Airborne transmission of highly pathogenic H7N1 influenza virus in ferrets

The Alarming Situation of Highly Pathogenic Avian Influenza Viruses in 2019-2023

Avian influenza viruses (AIVs) have the potential to cause severe illness in wild birds, domestic poultry, and humans. The ongoing circulation of highly pathogenic avian influenza viruses (HPAIVs) has presented significant challenges to global poultry industry and public health in recent years. This study aimed to elucidate the circulation of HPAIVs during 2019 to 2023. Specifically, we assess the alarming global spread and continuous evolution of HPAIVs. Moreover, we discuss their transmission and prevention strategies to provide valuable references for future prevention and control measures against AIVs.

Molecular Markers and Mechanisms of Influenza A Virus Cross-Species Transmission and New Host Adaptation

Influenza A viruses continue to be a serious health risk to people and result in a large-scale socio-economic loss. Avian influenza viruses typically do not replicate efficiently in mammals, but through the accumulation of mutations or genetic reassortment, they can overcome interspecies barriers, adapt to new hosts, and spread among them. Zoonotic influenza A viruses sporadically infect humans and exhibit limited human-to-human transmission. However, further adaptation of these viruses to humans may result in airborne transmissible viruses with pandemic potential. Therefore, we are beginning to understand genetic changes and mechanisms that may influence interspecific adaptation, cross-species transmission, and the pandemic potential of influenza A viruses. We also discuss the genetic and phenotypic traits associated with the airborne transmission of influenza A viruses in order to provide theoretical guidance for the surveillance of new strains with pandemic potential and the prevention of pandemics.

An immune-enhanced multivalent DNA nanovaccine to prevent H7 and H9 avian influenza virus in mice

H7 avian influenza virus has caused multiple human infections and poses a severe public health threat. In response to the highly variable nature of AIVs, a novel, easily regenerated DNA vaccine has great potential in treating or preventing avian influenza pandemics. Nevertheless, DNA vaccines have many disadvantages, such as weak immunogenicity and poor in vivo delivery. To further characterize and solve these issues and develop a novel H7 AIV DNA vaccine with enhanced stability and immunogenicity, we constructed nine AIV DNA plasmids, and the immunogenicity screened showed that mice immunized with pβH7N2SH9 elicited stronger hemagglutination-inhibiting (HI) antibodies than other eight plasmid DNAs. Then, to address the susceptibility to degradation and low transfection rate of DNA vaccine in vivo, we developed pβH7N2SH9/DGL NPs by encapsulating the pβH7N2SH9 within the dendrigraft poly-l-lysines nanoparticles. As expected, these NPs exhibited excellent physical and chemical properties, were capable of promote lymphocyte proliferation, and induce stronger humoral and cellular responses than the naked pβH7N2SH9, including higher levels of HI antibodies than naked pβH7N2SH9, as well as the production of cytokines, namely, IL-2, IFN-α. Taken together, our results suggest that the construction of an immune-enhanced H7-AIV DNA nanovaccine may be a promising strategy against most influenza viruses.

Emerging Infectious Diseases Are Virulent Viruses-Are We Prepared? An Overview

The recent pandemic caused by SARS-CoV-2 affected the global population, resulting in a significant loss of lives and global economic deterioration. COVID-19 highlighted the importance of public awareness and science-based decision making, and exposed global vulnerabilities in preparedness and response systems. Emerging and re-emerging viral outbreaks are becoming more frequent due to increased international travel and global warming. These viral outbreaks impose serious public health threats and have transformed national strategies for pandemic preparedness with global economic consequences. At the molecular level, viral mutations and variations are constantly thwarting vaccine efficacy, as well as diagnostic, therapeutic, and prevention strategies. Here, we discuss viral infectious diseases that were epidemic and pandemic, currently available treatments, and surveillance measures, along with their limitations.

What Have We Learned by Resurrecting the 1918 Influenza Virus?

The 1918 Spanish influenza pandemic was one of the deadliest infectious disease events in recorded history, resulting in approximately 50-100 million deaths worldwide. The origins of the 1918 virus and the molecular basis for its exceptional virulence remained a mystery for much of the 20th century because the pandemic predated virologic techniques to isolate, passage, and store influenza viruses. In the late 1990s, overlapping fragments of influenza viral RNA preserved in the tissues of several 1918 victims were amplified and sequenced. The use of influenza reverse genetics then permitted scientists to reconstruct the 1918 virus entirely from cloned complementary DNA, leading to new insights into the origin of the virus and its pathogenicity. Here, we discuss some of the advances made by resurrection of the 1918 virus, including the rise of innovative molecular research, which is a topic in the dual use debate.

TRIM21 restricts influenza A virus replication by ubiquitination-dependent degradation of M1

Tripartite motif-containing protein 21 (TRIM21), an E3 ubiquitin ligase, plays a critical role in the host antiviral response. However, the mechanism and antiviral spectrum of TRIM21 in influenza A virus (IAV) remain unclear. Here, we report that TRIM21 inhibits the replication of various IAV subtypes by targeting matrix protein 1 (M1) from H3/H5/H9, but not H1 and H7 M1. Mechanistically, TRIM21 binds to the residue R95 of M1 and facilitates K48 ubiquitination of M1 K242 for proteasome-dependent degradation, leading to the inhibition of H3, H5, and H9 IAV replication. Interestingly, the recombinant viruses with M1 R95K or K242R mutations were resistance to TRIM21 and exhibited more robust replication and severe pathogenicity. Moreover, the amino acid sequence M1 proteins, mainly from avian influenza such as H5N1, H7N9, H9N2, ranging from 1918 to 2022, reveals a gradual dominant accumulation of the TRIM21-driven R95K mutation when the virus jumps into mammals. Thus, TRIM21 in mammals' functions as a host restriction factor and drives a host adaptive mutation of influenza A virus.

Sequential Transmission of Influenza Viruses in Ferrets Does Not Enhance Infectivity and Does Not Predict Transmissibility in Humans

Airborne transmission in ferrets is a key component of pandemic risk assessment. However, some emerging avian influenza viruses transmit between ferrets but do not spread in humans. Therefore, we evaluated sequential rounds of airborne transmission as an approach to enhance the predictive accuracy of the ferret model. We reasoned that infection of ferrets via the respiratory route and onward transmission would more closely model transmission in humans. We hypothesized that pandemic and seasonal viruses would transmit efficiently over two rounds of transmission, while emerging avian viruses would fail to transmit in a second round. The 2009 pandemic H1N1 (pdm09) and seasonal H3N2 viruses were compared to avian-origin H7N9 and H3N8 viruses. Depending on the virus strain, transmission efficiency varied from 50 to 100% during the first round of transmission; the efficiency for each virus did not change during the second round, and viral replication kinetics in both rounds of transmission were similar. Both the H1N1pdm09 and H7N9 viruses acquired specific mutations during sequential transmission, while the H3N2 and H3N8 viruses did not; however, a global analysis of host-adaptive mutations revealed that minimal changes were associated with transmission of H1N1 and H3N2 viruses, while a greater number of changes occurred in the avian H3N8 and H7N9 viruses. Thus, influenza viruses that transmit in ferrets maintain their transmission efficiency through serial rounds of transmission. This answers the question of whether ferrets can propagate viruses through more than one round of airborne transmission and emphasizes that transmission in ferrets is necessary but not sufficient to infer transmissibility in humans. IMPORTANCE Airborne transmission in ferrets is used to gauge the pandemic potential of emerging influenza viruses; however, some emerging influenza viruses that transmit between ferrets do not spread between humans. Therefore, we evaluated sequential rounds of airborne transmission in ferrets as a strategy to enhance the predictive accuracy of the ferret model. Human influenza viruses transmitted efficiently (>83%) over two rounds of airborne transmission, demonstrating that, like humans, ferrets infected by the respiratory route can propagate the infection onward through the air. However, emerging avian influenza viruses with associated host-adaptive mutations also transmitted through sequential transmission. Thus, airborne transmission in ferrets is necessary but not sufficient to infer transmissibility in humans, and sequential transmission did not enhance pandemic risk assessment.

The future of zoonotic risk prediction

In the light of the urgency raised by the COVID-19 pandemic, global investment in wildlife virology is likely to increase, and new surveillance programmes will identify hundreds of novel viruses that might someday pose a threat to humans. To support the extensive task of laboratory characterization, scientists may increasingly rely on data-driven rubrics or machine learning models that learn from known zoonoses to identify which animal pathogens could someday pose a threat to global health. We synthesize the findings of an interdisciplinary workshop on zoonotic risk technologies to answer the following questions. What are the prerequisites, in terms of open data, equity and interdisciplinary collaboration, to the development and application of those tools? What effect could the technology have on global health? Who would control that technology, who would have access to it and who would benefit from it? Would it improve pandemic prevention? Could it create new challenges? This article is part of the theme issue 'Infectious disease macroecology: parasite diversity and dynamics across the globe'.

A(H1N1)pdm09 influenza viruses replicating in ferret upper or lower respiratory tract differed in onward transmission potential by air

BACKGROUND

A(H1N1)pdm09 influenza viruses replicate efficiently in respiratory epithelia and are transmitted via respiratory droplets and aerosols expelled by infected hosts. The relative onward transmission potential of influenza viruses replicating in the upper and lower respiratory epithelial cells has not been fully defined.

METHODS

Wild-type and barcoded A(H1N1)pdm09 viruses that differed by 2 synonymous mutations per gene segment were inoculated into ferrets via intra-nasal and intra-tracheal routes. Naïve recipients were exposed to the exhaled breath of inoculated donors for 8 hours on day 2 post-inoculation. Onward transmission potential of wild-type and barcoded genotypes were monitored by next generation sequencing.

RESULTS

Transmissible airborne particles were respired from the upper but not the lower respiratory epithelial cells of donor ferrets. There was limited mixing of viral populations replicating in the upper and lower respiratory tissues.

CONCLUSIONS

The ferret upper respiratory epithelium was mapped as the anatomic site that generated influenza virus-laden particles mediating onward transmission by air. Our results suggest that vaccines and antivirals should aim to reduce viral loads in the upper respiratory tract for prevention of influenza transmission.

Hemagglutinin Stability and Its Impact on Influenza A Virus Infectivity, Pathogenicity, and Transmissibility in Avians, Mice, Swine, Seals, Ferrets, and Humans

Genetically diverse influenza A viruses (IAVs) circulate in wild aquatic birds. From this reservoir, IAVs sporadically cause outbreaks, epidemics, and pandemics in wild and domestic avians, wild land and sea mammals, horses, canines, felines, swine, humans, and other species. One molecular trait shown to modulate IAV host range is the stability of the hemagglutinin (HA) surface glycoprotein. The HA protein is the major antigen and during virus entry, this trimeric envelope glycoprotein binds sialic acid-containing receptors before being triggered by endosomal low pH to undergo irreversible structural changes that cause membrane fusion. The HA proteins from different IAV isolates can vary in the pH at which HA protein structural changes are triggered, the protein causes membrane fusion, or outside the cell the virion becomes inactivated. HA activation pH values generally range from pH 4.8 to 6.2. Human-adapted HA proteins tend to have relatively stable HA proteins activated at pH 5.5 or below. Here, studies are reviewed that report HA stability values and investigate the biological impact of variations in HA stability on replication, pathogenicity, and transmissibility in experimental animal models. Overall, a stabilized HA protein appears to be necessary for human pandemic potential and should be considered when assessing human pandemic risk.

Plasmid-Based Reverse Genetics of Influenza A Virus

Reverse genetics is the process of generating an RNA virus from a cDNA copy. Reverse genetics systems have truly transformed our ability to manipulate and study negative-strand RNA viruses. Plasmid-based reverse genetics approaches for influenza viruses provide a better understanding of virulence, transmission, mechanisms of antiviral resistance, and the development of alternative vaccines and vaccination strategies. Studying the molecular changes that allow influenza A viruses (IAVs) to transmit among animal species is important to better understand their animal health and public health risks. In this chapter, the cloning of cDNA copies of IAV's RNA segments into a reverse genetics plasmid vector, the experimental procedures for studying viral polymerase activity, and the successful generation of recombinant IAVs are described.

Mammalian pathogenicity and transmissibility of low pathogenic avian influenza H7N1 and H7N3 viruses isolated from North America in 2018

Low pathogenic avian influenza (LPAI) H7 subtype viruses are infrequently, but persistently, associated with outbreaks in poultry in North America. These LPAI outbreaks provide opportunities for the virus to develop enhanced virulence and transmissibility in mammals and have previously resulted in both occasional acquisition of a highly pathogenic avian influenza (HPAI) phenotype in birds and sporadic cases of human infection. Two notable LPAI H7 subtype viruses caused outbreaks in 2018 in North America: LPAI H7N1 virus in chickens and turkeys, representing the first confirmed H7N1 infection in poultry farms in the United States, and LPAI H7N3 virus in turkeys, a virus subtype often associated with LPAI-to-HPAI phenotypes. Here, we investigated the replication capacity of representative viruses from these outbreaks in human respiratory tract cells and mammalian pathogenicity and transmissibility in the mouse and ferret models. We found that the LPAI H7 viruses replicated to high titre in human cells, reaching mean peak titres generally comparable to HPAI H7 viruses. Replication was efficient in both mammalian species, causing mild infection, with virus primarily limited to respiratory tract tissues. The H7 viruses demonstrated a capacity to transmit to naïve ferrets in a direct contact setting. These data support the need to perform routine risk assessments of LPAI H7 subtype viruses, even in the absence of confirmed human infection.

Avian-to-Human Receptor-Binding Adaptation of Avian H7N9 Influenza Virus Hemagglutinin

Since 2013, H7N9 avian influenza viruses (AIVs) have caused more than 1,600 human infections, posing a threat to public health. An emerging concern is whether H7N9 AIVs will cause pandemics among humans. Molecular analysis of hemagglutinin (HA), which is a critical determinant of interspecies transmission, shows that the current H7N9 AIVs are still dual-receptor tropic, indicating limited human-to-human transmission potency. Mutagenesis and structural studies reveal that a G186V substitution is sufficient for H7N9 AIVs to acquire human receptor-binding capacity, and a Q226L substitution would favor binding to both avian and human receptors only when paired with A138/V186/P221 hydrophobic residues. These data suggest a different evolutionary route of H7N9 viruses compared to other AIV-subtype HAs.

Might SARS-CoV-2 Have Arisen via Serial Passage through an Animal Host or Cell Culture?: A potential explanation for much of the novel coronavirus' distinctive genome

Despite claims from prominent scientists that SARS-CoV-2 indubitably emerged naturally, the etiology of this novel coronavirus remains a pressing and open question: Without knowing the true nature of a disease, it is impossible for clinicians to appropriately shape their care, for policy-makers to correctly gauge the nature and extent of the threat, and for the public to appropriately modify their behavior. Unless the intermediate host necessary for completing a natural zoonotic jump is identified, the dual-use gain-of-function research practice of viral serial passage should be considered a viable route by which the novel coronavirus arose. The practice of serial passage mimics a natural zoonotic jump, and offers explanations for SARS-CoV-2's distinctive spike-protein region and its unexpectedly high affinity for angiotensin converting enzyme (ACE2), as well as the notable polybasic furin cleavage site within it. Additional molecular clues raise further questions, all of which warrant full investigation into the novel coronavirus's origins and a re-examination of the risks and rewards of dual-use gain-of-function research.

The honeybee (Apis mellifera) developmental state shapes the genetic composition of the deformed wing virus-A quasispecies during serial transmission

The main biological threat to the western honeybee (Apis mellifera) is the parasitic mite Varroa destructor, largely because it vectors lethal epidemics of honeybee viruses that, in the absence of this mite, are relatively innocuous. The severe pathology is a direct consequence of excessive virus titres caused by this novel transmission route. However, little is known about how the virus adapts genetically during transmission and whether this influences the pathology. Here, we show that upon injection into honeybee pupae, the deformed wing virus type-A (DWV-A) quasispecies undergoes a rapid, extensive expansion of its sequence space, followed by strong negative selection towards a uniform, common shape by the time the pupae have completed their development, with no difference between symptomatic and asymptomatic adults in either DWV titre or genetic composition. This suggests that the physiological and molecular environment during pupal development has a strong, conservative influence on shaping the DWV-A quasispecies in emerging adults. There was furthermore no evidence of any progressive adaptation of the DWV-A quasispecies to serial intra-abdominal injection, simulating mite transmission, despite the generation of ample variation immediately following each transmission, suggesting that the virus either had already adapted to transmission by injection, or was unaffected by it.

Dual-Use and Infectious Disease Research

Despite rapid advance in the prevention, diagnosis, and treatment, infectious diseases remain a central challenge for global health policy. In the twenty-first century, the life sciences—including microbiology, virology, and immunology—have been marshalled as key tools in the fight against infectious disease, and the promotion of global health. Rapid advance in these fields, however, has given rise to the “dual-use dilemma,” when one and the same piece of scientific research or technology has the capacity to help or harm humanity. While not unique to fields that address infectious disease, contemporary cases of dual-use research are largely identified in the context of the life sciences. In this chapter I outline the debate about dual-use research in the life sciences, in particular the ethics of dual-use research. After a historical overview of the dual-use dilemma in the twenty-first century, I examine ethical issues in attempting to trade off the risks and benefits of dual-use research. I address how we select alternative, less risky experiments; translational issues arising for dual-use research; and political commitments to realise the benefits and mitigate the risks arising from such research. I then discuss the governance of dual-use research, before concluding with a brief discussion on priority setting in infectious disease research as a path forward for policymakers.

Adaptive amino acid substitutions enable transmission of an H9N2 avian influenza virus in guinea pigs

H9N2 is the most prevalent low pathogenic avian influenza virus (LPAIV) in domestic poultry in the world. Two distinct H9N2 poultry lineages, G1-like (A/quail/Hong Kong/G1/97) and Y280-like (A/Duck/Hong Kong/Y280/1997) viruses, are usually associated with binding affinity for both α 2,3 and α 2,6 sialic acid receptors (avian and human receptors), raising concern whether these viruses possess pandemic potential. To explore the impact of mouse adaptation on the transmissibility of a Y280-like virus A/Chicken/Hubei/214/2017(H9N2) (abbreviated as WT), we performed serial lung-to-lung passages of the WT virus in mice. The mouse-adapted variant (MA) exhibited enhanced pathogenicity and advantaged transmissibility after passaging in mice. Sequence analysis of the complete genomes of the MA virus revealed a total of 16 amino acid substitutions. These mutations distributed across 7 segments including PB2, PB1, PA, NP, HA, NA and NS1 genes. Furthermore, we generated a panel of recombinant or mutant H9N2 viruses using reverse genetics technology and confirmed that the PB2 gene governing the increased pathogenicity and transmissibility. The combinations of 340 K and 588 V in PB2 were important in determining the altered features. Our findings elucidate the specific mutations in PB2 contribute to the phenotype differences and emphasize the importance of monitoring the identified amino acid substitutions due to their potential threat to human health.

Influenza Viruses in Mice: Deep Sequencing Analysis of Serial Passage and Effects of Sialic Acid Structural Variation

Influenza A viruses have regularly jumped to new host species to cause epidemics or pandemics, an evolutionary process that involves variation in the viral traits necessary to overcome host barriers and facilitate transmission. Mice are not a natural host for influenza virus but are frequently used as models in studies of pathogenesis, often after multiple passages to achieve higher viral titers that result in clinical disease such as weight loss or death. Here, we examine the processes of influenza A virus infection and evolution in mice by comparing single nucleotide variations of a human H1N1 pandemic virus, a seasonal H3N2 virus, and an H3N2 canine influenza virus during experimental passage. We also compared replication and sequence variation in wild-type mice expressing N-glycolylneuraminic acid (Neu5Gc) with those seen in mice expressing only N-acetylneuraminic acid (Neu5Ac). Viruses derived from plasmids were propagated in MDCK cells and then passaged in mice up to four times. Full-genome deep sequencing of the plasmids, cultured viruses, and viruses from mice at various passages revealed only small numbers of mutational changes. The H3N2 canine influenza virus showed increases in frequency of sporadic mutations in the PB2, PA, and NA segments. The H1N1 pandemic virus grew well in mice, and while it exhibited the maintenance of some minority mutations, there was no clear evidence for adaptive evolution. The H3N2 seasonal virus did not establish in the mice. Finally, there were no clear sequence differences associated with the presence or absence of Neu5Gc.IMPORTANCE Mice are commonly used as a model to study the growth and virulence of influenza A viruses in mammals but are not a natural host and have distinct sialic acid receptor profiles compared to humans. Using experimental infections with different subtypes of influenza A virus derived from different hosts, we found that evolution of influenza A virus in mice did not necessarily proceed through the linear accumulation of host-adaptive mutations, that there was variation in the patterns of mutations detected in each repetition, and that the mutation dynamics depended on the virus examined. In addition, variation in the viral receptor, sialic acid, did not affect influenza virus evolution in this model. Overall, our results show that while mice provide a useful animal model for influenza virus pathology, host passage evolution will vary depending on the specific virus tested.

A Single Amino Acid Substitution at Residue 218 of Hemagglutinin Improves the Growth of Influenza A(H7N9) Candidate Vaccine Viruses

The circulating avian influenza A(H7N9) has caused recurrent epidemic waves with high mortality in China since 2013, in which the alarming fifth wave crossing 2016 and 2017 was highlighted by a large number of human infections and the emergence of highly pathogenic avian influenza (HPAI) A(H7N9) strains in human cases. We generated low-pathogenic reassortant CVVs derived from the emerging A(H7N9) with improved virus replication and protein yield in both MDCK cells and eggs by introducing a single substitution, G218E, into HA, which was associated with reducing HA receptor binding and subsequently balancing HA-NA functions. The in vitro and in vivo experiments demonstrated comparable antigenicity of the G218E CVVs with that of their wild-type (WT) counterparts, and both the WT and the G218E CVVs fully protected ferrets from parental HPAI virus challenge. With high yield traits and the anticipated antigenicity, the G218E CVVs should benefit preparedness against the threat of an A(H7N9) influenza pandemic.

The potential avian influenza pandemic remains a threat to public health, as the avian-origin influenza A(H7N9) virus has caused more than 1,560 laboratory-confirmed human infections since 2013, with nearly 40% mortality. Development of low-pathogenic candidate vaccine viruses (CVVs) for vaccine production is essential for pandemic preparedness. However, the suboptimal growth of CVVs in mammalian cells and chicken eggs is often a challenge. By introducing a single adaptive substitution, G218E, into the hemagglutinin (HA), we generated reassortant A(H7N9)-G218E CVVs that were characterized by significantly enhanced growth in both cells and eggs. These G218E CVVs retained the original antigenicity, as determined by a hemagglutination inhibition assay, and effectively protected ferrets from lethal challenge with the highly pathogenic parental virus. We found that the suboptimal replication of the parental H7 CVVs was associated with impeded progeny virus release as a result of strong HA receptor binding relative to weak neuraminidase (NA) cleavage of receptors. In contrast, the G218E-mediated growth improvement was attributed to relatively balanced HA and NA functions, resulted from reduced HA binding to both human- and avian-type receptors, and thus facilitated NA-mediated virus release. Our findings revealed that a single amino acid mutation at residue 218 of the HA improved the growth of A(H7N9) influenza virus by balancing HA and NA functions, shedding light on an alternative approach for optimizing certain influenza CVVs.

IMPORTANCE The circulating avian influenza A(H7N9) has caused recurrent epidemic waves with high mortality in China since 2013, in which the alarming fifth wave crossing 2016 and 2017 was highlighted by a large number of human infections and the emergence of highly pathogenic avian influenza (HPAI) A(H7N9) strains in human cases. We generated low-pathogenic reassortant CVVs derived from the emerging A(H7N9) with improved virus replication and protein yield in both MDCK cells and eggs by introducing a single substitution, G218E, into HA, which was associated with reducing HA receptor binding and subsequently balancing HA-NA functions. The in vitro and in vivo experiments demonstrated comparable antigenicity of the G218E CVVs with that of their wild-type (WT) counterparts, and both the WT and the G218E CVVs fully protected ferrets from parental HPAI virus challenge. With high yield traits and the anticipated antigenicity, the G218E CVVs should benefit preparedness against the threat of an A(H7N9) influenza pandemic.

Design and Validation with Influenza A Virus of an Aerosol Transmission Chamber for Ferrets

Background: The importance of aerosols in the spread of viruses like influenza is still a subject of debate. Indeed, most viruses can also be transmitted through direct contact and droplets. Therefore, the importance of the airborne route in a clinical context is difficult to determine. The aim of this study was to design a chamber system to study the airborne transmission of viruses between ferrets. Methods: A system composed of three chambers connected in series, each one housing one ferret and preventing direct contact, was designed. The chambers were designed to house the ferrets for several days and to study the transmission of viruses from an infected (index) ferret to two naïve ferrets via aerosols and droplets or aerosols only. A particle separator was designed that can be used to modulate the size of the particles traveling between the chambers. The chamber system was validated using standard dust as well as with ferrets infected with influenza A virus. Conclusions: The 50% efficiency cut-off of the separator could be modulated between a 5-µm and an 8-µm aerodynamic diameter. In the described setup, influenza A virus was transmitted through the aerosol route in two out of three experiments, and through aerosols and droplets in all three experiments.

Seasonal Genetic Drift of Human Influenza A Virus Quasispecies Revealed by Deep Sequencing

After a pandemic wave in 2009 following their introduction in the human population, the H1N1pdm09 viruses replaced the previously circulating, pre-pandemic H1N1 virus and, along with H3N2 viruses, are now responsible for the seasonal influenza type A epidemics. So far, the evolutionary potential of influenza viruses has been mainly documented by consensus sequencing data. However, like other RNA viruses, influenza A viruses exist as a population of diverse, albeit related, viruses, or quasispecies. Interest in this quasispecies nature has increased with the development of next generation sequencing (NGS) technologies that allow a more in-depth study of the genetic variability. NGS deep sequencing methodologies were applied to determine the whole genome genetic heterogeneity of the three categories of influenza A viruses that circulated in humans between 2007 and 2012 in France, directly from clinical respiratory specimens. Mutation frequencies and single nucleotide polymorphisms were used for comparisons to address the level of natural intrinsic heterogeneity of influenza A viruses. Clear differences in single nucleotide polymorphism profiles between seasons for a given subtype also revealed the constant genetic drift that human influenza A virus quasispecies undergo.

A novel framework for inferring parameters of transmission from viral sequence data

Transmission between hosts is a critical part of the viral lifecycle. Recent studies of viral transmission have used genome sequence data to evaluate the number of particles transmitted between hosts, and the role of selection as it operates during the transmission process. However, the interpretation of sequence data describing transmission events is a challenging task. We here present a novel and comprehensive framework for using short-read sequence data to understand viral transmission events, designed for influenza virus, but adaptable to other viral species. Our approach solves multiple shortcomings of previous methods for this purpose; for example, we consider transmission as an event involving whole viruses, rather than sets of independent alleles. We demonstrate how selection during transmission and noisy sequence data may each affect naive inferences of the population bottleneck, accounting for these in our framework so as to achieve a correct inference. We identify circumstances in which selection for increased viral transmission may or may not be identified from data. Applying our method to experimental data in which transmission occurs in the presence of strong selection, we show that our framework grants a more quantitative insight into transmission events than previous approaches, inferring the bottleneck in a manner that accounts for selection, both for within-host virulence, and for inherent viral transmissibility. Our work provides new opportunities for studying transmission processes in influenza, and by extension, in other infectious diseases.

Zoonotic Potential of Influenza A Viruses: A Comprehensive Overview

Influenza A viruses (IAVs) possess a great zoonotic potential as they are able to infect different avian and mammalian animal hosts, from which they can be transmitted to humans. This is based on the ability of IAV to gradually change their genome by mutation or even reassemble their genome segments during co-infection of the host cell with different IAV strains, resulting in a high genetic diversity. Variants of circulating or newly emerging IAVs continue to trigger global health threats annually for both humans and animals. Here, we provide an introduction on IAVs, highlighting the mechanisms of viral evolution, the host spectrum, and the animal/human interface. Pathogenicity determinants of IAVs in mammals, with special emphasis on newly emerging IAVs with pandemic potential, are discussed. Finally, an overview is provided on various approaches for the prevention of human IAV infections.

The Silver Lining in Gain-of-Function Experiments with Pathogens of Pandemic Potential

Half a decade after the contentious "gain-of-function" (GOF) debate of 2012 that followed experimentation showing that highly pathogenic avian influenza virus could become mammalian transmissible, it is possible to reflect on the arguments for and against this type of research. In this essay we argue that GOF-type experiments have already produced important information not available from any other source while also providing information on pathogenesis and the requirements for optimizing strains for vaccine production. We analyze the moral arguments against GOF and find them less compelling for a variety of reasons ranging from the uncertainty of risk-benefit analysis to the reduced likelihood of accidents given the enhanced biosafety and biosecurity protocols currently in place. In our view the most important consequence of the GOF debate is that it brought renewed attention to biosafety protocols and ushered innovation in answering the relevant biological questions with greater safety. We conclude that GOF experiments should go forward provided that necessary biosafety and biosecurity conditions are in place.

The Pandemic Threat of Emerging H5 and H7 Avian Influenza Viruses

The 1918 H1N1 Spanish Influenza pandemic was the most severe pandemic in modern history. Unlike more recent pandemics, most of the 1918 H1N1 virus' genome was derived directly from an avian influenza virus. Recent avian-origin H5 A/goose/Guangdong/1/1996 (GsGd) and Asian H7N9 viruses have caused several hundred human infections with high mortality rates. While these viruses have not spread beyond infected individuals, if they evolve the ability to transmit efficiently from person-to-person, specifically via the airborne route, they will initiate a pandemic. Therefore, this review examines H5 GsGd and Asian H7N9 viruses that have caused recent zoonotic infections with a focus on viral properties that support airborne transmission. Several GsGd H5 and Asian H7N9 viruses display molecular changes that potentiate transmission and/or exhibit ability for limited transmission between ferrets. However, the hemagglutinin of these viruses is unstable; this likely represents the most significant obstacle to the emergence of a virus capable of efficient airborne transmission. Given the global disease burden of an influenza pandemic, continued surveillance and pandemic preparedness efforts against H5 GsGd and Asian lineage H7N9 viruses are warranted.

Ethical and Philosophical Considerations for Gain-of-Function Policy: The Importance of Alternate Experiments

The Department of Health and Human Services Framework for Guiding Funding Decisions about Proposed Research Involving Enhanced Potential Pandemic Pathogens (PPPs) contains a series of principles for governing the funding and conduct of gain-of-function (GOF) research resulting in the creation of PPPs. In this article, I address one of these principles, governing the replacement of GOF research with alternate experiments. I argue that the principle fails to address the way that different experiments can promote the same values as those promoted by GOF research resulting in PPPs. I then address some objections to this claim, and provide policy recommendations moving forward.

Why Do Exceptionally Dangerous Gain-of-Function Experiments in Influenza?

This chapter makes the case against performing exceptionally dangerous gain-of-function experiments that are designed to create potentially pandemic and novel strains of influenza, for example, by enhancing the airborne transmissibility in mammals of highly virulent avian influenza strains. This is a question of intense debate over the last 5 years, though the history of such experiments goes back at least to the synthesis of viable influenza A H1N1 (1918) based on material preserved from the 1918 pandemic. This chapter makes the case that experiments to create potential pandemic pathogens (PPPs) are nearly unique in that they present biosafety risks that extend well beyond the experimenter or laboratory performing them; an accidental release could, as the name suggests, lead to global spread of a virulent virus, a biosafety incident on a scale never before seen. In such cases, biosafety considerations should be uppermost in the consideration of alternative approaches to experimental objectives and design, rather than being settled after the fact, as is appropriately done for most research involving pathogens. The extensive recent discussion of the magnitude of risks from such experiments is briefly reviewed. The chapter argues that, while there are indisputably certain questions that can be answered only by gain-of-function experiments in highly pathogenic strains, these questions are narrow and unlikely to meaningfully advance public health goals such as vaccine production and pandemic prediction. Alternative approaches to experimental influenza virology and characterization of existing strains are in general completely safe, higher throughput, more generalizable, and less costly than creation of PPP in the laboratory and can thereby better inform public health. Indeed, virtually every finding of recent PPP experiments that has been cited for its public health value was predated by similar findings using safe methodologies. The chapter concludes that the unique scientific and public health value of PPP experiments is inadequate to justify the unique risks they entail and that researchers would be well-advised to turn their talents to other methodologies that will be safe and more rewarding scientifically.

Improvement of a rapid diagnostic application of monoclonal antibodies against avian influenza H7 subtype virus using Europium nanoparticles

The development of a sensitive and rapid diagnostic test is needed for early detection of avian influenza (AI) H7 subtype. In this study, novel monoclonal antibodies (mAbs) against influenza A H7N9 recombinant hemagglutinin (rHA)1 were developed and applied to a Europium nanoparticle–based rapid fluorescent immunochromatographic strip test (FICT) to improve the sensitivity of the rapid diagnostic system. Two antibodies (2F4 and 6D7) exhibited H7 subtype specificity in a dot-FICT assay by optimization of the conjugate and the pH of the lysis buffer. The subtype specificity was confirmed by an immunofluorescence assay and Western blot analysis. The limit of detection of the FICT employing novel mAbs 31 ng/mL for H7N9 rHA1 and 40 hemagglutination units/mL for H7 subtype virus. Sensitivity was improved 25-fold using Europium as confirmed by comparison of colloidal gold-based rapid diagnostic kit using the 2F4 and 6D7 mAbs.

Disparate foundations of scientists' policy positions on contentious biomedical research

What drives scientists' position taking on matters where empirical answers are unavailable or contradictory? We examined the contentious debate on whether to limit experiments involving the creation of potentially pandemic pathogens. Hundreds of scientists, including Nobel laureates, have signed petitions on the debate, providing unique insights into how scientists take a public stand on important scientific policies. Using 19,257 papers published by participants, we reconstructed their collaboration networks and research specializations. Although we found significant peer associations overall, those opposing "gain-of-function" research are more sensitive to peers than are proponents. Conversely, specializing in fields directly related to gain-of-function research (immunology, virology) predicts public support better than specializing in fields related to potential pathogenic risks (such as public health) predicts opposition. These findings suggest that different social processes might drive support compared with opposition. Supporters are embedded in a tight-knit scholarly community that is likely both more familiar with and trusting of the relevant risk mitigation practices. Opponents, on the other hand, are embedded in a looser federation of widely varying academic specializations with cognate knowledge of disease and epidemics that seems to draw more heavily on peers. Understanding how scientists' social embeddedness shapes the policy actions they take is important for helping sides interpret each other's position accurately, avoiding echo-chamber effects, and protecting the role of scientific expertise in social policy.

Plasmid-Based Reverse Genetics of Influenza A Virus

Influenza A viruses have broad host range with a recognized natural reservoir in wild aquatic birds. From this reservoir, novel strains occasionally emerge with the potential to establish stable lineages in other avian and mammalian species, including humans. Understanding the molecular changes that allow influenza A viruses to change host range is essential to better assess their animal and public health risks. Reverse genetics systems have transformed the ability to manipulate and study negative strand RNA viruses. In the particular case of influenza A viruses, plasmid-based reverse genetics approaches have allowed for a better understanding of, among others, virulence, transmission, mechanisms of antiviral resistance, and the development of alternative vaccines and vaccination strategies. In this chapter we describe the cloning of cDNA copies of viral RNA segments derived from a type A influenza virus into reverse genetics plasmid vectors and the experimental procedures for the successful generation of recombinant influenza A viruses.

Immunogenicity and Safety of an AS03-Adjuvanted H7N9 Pandemic Influenza Vaccine in a Randomized Trial in Healthy Adults

Background

Almost 700 cases of human infection with avian influenza A/H7N9 have been reported since 2013. Pandemic preparedness strategies include H7N9 vaccine development.

Methods

We evaluated an inactivated H7N9 vaccine in an observer-blind study in healthy adults aged 18–64 years. Participants (420) were randomized to receive 1 of 4 AS03-adjuvanted vaccines (low or medium dose of hemagglutinin with AS03_A or AS03_B), one nonadjuvanted vaccine, or placebo. The coprimary immunogenicity objective determined whether adjuvanted vaccines elicited an immune response against the vaccine-homologous virus, 21 days after the second vaccine dose per US and European licensure criteria in the per-protocol cohort (n = 389).

Results

All adjuvanted vaccines met regulatory acceptance criteria. In groups receiving adjuvanted formulations, seroconversion rates were ≥85.7%, seroprotection rates ≥91.1%, and geometric mean titers ≥92.9% versus 23.2%, 28.6%, and 17.2 for the nonadjuvanted vaccine. The AS03 adjuvant enhanced immune response at antigen-sparing doses. Injection site pain occurred more frequently with adjuvanted vaccines (in ≤98.3% of vaccinees) than with the nonadjuvanted vaccine (40.7%) or placebo (20.0%). None of the 20 serious adverse events reported were related to vaccination.

Conclusions

Two doses of AS03-adjuvanted H7N9 vaccine were well tolerated and induced a robust antibody response at antigen-sparing doses in healthy adults.

Clinical Trials Registration

NCT01999842.

Complexities in Ferret Influenza Virus Pathogenesis and Transmission Models

Ferrets are widely employed to study the pathogenicity, transmissibility, and tropism of influenza viruses. However, inherent variations in inoculation methods, sampling schemes, and experimental designs are often overlooked when contextualizing or aggregating data between laboratories, leading to potential confusion or misinterpretation of results. Here, we provide a comprehensive overview of parameters to consider when planning an experiment using ferrets, collecting data from the experiment, and placing results in context with previously performed studies. This review offers information that is of particular importance for researchers in the field who rely on ferret data but do not perform the experiments themselves. Furthermore, this review highlights the breadth of experimental designs and techniques currently available to study influenza viruses in this model, underscoring the wide heterogeneity of protocols currently used for ferret studies while demonstrating the wealth of information which can benefit risk assessments of emerging influenza viruses.

Pathogenesis and Phylogenetic Analyses of Two Avian Influenza H7N1 Viruses Isolated from Wild Birds

The emergence of human infections with a novel H7N9 influenza strain has raised global concerns about a potential human pandemic. To further understand the character of other influenza viruses of the H7 subtype, we selected two H7N1 avian influenza viruses (AIVs) isolated from wild birds during routine surveillance in China: A/Baer's Pochard/Hunan/414/2010 (BP/HuN/414/10) (H7N1) and A/Common Pochard/Xianghai/420/2010 (CP/XH/420/10) (H7N1). To better understand the molecular characteristics of these two isolated H7N1 viruses, we sequenced and phylogenetically analyzed their entire genomes. The results showed that the two H7N1 strains belonged to a Eurasian branch, originating from a common ancestor. Phylogenetic analysis of their hemagglutinin (HA) genes showed that BP/HuN/414/10 and CP/XH/420/10 have a more distant genetic relationship with A/Shanghai/13/2013 (H7N9), with similarities of 91.6 and 91.4%, respectively. To assess the replication and pathogenicity of these viruses in different hosts, they were inoculated in chickens, ducks and mice. Although, both CP/XH/420/10 and BP/HuN/414/10 can infect chickens, ducks and mice, they exhibited different replication capacities in these animals. The results of this study demonstrated that two low pathogenic avian influenza (LPAI) H7N1 viruses of the Eurasian branch could infect mammals and may even have the potential to infect humans. Therefore, it is important to monitor H7 viruses in both domestic and wild birds.

Glycomaterials for probing host-pathogen interactions and the immune response

The initial engagement of host cells by pathogens is often mediated by glycan structures presented on the cell surface. Various components of the glycocalyx can be targeted by pathogens for adhesion to facilitate infection. Glycans also play integral roles in the modulation of the host immune response to infection. Therefore, understanding the parameters that define glycan interactions with both pathogens and the various components of the host immune system can aid in the development of strategies to prevent, interrupt, or manage infection. Glycomaterials provide a unique and powerful tool with which to interrogate the compositional and functional complexity of the glycocalyx. The objective of this review is to highlight some key contributions from this area of research in deciphering the mechanisms of pathogenesis and the associated host response.

Evaluating influenza vaccines: progress and perspectives

Severe influenza infections are responsible for 3–5 million cases worldwide and 250,000–500,000 deaths per year. Although vaccination is the primary and most effective means of inducing protection against influenza viruses, it also presents limitations. This review outlines the promising steps that have been taken toward the development of a broadly protective influenza virus vaccine through the use of new technologies. The future challenge is to develop a broadly protective vaccine that is able to induce long-term protection against antigenically variant influenza viruses, regardless of antigenic shift and drift, and thus to protect against seasonal and pandemic influenza viruses.

A North American H7N3 Influenza Virus Supports Reassortment with 2009 Pandemic H1N1 and Induces Disease in Mice without Prior Adaptation

Reassortment between H5 or H9 subtype avian and mammalian influenza A viruses (IAV) can generate a novel virus that causes disease and transmits between mammals. Such information is currently not available for H7 subtype viruses. We evaluated the ability of a low-pathogenicity North American avian H7N3 virus (A/shorebird/Delaware/22/2006) to reassort with mammalian or avian viruses using a plasmid-based competition assay. In addition to genome segments derived from an avian H7N9 virus, the H7N3 virus reassorted efficiently with the PB2, NA, and M segments from the 2009 pandemic H1N1 (PH1N1) virus.In vitro and in vivo evaluation of the H7N3:PH1N1 (7 + 1) reassortant viruses revealed that the PB2, NA, or M segments from PH1N1 largely do not attenuate the H7N3 virus, whereas the PB1, PA, NP, or NS genome segments from PH1N1 do. Additionally, we assessed the functionality of the H7N3:PH1N1 7 + 1 reassortant viruses by measuring the inflammatory response in vivo We found that infection with wild-type H7N3 resulted in increased inflammatory cytokine production relative to that seen with the PH1N1 strain and that the increase was further exacerbated by substitution of PH1N1 PB2 but not NA or M. Finally, we assessed if any adaptations occurred in the individually substituted segments after in vivo inoculation and found no mutations, suggesting that PH1N1 PB2, NA, and M are genetically stable in the background of this H7N3 virus. Taking the data together, we demonstrate that a North American avian H7N3 IAV is genetically and functionally compatible with multiple gene segments from the 2009 pandemic influenza virus strain without prior adaptation.

IMPORTANCE

The 2009 pandemic H1N1 virus continues to circulate and reassort with other influenza viruses, creating novel viruses with increased replication and transmission potential in humans. Previous studies have found that this virus can also reassort with H5N1 and H9N2 avian influenza viruses. We now show that several genome segments of the 2009 H1N1 virus are also highly compatible with a low-pathogenicity avian H7N3 virus and that these reassortant viruses are stable and not attenuated in an animal model. These results highlight the potential for reassortment of H1N1 viruses with avian influenza virus and emphasize the need for continued surveillance of influenza viruses in areas of cocirculation between avian, human, and swine viruses.

Mammalian Pathogenesis and Transmission of H7N9 Influenza Viruses from Three Waves, 2013-2015

Three waves of human infection with H7N9 influenza viruses have concluded to date, but only viruses within the first wave (isolated between March and September 2013) have been extensively studied in mammalian models. While second- and third-wave viruses remain closely linked phylogenetically and antigenically, even subtle molecular changes can impart critical shifts in mammalian virulence. To determine if H7N9 viruses isolated from humans during 2013 to 2015 have maintained the phenotype first identified among 2013 isolates, we assessed the ability of first-, second-, and third-wave H7N9 viruses isolated from humans to cause disease in mice and ferrets and to transmit among ferrets. Similar to first-wave viruses, H7N9 viruses from 2013 to 2015 were highly infectious in mice, with lethality comparable to that of the well-studied A/Anhui/1/2013 virus. Second- and third-wave viruses caused moderate disease in ferrets, transmitted efficiently to cohoused, naive contact animals, and demonstrated limited transmissibility by respiratory droplets. All H7N9 viruses replicated efficiently in human bronchial epithelial cells, with subtle changes in pH fusion threshold identified between H7N9 viruses examined. Our results indicate that despite increased genetic diversity and geographical distribution since their initial detection in 2013, H7N9 viruses have maintained a pathogenic phenotype in mammals and continue to represent an immediate threat to public health.

IMPORTANCE

H7N9 influenza viruses, first isolated in 2013, continue to cause human infection and represent an ongoing public health threat. Now entering the fourth wave of human infection, H7N9 viruses continue to exhibit genetic diversity in avian hosts, necessitating continuous efforts to monitor their pandemic potential. However, viruses isolated post-2013 have not been extensively studied, limiting our understanding of potential changes in virus-host adaptation. In order to ensure that current research with first-wave H7N9 viruses still pertains to more recently isolated strains, we compared the relative virulence and transmissibility of H7N9 viruses isolated during the second and third waves, through 2015, in the mouse and ferret models. Our finding that second- and third-wave viruses generally exhibit disease in mammals comparable to that of first-wave viruses strengthens our ability to extrapolate research from the 2013 viruses to current public health efforts. These data further contribute to our understanding of molecular determinants of pathogenicity, transmissibility, and tropism.

Influenza: from zoonosis to pandemic

Probable epidemic influenza outbreaks have been described as early as the 5th century BC, as part of the Cough of Perinthus associated with the winter solstice, in Hippocrates' Corpus Hippocraticum “Of the Epidemics” [1]. The word “influenza” was first introduced in the 16th century, defining the illness of the cold season that in the early 1930s was shown to be caused by “filterable agents”, since then called influenza viruses. Three types or genera of influenza viruses have been recognised (influenza A, B and C viruses), while a fourth genus has been recently proposed [2]. Today, we distinguish three manifestations of influenza in humans, which may impose from mild to severe morbidity and mortality burdens: zoonotic, pandemic and seasonal influenza, all caused by an infection with an influenza virus. Although our arsenal of intervention strategies for influenza has advanced in the past decades and a growing public conception has arisen of influenza as a usually seasonal inconvenience, threats posed by influenza A viruses, with their different manifestations and associated complications, are continuously knocking on our door.

Global surveillance and advances in vaccine technology are essential to answer the threat of influenza pandemicshttp://ow.ly/Yt3e4

Introduction and enzootic of A/H5N1 in Egypt: Virus evolution, pathogenicity and vaccine efficacy ten years on

It is almost a decade since the highly pathogenic H5N1 avian influenza virus (A/H5N1) of clade 2.2.1 was introduced to Egypt in 2005, most likely, via wild birds; marking the longest endemic status of influenza viruses in poultry outside Asia. The endemic A/H5N1 in Egypt still compromises the poultry industry, poses serious hazards to public health and threatens to become potentially pandemic. The control strategies adopted for A/H5N1 in Egyptian poultry using diverse vaccines in commercialized poultry neither eliminated the virus nor did they decrease its evolutionary rate. Several virus clades have evolved, a few of them disappeared and others prevailed. Disparate evolutionary traits in both birds and humans were manifested by accumulation of clade-specific mutations across viral genomes driven by a variety of selection pressures. Viruses in vaccinated poultry populations displayed higher mutation rates at the immunogenic epitopes, promoting viral escape and reducing vaccine efficiency. On the other hand, viruses isolated from humans displayed changes in the receptor binding domain, which increased the viral affinity to bind to human-type glycan receptors. Moreover, viral pathogenicity exhibited several patterns in different hosts. This review aims to provide an overview of the viral evolution, pathogenicity and vaccine efficacy of A/H5N1 in Egypt during the last ten years.

Influenza A virus transmission via respiratory aerosols or droplets as it relates to pandemic potential

Many respiratory viruses of humans originate from animals. For instance, there are now eight paramyxoviruses, four coronaviruses and four orthomxoviruses that cause recurrent epidemics in humans but were once confined to other hosts. In the last decade, several members of the same virus families have jumped the species barrier from animals to humans. Fortunately, these viruses have not become established in humans, because they lacked the ability of sustained transmission between humans. However, these outbreaks highlighted the lack of understanding of what makes a virus transmissible. In part triggered by the relatively high frequency of occurrence of influenza A virus zoonoses and pandemics, the influenza research community has started to investigate the viral genetic and biological traits that drive virus transmission via aerosols or respiratory droplets between mammals. Here we summarize recent discoveries on the genetic and phenotypic traits required for airborne transmission of zoonotic influenza viruses of subtypes H5, H7 and H9 and pandemic viruses of subtypes H1, H2 and H3. Increased understanding of the determinants and mechanisms of respiratory virus transmission is not only key from a basic scientific perspective, but may also aid in assessing the risks posed by zoonotic viruses to human health, and preparedness for such risks.

Respiratory nanoparticle-based vaccines and challenges associated with animal models and translation

Vaccine development has had a huge impact on human health. However, there is a significant need to develop efficacious vaccines for several existing as well as emerging respiratory infectious diseases. Several challenges need to be overcome to develop efficacious vaccines with translational potential. This review focuses on two aspects to overcome some barriers - 1) the development of nanoparticle-based vaccines, and 2) the choice of suitable animal models for respiratory infectious diseases that will allow for translation. Nanoparticle-based vaccines, including subunit vaccines involving synthetic and/or natural polymeric adjuvants and carriers, as well as those based on virus-like particles offer several key advantages to help overcome the barriers to effective vaccine development. These include the ability to deliver combinations of antigens, target the vaccine formulation to specific immune cells, enable cross-protection against divergent strains, act as adjuvants or immunomodulators, allow for sustained release of antigen, enable single dose delivery, and potentially obviate the cold chain. While mouse models have provided several important insights into the mechanisms of infectious diseases, they are often a limiting step in translation of new vaccines to the clinic. An overview of different animal models involved in vaccine research for respiratory infections, with advantages and disadvantages of each model, is discussed. Taken together, advances in nanotechnology, combined with the right animal models for evaluating vaccine efficacy, has the potential to revolutionize vaccine development for respiratory infections.

One health, multiple challenges: The inter-species transmission of influenza A virus

Influenza A viruses are amongst the most challenging viruses that threaten both human and animal health. Influenza A viruses are unique in many ways. Firstly, they are unique in the diversity of host species that they infect. This includes waterfowl (the original reservoir), terrestrial and aquatic poultry, swine, humans, horses, dog, cats, whales, seals and several other mammalian species. Secondly, they are unique in their capacity to evolve and adapt, following crossing the species barrier, in order to replicate and spread to other individuals within the new species. Finally, they are unique in the frequency of inter-species transmission events that occur. Indeed, the consequences of novel influenza virus strain in an immunologically naïve population can be devastating. The problems that influenza A viruses present for human and animal health are numerous. For example, influenza A viruses in humans represent a major economic and disease burden, whilst the poultry industry has suffered colossal damage due to repeated outbreaks of highly pathogenic avian influenza viruses. This review aims to provide a comprehensive overview of influenza A viruses by shedding light on interspecies virus transmission and summarising the current knowledge regarding how influenza viruses can adapt to a new host.

Adaptation of H9N2 AIV in guinea pigs enables efficient transmission by direct contact and inefficient transmission by respiratory droplets

H9N2 avian influenza viruses circulate worldwide in poultry and have sporadically infected humans, raising concern whether H9N2 viruses have pandemic potential. Here, we use a guinea pig model to examine whether serial passage results in adaptive viral changes that confer a transmissible phenotype to a wild-type H9N2 virus. After nine serial passages of an H9N2 virus through guinea pigs, productive transmission by direct contact occurred in 2/3 guinea pig pairs. The efficiency of transmission by direct contact increased following the fifteenth passage and occurred in 3/3 guinea pig pairs. In contrast, airborne transmission of the passaged virus was less efficient and occurred in 1/6 guinea pig pairs and 0/6 ferret pairs after the fifteenth passage. Three amino acid substitutions, HA1-Q227P, HA2-D46E, and NP-E434K, were sufficient for contact transmission in guinea pigs (2/3 pairs). The two HA amino acid substitutions enhanced receptor binding to α2,3-linked sialic acid receptors. Additionally, the HA2-D46E substitution increased virus thermostability whereas the NP-E434K mutation enhanced viral RNA polymerase activity in vitro. Our findings suggest that adaptive changes that enhance viral receptor binding, thermostability, and replicative capacity in mammalian cells can collectively enhance the transmissibility of H9N2 AIVs by direct contact in the guinea pig model.

Mouse adaptation of influenza B virus increases replication in the upper respiratory tract and results in droplet transmissibility in ferrets

To investigate the molecular changes that allow influenza B viruses to adapt to new mammalian hosts, influenza B/Florida/04/2006 was serially passaged in BALB/c mice until highly virulent. The viral factors underlying this transition were then investigated in mice and ferrets. Five viruses, including the wild-type virus (P0), three intermediate viruses (P5, P9, and P12), and a lethal mouse-adapted virus (P17 (MA)), harbored one to five amino acid substitutions in the hemagglutinin, M, NP, and PA segments suggesting that these mutations enhance virulence. The P17 (MA) virus replicated significantly more efficiently than the P0 virus both in vitro and in vivo (P < 0.0001), and was highly virulent (MLD50: 10(5.25)TCID50) while the P0, P5, and P9 viruses did not kill any infected mice (MLD50 > 10(6.0)TCID50). Furthermore, the P17 (MA) virus grew to greater titers in the ferret upper respiratory tract compared with the P0 and intermediate viruses, and only the P17 (MA) virus was transmissible between ferrets via both direct and aerosol contact. To our knowledge, this is the first study to demonstrate ferret-to-ferret transmission of influenza B virus and to delineate factors that may affect its transmission.

Quantifying the risk of pandemic influenza virus evolution by mutation and re-assortment

Large outbreaks of zoonotic influenza A virus (IAV) infections may presage an influenza pandemic. However, the likelihood that an airborne-transmissible variant evolves upon zoonotic infection or co-infection with zoonotic and seasonal IAVs remains poorly understood, as does the relative importance of accumulating mutations versus re-assortment in this process. Using discrete-time probabilistic models, we determined quantitative probability ranges that transmissible variants with 1-5 mutations and transmissible re-assortants evolve after a given number of zoonotic IAV infections. The systematic exploration of a large population of model parameter values was designed to account for uncertainty and variability in influenza virus infection, epidemiological and evolutionary processes. The models suggested that immunocompromised individuals are at high risk of generating IAV variants with pandemic potential by accumulation of mutations. Yet, both immunocompetent and immunocompromised individuals could generate high viral loads of single and double mutants, which may facilitate their onward transmission and the subsequent accumulation of additional 1-2 mutations in newly-infected individuals. This may result in the evolution of a full transmissible genotype along short chains of contact transmission. Although co-infection with zoonotic and seasonal IAVs was shown to be a rare event, it consistently resulted in high viral loads of re-assortants, which may facilitate their onward transmission among humans. The prevention or limitation of zoonotic IAV infection in immunocompromised and contact individuals, including health care workers, as well as vaccination against seasonal IAVs-limiting the risk of co-infection-should be considered fundamental tools to thwart the evolution of a novel pandemic IAV by accumulation of mutations and re-assortment.

A Unique Multibasic Proteolytic Cleavage Site and Three Mutations in the HA2 Domain Confer High Virulence of H7N1 Avian Influenza Virus in Chickens

In 1999, after circulation for a few months in poultry in Italy, low-pathogenic (LP) avian influenza (AI) H7N1 virus mutated into a highly pathogenic (HP) form by acquisition of a unique multibasic cleavage site (mCS), PEIPKGSRVRR*GLF (asterisk indicates the cleavage site), in the hemagglutinin (HA) and additional alterations with hitherto unknown biological function. To elucidate these virulence-determining alterations, recombinant H7N1 viruses carrying specific mutations in the HA of LPAI A/chicken/Italy/473/1999 virus (Lp) and HPAI A/chicken/Italy/445/1999 virus (Hp) were generated. Hp with a monobasic CS or carrying the HA of Lp induced only mild or no disease in chickens, thus resembling Lp. Conversely, Lp with the HA of Hp was as virulent and transmissible as Hp. While Lp with a multibasic cleavage site (Lp_CS445) was less virulent than Hp, full virulence was exhibited when HA2 was replaced by that of Hp. In HA2, three amino acid differences consistently detected between LP and HP H7N1 viruses were successively introduced into Lp_CS445. Q450L in the HA2 stem domain increased virulence and transmission but was detrimental to replication in cell culture, probably due to low-pH activation of HA. A436T and/or K536R restored viral replication in vitro and in vivo. Viruses possessing A436T and K536R were observed early in the HPAI outbreak but were later superseded by viruses carrying all three mutations. Together, besides the mCS, stepwise mutations in HA2 increased the fitness of the Italian H7N1 virus in vivo. The shift toward higher virulence in the field was most likely gradual with rapid optimization.

IMPORTANCE

In 1999, after 9 months of circulation of low-pathogenic (LP) avian influenza virus (AIV), a devastating highly pathogenic (HP) H7N1 AIV emerged in poultry, marking the largest epidemic of AIV reported in a Western country. The HPAIV possessed a unique multibasic cleavage site (mCS) complying with the minimum motif for HPAIV. The main finding in this report is the identification of three mutations in the HA2 domain that are required for replication and stability, as well as for virulence, transmission, and tropism of H7N1 in chickens. In addition to the mCS, Q450L was required for full virulence and transmissibility of the virus. Nonetheless, it was detrimental to virus replication and required A436T and/or K536R to restore replication, systemic spread, and stability. These results are important for better understanding of the evolution of highly pathogenic avian influenza viruses from low-pathogenic precursors.

Molecular pathogenesis of H5 highly pathogenic avian influenza: the role of the haemagglutinin cleavage site motif

The emergence of H5N1 highly pathogenic avian influenza has caused a heavy socio‐economic burden through culling of poultry to minimise human and livestock infection. Although human infections with H5N1 have to date been limited, concerns for the pandemic potential of this zoonotic virus have been greatly intensified following experimental evidence of aerosol transmission of H5N1 viruses in a mammalian infection model. In this review, we discuss the dominance of the haemagglutinin cleavage site motif as a pathogenicity determinant, the host‐pathogen molecular interactions driving cleavage activation, reverse genetics manipulations and identification of residues key to haemagglutinin cleavage site functionality and the mechanisms of cell and tissue damage during H5N1 infection. We specifically focus on the disease in chickens, as it is in this species that high pathogenicity frequently evolves and from which transmission to the human population occurs. With >75% of emerging infectious diseases being of zoonotic origin, it is necessary to understand pathogenesis in the primary host to explain spillover events into the human population. © 2015 The Authors. Reviews in Medical Virology published by John Wiley & Sons Ltd.

Mapping influenza transmission in the ferret model to transmission in humans

The controversy surrounding 'gain-of-function' experiments on high-consequence avian influenza viruses has highlighted the role of ferret transmission experiments in studying the transmission potential of novel influenza strains. However, the mapping between influenza transmission in ferrets and in humans is unsubstantiated. We address this gap by compiling and analyzing 240 estimates of influenza transmission in ferrets and humans. We demonstrate that estimates of ferret secondary attack rate (SAR) explain 66% of the variation in human SAR estimates at the subtype level. Further analysis shows that ferret transmission experiments have potential to identify influenza viruses of concern for epidemic spread in humans, though small sample sizes and biological uncertainties prevent definitive classification of human transmissibility. Thus, ferret transmission experiments provide valid predictions of pandemic potential of novel influenza strains, though results should continue to be corroborated by targeted virological and epidemiological research.

DOI:http://dx.doi.org/10.7554/eLife.07969.001

Every year, thousands of people develop influenza (flu). After being infected by the influenza virus, the immune systems of most people adapt to fight off the virus if it is encountered again. However, there are many different strains of influenza, and new strains constantly evolve. Therefore, although someone may have developed resistance to one previously encountered strain, they can still become ill if another strain infects them.

Different strains of the influenza virus have different abilities to spread between people and make them ill. One way that scientists assess whether a particular strain of influenza is a threat to people is by studying ferrets, which develop many of the same flu symptoms as humans. However, questions have been raised over how accurately ferret studies reflect whether a particular virus strain will spread between humans.

Controversy has also arisen over experiments in which ferrets are infected with genetically engineered strains of influenza that mimic how a strain that has evolved in birds could adapt to cause a pandemic in humans. In 2014, the United States government suggested that such research should be temporarily stopped until more is known about the risks and usefulness of these studies. Now, Buhnerkempe, Gostic et al. have compared the results of 240 ferret and human studies that aimed to assess how easily strains of influenza spread. Specifically, the studies looked at how often a healthy ferret or human became ill when exposed to an animal or human infected with a particular strain of influenza.

The results of the ferret transmission studies matched well with transmission patterns observed in human studies. Ferret studies that assessed how the influenza virus is transmitted through the air via sneezes and coughs were particularly good at predicting how the virus spreads in humans. But Buhnerkempe, Gostic et al. caution that ferret studies are not always accurate, partly because they involve small numbers of animals, which can skew the results. There also needs to be more effort to standardize the procedures and measurements used in ferret studies.

Still, the analysis suggests that overall, ferret studies are a useful tool for making an initial prediction of which influenza strains may cause a pandemic in humans, which can then be verified using other methods.

DOI:http://dx.doi.org/10.7554/eLife.07969.002