled with high environmental contamination. H5N1-21 was frequently detected in water sources, serving as likely sources of infection for ducks, but inhalable dust and aerosols represented low transmission risks. In contrast, chickens inoculated with the highest dose exhibited lower rates of infection compared to ducks. There was no evidence for experimental H5N1-21 transmission to any naive chickens, in two stocking density scenarios, coupled with minimal and infrequent contamination being detected in the chicken environment. Systemic viral dissemination to multiple organs reflected the pathogenesis and high mortalities in both species. In summary, the H5N1-21 virus is highly infectious and transmissible in anseriformes, yet comparatively poorly adapted to galliformes, supporting strong host preferences for wild waterfowl. Key environmental matrices were also identified as being important in the epidemiological spread of this virus during the continuing epizootic.

The Role of Airborne Particles in the Epidemiology of Clade 2.3.4.4b H5N1 High Pathogenicity Avian Influenza Virus in Commercial Poultry Production Units

Since October 2021, Europe has experienced the largest avian influenza virus (AIV) epizootic, caused by clade 2.3.4.4b H5N1 high pathogenicity AIV (HPAIV), with over 284 poultry infected premises (IPs) and 2480 dead H5N1-positive wild birds detected in Great Britain alone. Many IPs have presented as geographical clusters, raising questions about the lateral spread between premises by airborne particles. Airborne transmission over short distances has been observed for some AIV strains. However, the risk of airborne spread of this strain remains to be elucidated. We conducted extensive sampling from IPs where clade 2.3.4.4b H5N1 HPAIVs were confirmed during the 2022/23 epizootic, each representing a major poultry species (ducks, turkeys, and chickens). A range of environmental samples were collected inside and outside houses, including deposited dust, feathers, and other potential fomites. Viral RNA (vRNA) and infectious viruses were detected in air samples collected from inside and outside but in close proximity to infected houses, with vRNA alone being detected at greater distances (≤10 m) outside. Some dust samples collected outside of the affected houses contained infectious viruses, while feathers from the affected houses, located up to 80 m away, only contained vRNA. Together, these data suggest that airborne particles harboring infectious HPAIV can be translocated short distances (<10 m) through the air, while macroscopic particles containing vRNA might travel further (≤80 m). Therefore, the potential for airborne transmission of clade 2.3.4.4b H5N1 HPAIV between premises is considered low. Other factors, including indirect contact with wild birds and the efficiency of biosecurity, represent greater importance in disease incursion.

Pathogenesis and infection dynamics of high pathogenicity avian influenza virus (HPAIV) H5N6 (clade 2.3.4.4b) in pheasants and onward transmission to chickens

High pathogenicity avian influenza viruses clade 2.3.4.4 H5 have spread among wild birds worldwide during recent years causing annual die-offs among wild birds and outbreaks in poultry in multiple European countries. The outbreaks significantly impact the poultry and game bird sectors. Infected game birds may act as a bridging species potentially enabling spread of virus into commercial and backyard premises. In this study, the pathogenesis and transmission of a HPAIV clade 2.3.4.4b H5N6 virus was investigated in pheasants and chickens. Efficient virus transmission was detected between pheasants over multiple rounds of naïve pheasant introductions and onwards to chickens. Mortality of up to 100% was observed for both infected pheasants and chickens. Intra-species transmission from chicken to chicken was less efficient. The study confirmed that clade 2.3.4.4b H5N6 HPAIV is highly virulent in pheasants and emphasises the role of pheasants as a bridging host for the infection of commercial poultry.

Emergence of High Pathogenicity Avian Influenza Virus H5N1 Clade 2.3.4.4b in Wild Birds and Poultry in Botswana

Numerous outbreaks of high-pathogenicity avian influenza (HPAI) were reported during 2020-2021. In Africa, H5Nx has been detected in Benin, Burkina Faso, Nigeria, Senegal, Lesotho, Namibia and South Africa in both wild birds and poultry. Botswana reported its first outbreak of HPAI to the World Organisation for Animal Health (WOAH) in 2021. An H5N1 virus was detected in a fish eagle, doves, and chickens. Full genome sequence analysis revealed that the virus belonged to clade 2.3.4.4b and showed high identity within haemagglutinin (HA) and neuraminidase proteins (NA) for viruses identified across a geographically broad range of locations. The detection of H5N1 in Botswana has important implications for disease management, wild bird conservation, tourism, public health, economic empowerment of vulnerable communities and food security in the region.

Spatial epidemiology of highly pathogenic avian influenza subtype H5N6 in Gyeonggi Province, South Korea, 2016-2017

Over four months in the winter of 2016-2017, 343 poultry farms in South Korea reported highly pathogenic avian influenza (HPAI) H5N6 occurrences, leading to the culling of 40 million poultry. Our study aimed to describe the spatial epidemiology of the 2016-2017 HPAI H5N6 outbreak in Gyeonggi Province, the most affected area in South Korea, comprising 35.9% (123) of the HPAI-infected poultry farms, to identify spatial risk factors for the increased probability of HPAI H5N6 occurrence, and to delineate areas with the highest likelihood of infection among different target poultry species. Although the poultry density was risk factor for the all species, rice paddy was only identified as risk factor for chicken and duck farms, not other species farms suggesting different biosecurity measures are required depending on the species. Although spatial effects of HPAI occurrence tended to be clustered within 16 km, the cluster range was reduced to 7 km when considering the identified risk factors, indicating a more geographically focused outbreak response when taking risk factors into account. The areas identified with the highest likelihood of infection can provide evidence, with accessibility to policymakers, to improve risk-based surveillance for HPAI. Our findings provide epidemiological understanding helpful in improving surveillance activity and assisting in the design of more cost-effective intervention policies related to future HPAI outbreaks in South Korea. This article is protected by copyright. All rights reserved.

Detection of Highly Pathogenic Avian Influenza Virus H5N1 Clade 2.3.4.4b in Great Skuas: A Species of Conservation Concern in Great Britain

The UK and Europe have seen successive outbreaks of highly pathogenic avian influenza across the 2020/21 and 2021/22 autumn/winter seasons. Understanding both the epidemiology and transmission of these viruses in different species is critical to aid mitigating measures where outbreaks cause extensive mortalities in both land- and waterfowl. Infection of different species can result in mild or asymptomatic outcomes, or acute infections that result in high morbidity and mortality levels. Definition of disease outcome in different species is of great importance to understanding the role different species play in the maintenance and transmission of these pathogens. Further, the infection of species that have conservation value is also important to recognise and characterise to understand the impact on what might be limited wild populations. Highly pathogenic avian influenza virus H5N1 clade 2.3.4.4b has been detected in great skuas (Stercorarius skua) across different colonies on islands off the shore of Scotland, Great Britain during summer 2021. A large number of great skuas were observed as developing severe clinical disease and dying during the epizootic and mortalities were estimated to be high where monitored. Of eight skuas submitted for post-mortem examination, seven were confirmed as being infected with this virus using a range of diagnostic assays. Here we overview the outbreak event that occurred in this species, listed as species of conservation concern in Great Britain and outline the importance of this finding with respect to virus transmission and maintenance.

Encephalitis and Death in Wild Mammals at a Rehabilitation Center after Infection with Highly Pathogenic Avian Influenza A(H5N8) Virus, United Kingdom

We report a disease and mortality event involving swans, seals, and a fox at a wildlife rehabilitation center in the United Kingdom during late 2020. Five swans had onset of highly pathogenic avian influenza virus infection while in captivity. Subsequently, 5 seals and a fox died (or were euthanized) after onset of clinical disease. Avian-origin influenza A virus subtype H5N8 was retrospectively determined as the cause of disease. Infection in the seals manifested as seizures, and immunohistochemical and molecular testing on postmortem samples detected a neurologic distribution of viral products. The fox died overnight after sudden onset of inappetence, and postmortem tissues revealed neurologic and respiratory distribution of viral products. Live virus was isolated from the swans, seals, and the fox, and a single genetic change was detected as a potential adaptive mutation in the mammalian-derived viral sequences. No human influenza-like illness was reported in the weeks after the event.