Cocirculation of avian H9N2 and contemporary "human" H3N2 influenza A viruses in pigs in southeastern China: potential for genetic reassortment?

A nonpathogenic duck-origin H9N2 influenza A virus adapts to high pathogenicity in mice

H9N2 influenza viruses continue to circulate in wild birds and poultry in Eurasian countries and have repeatedly infected mammals, including pigs and humans, posing a significant threat to public health. To understand the adaptation of H9N2 influenza viruses to mammals, we serially passaged a nonpathogenic duck-origin H9N2 influenza virus, A/duck/Jiangsu/1/2008 (DK1), in mouse lungs. Increased virulence was detectable after five sequential passages, and a highly pathogenic mouse-adapted strain (DK1-MA) with a 50% mouse lethal dose of 10(2.37) 50% egg infectious dose was obtained after 18 passages. DK1-MA grew faster and reached significantly higher titers than DK1 in mouse lungs and could sporadically spread to other organs. Moreover, DK1-MA induced a greater magnitude of pulmonary edema and higher levels of inflammatory cellular infiltration in bronchoalveolar lavage fluids than DK1 did. Genomic sequence alignment revealed eight amino acid substitutions (HA-L80F, HA-N193D, NA-A27T, PB2-F404L, PA-D3V, PA-S225R, NP-V105M, M1-A166V) in six viral proteins of DK1-MA compared with DK1 virus. Except for HA-L80F, the other seven substitutions were all located in known functional regions involved in interaction of viral proteins or interaction between the virus and host factors. Taken together, our results suggest that multiple amino acid substitutions may be involved in the adaptation of H9N2 avian influenza virus to mice, resulting in lethal infection, enhanced viral replication, severe pulmonary edema, and excessive inflammatory cellular infiltration in lungs. These observations provide helpful insights into the pathogenic potential of H9N2 avian influenza viruses that could pose threats to human health in the future.

Antigenic and molecular characterization of avian influenza A(H9N2) viruses, Bangladesh

Human infection with avian influenza A(H9N2) virus was identified in Bangladesh in 2011. Surveillance for influenza viruses in apparently healthy poultry in live-bird markets in Bangladesh during 2008-2011 showed that subtype H9N2 viruses are isolated year-round, whereas highly pathogenic subtype H5N1 viruses are co-isolated with subtype H9N2 primarily during the winter months. Phylogenetic analysis of the subtype H9N2 viruses showed that they are reassortants possessing 3 gene segments related to subtype H7N3; the remaining gene segments were from the subtype H9N2 G1 clade. We detected no reassortment with subtype H5N1 viruses. Serologic analyses of subtype H9N2 viruses from chickens revealed antigenic conservation, whereas analyses of viruses from quail showed antigenic drift. Molecular analysis showed that multiple mammalian-specific mutations have become fixed in the subtype H9N2 viruses, including changes in the hemagglutinin, matrix, and polymerase proteins. Our results indicate that these viruses could mutate to be transmissible from birds to mammals, including humans.

Characterization of the sialic acid binding activity of influenza A viruses using soluble variants of the H7 and H9 hemagglutinins

Binding of influenza viruses to target cells is mediated by the viral surface protein hemagglutinin. To determine the presence of binding sites for influenza A viruses on cells and tissues, soluble hemagglutinins of the H7 and H9 subtype were generated by connecting the hemagglutinin ectodomain to the Fc portion of human immunoglobulin G (H7Fc and H9Fc). Both chimeric proteins bound to different cells and tissues in a sialic acid-dependent manner. Pronounced differences were observed between H7Fc and H9Fc, in the binding both to different mammalian and avian cultured cells and to cryosections of the respiratory epithelium of different virus host species (turkey, chicken and pig). Binding of the soluble hemagglutinins was similar to the binding of virus particles, but showed differences in the binding pattern when compared to two sialic acid-specific plant lectins. These findings were substantiated by a comparative glycan array analysis revealing a very narrow recognition of sialoglycoconjugates by the plant lectins that does not reflect the glycan structures preferentially recognized by H7Fc and H9Fc. Thus, soluble hemagglutinins may serve as sialic acid-specific lectins and are a more reliable indicator of the presence of binding sites for influenza virus HA than the commonly used plant lectins.

Antigenic mapping of the hemagglutinin of an H9N2 avian influenza virus reveals novel critical amino acid positions in antigenic sites

H9N2 influenza virus is undergoing extensive genetic and antigenic evolution, warranting detailed antigenic mapping of its hemagglutinin (HA). Through examining antibody escape mutants of an Asian avian H9N2 virus, we identified 9 critical amino acid positions in H9 antigenic sites. Five of these positions, 164, 167, 168, 196, and 207, have not been reported previously and, thus, represent novel molecular markers for monitoring the antigenic change of H9N2 virus.

Influenza A virus reassortment

Reassortment is the process by which influenza viruses swap gene segments. This genetic exchange is possible due to the segmented nature of the viral genome and occurs when two differing influenza viruses co-infect a cell. The viral diversity generated through reassortment is vast and plays an important role in the evolution of influenza viruses. Herein we review recent insights into the contribution of reassortment to the natural history and epidemiology of influenza A viruses, gained through population scale phylogenic analyses. We describe methods currently used to study reassortment in the laboratory, and we summarize recent progress made using these experimental approaches to further our understanding of influenza virus reassortment and the contexts in which it occurs.

Swine and influenza: a challenge to one health research

The challenge of increasing swine production and a rising number of novel and known swine influenza viruses has prompted a considerable boost in research into how and why pigs have become such significant hosts for influenza viruses. The ecology of influenza A viruses is rather complicated, involving multiple host species and a segmented genome. Wild aquatic birds are the reservoir for the majority of influenza A viruses, but novel influenza viruses were recently identified in bats. Occasionally, influenza A viruses can be transmitted to mammals from avian species and this event could lead to the generation of human pandemic strains. Swine are thought to be "mixing vessels" because they are susceptible to infection with both avian and mammalian influenza viruses; and novel influenza viruses can be generated in pigs by reassortment. At present, it is difficult to predict which viruses might cause a human pandemic. Therefore, both human and veterinary research needs to give more attention to the potential cross-species transmission capacity of influenza A viruses.

Enhancement of influenza virus transmission by gene reassortment

Influenza A virus is characterized by a genome composed of eight single-stranded, negative sense RNA segments, which allow for reassortment between different strains when they co-infect the same host cell. Reassortment is an important driving force for the evolution of influenza viruses. The ability of reassortment allows influenza virus to endlessly reinvent itself and pose a constant threat to the health of humans and other animals. Of the four human influenza pandemics since the beginning of the last century, three of them were caused by reassortant viruses bearing genes of avian, human or swine influenza virus origin. In the past decade, great efforts have been made to understand the transmissibility of influenza viruses. The use of reverse genetics technology has made it substantially easier to generate reassortant viruses and evaluate the contribution of individual virus gene on virus transmissibility in animal models such as ferrets and guinea pigs. H5, H7, and H9 avian influenza viruses represent the top three subtypes that are candidates to cause the next human influenza pandemic. Many studies have been conducted to determine whether the transmission of these avian influenza viruses could be enhanced by acquisition of gene segments from human influenza viruses. Moreover, the 2009 pdmH1N1 viruses and the triple reassortant swine influenza viruses were extensively studied to identify the gene segments that contribute to their transmissibility. These studies have greatly deepened our understanding of the transmissibility of reassortant influenza viruses, which, in turn, has improved our ability to be prepared for reassortant influenza virus with enhanced transmissibility and pandemic potential.

Evolution and ecology of influenza A viruses

Wild aquatic bird populations have long been considered the natural reservoir for influenza A viruses with virus transmission from these birds seeding other avian and mammalian hosts. While most evidence still supports this dogma, recent studies in bats have suggested other reservoir species may also exist. Extensive surveillance studies coupled with an enhanced awareness in response to H5N1 and pandemic 2009 H1N1 outbreaks is also revealing a growing list of animals susceptible to infection with influenza A viruses. Although in a relatively stable host-pathogen interaction in aquatic birds, antigenic, and genetic evolution of influenza A viruses often accompanies interspecies transmission as the virus adapts to a new host. The evolutionary changes in the new hosts result from a number of processes including mutation, reassortment, and recombination. Depending on host and virus these changes can be accompanied by disease outbreaks impacting wildlife, veterinary, and public health.

Active surveillance for influenza A virus among swine, midwestern United States, 2009-2011

Veterinary diagnostic laboratories identify and characterize influenza A viruses primarily through passive surveillance. However, additional surveillance programs are needed. To meet this need, an active surveillance program was conducted at pig farms throughout the midwestern United States. From June 2009 through December 2011, nasal swab samples were collected monthly from among 540 groups of growing pigs and tested for influenza A virus by real-time reverse transcription PCR. Of 16,170 samples, 746 were positive for influenza A virus; of these, 18.0% were subtype H1N1, 16.0% H1N2, 7.6% H3N2, and 14.5% (H1N1)pdm09. An influenza (H3N2) and (H1N1)pdm09 virus were identified simultaneously in 8 groups. This active influenza A virus surveillance program provided quality data and increased the understanding of the current situation of circulating viruses in the midwestern US pig population.

Alternative reassortment events leading to transmissible H9N1 influenza viruses in the ferret model

Influenza A H9N2 viruses are common poultry pathogens that occasionally infect swine and humans. It has been shown previously with H9N2 viruses that reassortment can generate novel viruses with increased transmissibility. Here, we demonstrate the modeling power of a novel transfection-based inoculation system to select reassortant viruses under in vivo selective pressure. Plasmids containing the genes from an H9N2 virus and a pandemic H1N1 (pH1N1) virus were transfected into HEK 293T cells to potentially generate the full panel of possible H9 reassortants. These cells were then used to inoculate ferrets, and the population dynamics were studied. Two respiratory-droplet-transmissible H9N1 viruses were selected by this method, indicating a selective pressure in ferrets for the novel combination of surface genes. These results show that a transfection-based inoculation system is a fast and efficient method to model reassortment and highlight the risk of reassortment between H9N2 and pH1N1 viruses.

Avian influenza A (H5N1) virus antibodies in pigs and residents of swine farms, southern China

BACKGROUND

Since 1997, the H5 avian influenza viruses (AIVs) circulating in China have become an international concern. Clade 2.3.2 of H5N1 AIVs is genetically distinct from the viruses isolated before 2007 and antigenically different from the vaccine strains widely used in China. Swine farms in rural China are thought to play an important role in AIVs ecology.

OBJECTIVES

A seroepidemiological study was undertaken among swine farm residents and pigs to understand the prevalence of antibodies against H5N1 AIVs in southern China.

STUDY DESIGN

During the period March 24, 2008 to December 25, 2012,serum samples were collected from 1606 swine farm residents on 40 swine farms in southern China. A total of 1980 pigs' serum samples were collected in the same swine farms where swine workers' serum samples were collected from March 2009 to March 2013. For a control group, 104 serum samples were collected from healthy city residents in Nanchang. All the serum samples were collected to perform hemagglutination inhibition (HI) and (neutralization) NT assays to investigate the prevalence of H5N1 AIV infections in southern China.

RESULTS

Sixteen human samples were positive by HI assay and 10 of these were also positive by NT assay against H5N1. No serum samples from human control and pigs were HI positive for H5N1 AIV.

DISCUSSION

Our results demonstrate minimal transmission H5N1 AIV from birds to pigs in the swine farms studied and the risk of poultry-to-human and poultry-to-pig transmission for at least clades 2.3.2 seemed very low. This study provides the first data regarding antibodies against H5N1 AIV in humans and pigs on swine farms in China. The findings of this study can serve as a baseline for additional serologic studies to assess transmission of H5N1 viruses between avian species, pigs and swine workers.

Serological surveillance of H5 and H9 avian influenza A viral infections among pigs in Southern China

Pigs are susceptible to both human and avian influenza viruses (AIV). Moreover, they are suspected of being the intermediate hosts or mixing vessels of pandemic influenza viruses. Researchers suspect that the influenza viruses are able to undergo reassortment or to adapt to various mammalian hosts while they incubate in pigs. For the present report, we conducted a serological surveillance of pigs in southern China from 2008 to 2012 to establish the prevalence of antibodies against H5N1 and H9N2 AIV. A total of one hundred pig farms from the Guangdong, Zhejiang, Fujian, and Yunnan Provinces were sampled, yielding a total of 3960 serum specimens. The haemagglutination inhibition (HI) tests revealed no evidence of H5 infection when the Clade 2.3.2 virus was used as the antigen, but a 4.6% positive rate of H9 infection was observed when using the Beijing/1/94-like virus as the antigen. The positive sera for H9 infection were further verified with neutralization tests, which confirmed a 3.7% rate of positive sera of H9 infection. In summary, the results imply that the swine populations in southern China had not been affected greatly by the H5N1 avian influenza virus. Nevertheless, these swine H9N2 influenza viruses might pose a threat to human health, and so researchers should continue to carry out swine influenza virus surveillance in China.

Diagnostics and surveillance for Swine influenza

Collective knowledge regarding the occurrence of influenza among swine is incomplete due to inconsistent surveillance of swine populations. In this chapter, we review what surveillance activities exist and some of the practical challenges encountered. Furthermore, to support robust surveillance activities, accurate laboratory assays are needed for the detection of the virus and viral nucleic acids within clinical samples, or for antiviral antibodies in serum samples. The most common influenza diagnostic assays used for swine are explained and their use as surveillance tools evaluated.

H9N2 influenza viruses from birds used in falconry

Background

H9N2 avian influenza viruses continue to spread in poultry and wild birds throughout Eurasia.

Objectives

To characterize H9N2 influenza viruses from pheasants, quail, and white‐bellied bustards (WBBs) used to train falcons in the United Arab Emirates (UAE).

Methods

Four H9N2 viruses were isolated from pheasants, quail, and WBB used for falconry in the UAE, and antigenic, molecular, phylogenetic analysis, and invivo characterization of H9N2 viruses were performed.

Results and conclusions

The pheasant and WBB isolates were antigenically and molecularly clearly related and along with the quail isolates contained multiple “avian–human” substitutions. The release of smuggled H9N2‐infected birds for falconry may contribute to the spread of these viruses to wild birds, domestic poultry, and humans.

Full Genome Sequence of a Natural Reassortant H9N2 Avian Influenza Virus Isolated from Domestic Ducks in Jiangsu Province, China

In this study, the complete genomic sequence of a novel reassortant H9N2 avian influenza virus (AIV) from domestic ducks in eastern China was reported. Phylogenetic analysis showed that seven of the eight genes were all highly homologous to the chicken-origin H9N2 viruses, whereas the PB2 gene was homologous to the human-origin H1N1 virus, which suggested that domestic ducks might play a key role in the genetic reassortment and evolution of H9N2 AIVs in eastern China.

Complete Genome Sequence of a New H9N2 Avian Influenza Virus Isolated in China

The complete genomic sequence of a new H9N2 avian influenza virus (AIV), isolated in northwestern China, was determined. Sequence and phylogenetic analyses based on the sequences of eight genomic segments revealed that the isolate is phylogenetically related to the Y280-like sublineage.

Experimental infection of non-human primates with avian influenza virus (H9N2)

Several cases of humans infected with the H9N2 avian influenza virus (AIV) have been described since 1999; however, the infectivity and pathogenicity of H9N2 in humans is not well defined. A non-human primate model in rhesus macaques was developed to study H9N2 virus infections as a means of better understanding the pathogenesis and virulence of this virus, in addition to testing antiviral drugs. Rhesus macaques inoculated with H9N2 AIV presented with biphasic fever and viral pneumonia. H9N2 was recovered from nasal washes and pharyngeal samples up to days 7-9 postinfection, followed by an increase in HI (hemagglutination inhibition) antibody titers. Tissue tropism and immunohistochemistry indicated that H9N2 AIV replicated in the upper respiratory tract (turbinate, trachea, and bronchus) and in all lobes of the lung. Our data suggest that rhesus macaques are a suitable animal model to study H9N2 influenza virus infections, particularly in the context of viral evolution and pathogenicity.

Inferring patterns of influenza transmission in swine from multiple streams of surveillance data

Swine populations are known to be an important source of new human strains of influenza A, including those responsible for global pandemics. Yet our knowledge of the epidemiology of influenza in swine is dismayingly poor, as highlighted by the emergence of the 2009 pandemic strain and the paucity of data describing its origins. Here, we analyse a unique dataset arising from surveillance of swine influenza at a Hong Kong abattoir from 1998 to 2010. We introduce a state-space model that estimates disease exposure histories by joint inference from multiple modes of surveillance, integrating both virological and serological data. We find that an observed decrease in virus isolation rates is not due to a reduction in the regional prevalence of influenza. Instead, a more likely explanation is increased infection of swine in production farms, creating greater immunity to disease early in life. Consistent with this, we find that the weekly risk of exposure on farms equals or exceeds the exposure risk during transport to slaughter. We discuss potential causes for these patterns, including competition between influenza strains and shifts in the Chinese pork industry, and suggest opportunities to improve knowledge and reduce prevalence of influenza in the region.

Chimaeric VP2 proteins from infectious bursal disease virus containing the N-terminal M2e of H9 subtype avian influenza virus induce neutralizing antibody responses to both viruses

Subunit vaccines capable of inducing antibody against both infectious bursal disease virus (IBDV) and H9 subtype avian influenza virus (AIV) were developed. The VP2 protein of IBDV was used as a cargo protein to display a 12-amino-acid immunodominant epitope derived from the N-terminal M2 extracellular domain (nM2e) of the H9 subtype AIV. Two chimaeric proteins were constructed by insertion of one copy of the nM2e into the PBC region (VP2BCnM2e(H9)) or by fusing four copies of nM2e to the carboxyl terminal (VP2-4nM2e(H9)) of VP2. Genes that encoded the VP2 chimaeras were subsequently cloned into a baculovirus vector and expressed in Spodoptera frugiperda cells. The recombinant proteins were used to vaccinate chickens at day 0 and again after 4 weeks. Blood was collected at 2-week intervals after primary and secondary vaccination to detect the antibody titre against VP2 or the nM2e via indirect enzyme-linked immunosorbent assay. Virus neutralization tests were also performed to measure anti-IBDV or anti-H9 AIV neutralizing antibodies in chick embryo fibroblasts. Oropharyngeal and cloacal swabs were collected 3, 5 and 7 days post H9 subtype AIV infection for virus isolation. Vaccination with VP2-4nM2e(H9) induced higher levels of antibody responses against IBDV or H9 subtype AIV, and provided better protection against an IBDV virulent challenge compared with vaccination with VP2BCnM2e(H9) vaccine, the wild-type VP2 subunit vaccine or the IBDV subunit commercial vaccines. Both chimaeric VP2 vaccines showed poor efficacy in inhibiting H9 virus replication post challenge. In summary, chimaeric proteins that contain the nM2e epitope were able to induce both IBDV and H9 subtype AIV-neutralizing antibody responses.

Genetic variation and phylogenetic analysis of hemagglutinin genes of H9 avian influenza viruses isolated in China during 2010-2012

Genetic variation and phylogenetic relationships of H9 avian influenza viruses (AIVs) were analyzed based on hemagglutinin (HA) gene sequences of 84 Chinese H9 reference viruses recently available in GenBank, 3 widely used vaccine strains and 29 novel isolates. The novel isolates were obtained from vaccinated poultry flocks in 11 provinces of China during 2010 to 2012. The nucleotide homologies of HA genes of these isolates ranged from 87.8-99.8%, and from 89.8-93.2% as compared with the vaccine strains. Among the 29 novel isolates and the 84 reference viruses, 69.9% of the them belonged to the lineage h9.4.2.5 and had the dominant PSRSSR↓GLF motifs in the HA cleavage sites, while 27.4% of the them belonged to the newly emerging lineage h9.4.2.6 and had the dominant PARSSR↓GLF motifs, no consecutive basic amino acids insertion, showing the characteristic feature of low-pathogenic AIV. All the lineage h9.4.2.5 viruses and 75% of the lineage h9.4.2.6 viruses had the substitution Q226L (in H3 numbering). Additional potential glycosylation site at residues 313-315 (NCS) were found merely in all the lineage h9.4.2.5 viruses. Our results demonstrated that lineage h9.4.2.5 was more dominant than other lineages as it harbored more viruses that widely distributed in China in recent years. New lineage h9.4.2.6 previously existed mainly in South China had emerged in North China. Updated vaccine and increased veterinary biosecurity on poultry farms and trade markets are needed to prevent and control avian influenza.

Connecting the study of wild influenza with the potential for pandemic disease

Continuing outbreaks of pathogenic (H5N1) and pandemic (SOIVH1N1) influenza have underscored the need to understand the origin, characteristics, and evolution of novel influenza A virus (IAV) variants that pose a threat to human health. In the last 4-5years, focus has been placed on the organization of large-scale surveillance programs to examine the phylogenetics of avian influenza virus (AIV) and host-virus relationships in domestic and wild animals. Here we review the current gaps in wild animal and environmental surveillance and the current understanding of genetic signatures in potentially pandemic strains.

Susceptibility and intra-species transmission of the H9N2 G1 prototype lineage virus in Japanese quail and turkeys

Avian influenza viruses of the H9N2 subtype have circulated in the poultry population in Asia, Far and Middle East since the mid-1990 s. One of the most widespread lineages established in poultry is the G1 lineage. This lineage has undergone further evolution and reassortment since its first detection in 1997 and G1-like H9N2 viruses still circulate. In this study we have investigated the susceptibility of quail and turkeys to the H9N2 G1-lineage prototype strain (A/quail/Hong Kong/G1/97). Contact transmission experiments were carried out in both avian species. Animals were infected oro-nasally with increasing doses of the virus (10(3)-10(6) EID 50/0.1 ml) and sentinel birds were introduced 4 days post infection (pi) in each experimental group. Quail were more susceptible than turkeys, as they were readily infected with lower challenge doses. Interestingly, infection of turkeys was associated with worse clinical condition. Transmission was detected in both species. Quail infected with a dose less than or equal to 10(4) EID50 transmitted the virus to the sentinels without showing any signs of disease. These findings reinforce the hypothesis that quail may ensure the perpetuation of H9N2 viruses in poultry, acting as a silent reservoir.

Influenza HA subtypes demonstrate divergent phenotypes for cleavage activation and pH of fusion: implications for host range and adaptation

The influenza A virus (IAV) HA protein must be activated by host cells proteases in order to prime the molecule for fusion. Consequently, the availability of activating proteases and the susceptibility of HA to protease activity represents key factors in facilitating virus infection. As such, understanding the intricacies of HA cleavage by various proteases is necessary to derive insights into the emergence of pandemic viruses. To examine these properties, we generated a panel of HAs that are representative of the 16 HA subtypes that circulate in aquatic birds, as well as HAs representative of the subtypes that have infected the human population over the last century. We examined the susceptibility of the panel of HA proteins to trypsin, as well as human airway trypsin-like protease (HAT) and transmembrane protease, serine 2 (TMPRSS2). Additionally, we examined the pH at which these HAs mediated membrane fusion, as this property is related to the stability of the HA molecule and influences the capacity of influenza viruses to remain infectious in natural environments. Our results show that cleavage efficiency can vary significantly for individual HAs, depending on the protease, and that some HA subtypes display stringent selectivity for specific proteases as activators of fusion function. Additionally, we found that the pH of fusion varies by 0.7 pH units among the subtypes, and notably, we observed that the pH of fusion for most HAs from human isolates was lower than that observed from avian isolates of the same subtype. Overall, these data provide the first broad-spectrum analysis of cleavage-activation and membrane fusion characteristics for all of the IAV HA subtypes, and also show that there are substantial differences between the subtypes that may influence transmission among hosts and establishment in new species.

IAV is associated with significant morbidity and mortality, and represents a challenging public health threat that affects social and economic welfare each year, particularly during IAV pandemics. Although we know that all human strains derive, either directly or via intermediate hosts, from avian viral sources, we know very little about the phenotypic characteristics of the 16 HA subtypes that circulate in aquatic birds and have potential to infect mammals. HA membrane fusion properties, in conjunction with the characteristics for protease activation of HA, a requirement for fusion, are critical factors involved in the ecology and transmission of IAVs, and need to be understood if we are to derive explanations for how pandemic viruses emerge in humans. We examined the cleavage-activation and membrane fusion characteristics for the 16 HA subtypes by transiently expressing HA proteins in cells. Our findings show that the cleavability of the HAs vary considerably between subtypes and depending on the protease. Additionally, analysis of the pH of fusion for each subtype showed that HA stability varied significantly among the subtypes, as well as within subtypes from viruses isolated from different species. Overall, these data have implications for host range, potential for adaptation, and persistence in natural environments.

Evidence for avian H9N2 influenza virus infections among rural villagers in Cambodia

BACKGROUND

Southeast Asia remains a critical region for the emergence of novel and/or zoonotic influenza, underscoring the importance of extensive sampling in rural areas where early transmission is most likely to occur.

METHODS

In 2008, 800 adult participants from eight sites were enrolled in a prospective population-based study of avian influenza (AI) virus transmission where highly pathogenic avian influenza (HPAI) H5N1 virus had been reported in humans and poultry from 2006 to 2008. From their enrollment sera and questionnaires, we report risk factor findings for serologic evidence of previous infection with 18 AI virus strains.

RESULTS

Serologic assays revealed no evidence of previous infection with 13 different low-pathogenic AI viruses or with HPAI avian-like A/Cambodia/R0404050/2007(H5N1). However, 21 participants had elevated antibodies against avian-like A/Hong Kong/1073/1999(H9N2), validated with a monoclonal antibody blocking ELISA assay specific for avian H9.

CONCLUSIONS

Although cross-reaction from antibodies against human influenza viruses cannot be completely excluded, the study data suggest that a number of participants were previously infected with the avian-like A/Hong Kong/1073/1999(H9N2) virus, likely due to as yet unidentified environmental exposures. Prospective data from this cohort will help us better understand the serology of zoonotic influenza infection in a rural cohort in SE Asia.

Infection dynamics of pandemic 2009 H1N1 influenza virus in a two-site swine herd

Influenza A viruses are common causes of respiratory disease in pigs and can be transmitted among multiple host species, including humans. The current lack of published information on infection dynamics of influenza viruses within swine herds hinders the ability to make informed animal health, biosecurity and surveillance programme decisions. The objectives of this serial cross-sectional study were to describe the infection dynamics of influenza virus in a two-site swine system by estimating the prevalence of influenza virus in animal subpopulations at the swine breeding herd and describing the temporal pattern of infection in a selected cohort of growing pigs weaned from the breeding herd. Nasal swab and blood samples were collected at approximately 30-day intervals from the swine breeding herd (Site 1) known to be infected with pandemic 2009 H1N1 influenza virus. Sows, gilts and neonatal pigs were sampled at each sampling event, and samples were tested for influenza virus genome using matrix gene RRT-PCR. Influenza virus was detected in neonatal pigs, but was not detected in sow or gilt populations via RRT-PCR. A virus genetically similar to that detected in the neonatal pig population at Site 1 was also detected at the wean-to-finish site (Site 2), presumably following transportation of infected weaned pigs. Longitudinal sampling of nasal swabs and oral fluids revealed that influenza virus persisted in the growing pigs at Site 2 for at least 69 days. The occurrence of influenza virus in neonatal pigs, but not breeding females, at Site 1 emphasizes the potential for virus maintenance in this dynamic subpopulation, the importance of including this subpopulation in surveillance programmes and the potential transport of influenza virus between sites via the movement of weaned pigs.

Complete genome sequence of an H9N2 avian influenza virus isolated from egret in Lake Dongting wetland

We isolated a recombinant H9N2 avian influenza virus (AIV) from fresh egret feces in the Ardeidae protection region of the Dongting Lake wetland area in China, and it was designated A/Egret/Hunan/1/2012(H9N2). This is the first report of isolating H9N2 AIV from wild birds in the Dongting Lake wetland. Its eight gene segments are generated by reassortment of gene segments of different AIV subtypes. These results are helpful for understanding the epidemiology and evolution of AIV in wild birds during migration.

Cross talk between animal and human influenza viruses

Although outbreaks of highly pathogenic avian influenza in wild and domestic birds have been posing the threat of a new influenza pandemic for the past decade, the first pandemic of the twenty-first century came from swine viruses. This fact emphasizes the complexity of influenza viral ecology and the difficulty of predicting influenza viral dynamics. Complete control of influenza viruses seems impossible. However, we must minimize the impact of animal and human influenza outbreaks by learning lessons from past experiences and recognizing the current status. Here, we review the most recent influenza virology data in the veterinary field, including aspects of zoonotic agents and recent studies that assess the pandemic potential of H5N1 highly pathogenic avian influenza viruses.

History of Swine influenza viruses in Asia

The pig is one of the main hosts of influenza A viruses and plays important roles in shaping the current influenza ecology. The occurrence of the 2009 H1N1 pandemic influenza virus demonstrated that pigs could independently facilitate the genesis of a pandemic influenza strain. Genetic analyses revealed that this virus was derived by reassortment between at least two parent swine influenza viruses (SIV), from the northern American triple reassortant H1N2 (TR) and European avian-like H1N1 (EA) lineages. The movement of live pigs between different continents and subsequent virus establishment are preconditions for such a reassortment event to occur. Asia, especially China, has the largest human and pig populations in the world, and seems to be the only region frequently importing pigs from other continents. Virological surveillance revealed that not only classical swine H1N1 (CS), and human-origin H3N2 viruses circulated, but all of the EA, TR and their reassortant variants were introduced into and co-circulated in pigs in this region. Understanding the long-term evolution and history of SIV in Asia would provide insights into the emergence of influenza viruses with epidemic potential in swine and humans.

Genetic Analysis of Avian Influenza Viruses: Cocirculation of Avian Influenza Viruses with Allele A and B Nonstructural Gene in Northern Pintail (Anas acuta) Ducks Wintering in Japan

The pandemic influenza virus strains of 1918 (H1N1), 1957 (H2N2), 1968 (H3N2), and 2009 (H1N1) have genes related to avian influenza viruses (AIVs). The nonstructural (NS) gene of AIVs plays a significant role in host-viral interaction. However, little is known about the degree of diversity of this gene in Northern pintail (Anas acuta) ducks wintering in Japan. This study describes characteristics of pintail-originated H1N1, H1N2, H1N3, H5N2, H5N3, H5N9, and H7N7 viruses. Most of the viruses were revealed to be avian strains and not related to pandemic and seasonal flu strains. Nevertheless, the NP genes of 62.5% (5/8) viruses were found closely related to a A/swine/Korea/C12/08, indicating exchange of genetic material and ongoing mammalian-linked evolution of AIVs. Besides, all the viruses, except Aomori/422/07 H1N1, contain PSIQSR∗GLF motif usually found in avian, porcine, and human H1 strains. The Aomori/422/07 H1N1 has a PSVQSR∗GLF motif identical to a North American strain. This findings linked to an important intercontinental, Asian-American biogeographical interface. Phylogenetically all the viruses were clustered in Eurasian lineage. Cocirculation of allele A and B (NS gene) viruses was evident in the study implying the existence of a wide reservoir of influenza A viruses in pintail wintering in Japan.

Matriptase, HAT, and TMPRSS2 activate the hemagglutinin of H9N2 influenza A viruses

Influenza A viruses of the subtype H9N2 circulate worldwide and have become highly prevalent in poultry in many countries. Moreover, they are occasionally transmitted to humans, raising concern about their pandemic potential. Influenza virus infectivity requires cleavage of the surface glycoprotein hemagglutinin (HA) at a distinct cleavage site by host cell proteases. H9N2 viruses vary remarkably in the amino acid sequence at the cleavage site, and many isolates from Asia and the Middle East possess the multibasic motifs R-S-S-R and R-S-R-R, but are not activated by furin. Here, we investigated proteolytic activation of the early H9N2 isolate A/turkey/Wisconsin/1/66 (H9-Wisc) and two recent Asian isolates, A/quail/Shantou/782/00 (H9-782) and A/quail/Shantou/2061/00 (H9-2061), containing mono-, di-, and tribasic HA cleavage sites, respectively. All H9N2 isolates were activated by human proteases TMPRSS2 (transmembrane protease, serine S1 member 2) and HAT (human airway trypsin-like protease). Interestingly, H9-782 and H9-2061 were also activated by matriptase, a protease widely expressed in most epithelia with high expression levels in the kidney. Nephrotropism of H9N2 viruses has been observed in chickens, and here we found that H9-782 and H9-2061 were proteolytically activated in canine kidney (MDCK-II) and chicken embryo kidney (CEK) cells, whereas H9-Wisc was not. Virus activation was inhibited by peptide-mimetic inhibitors of matriptase, strongly suggesting that matriptase is responsible for HA cleavage in these kidney cells. Our data demonstrate that H9N2 viruses with R-S-S-R or R-S-R-R cleavage sites are activated by matriptase in addition to HAT and TMPRSS2 and, therefore, can be activated in a wide range of tissues what may affect virus spread, tissue tropism and pathogenicity.

Phylogenetic diversity and genotypic complexity of H1N1 subtype swine influenza viruses isolated in mainland China

Background

After the occurrence of 2009 pandemic H1N1, close attention has been paid to the H1N1 subtype swine influenza viruses (H1N1 SIV) by scientific communities in many countries. A large-scale sequence analysis of the NCBI Influenza Virus Resource Database on H1N1 SIVs submitted primarily by scientists in China during 1992 to 2011 was performed. The aims of this study were to elucidate the genetic and evolutionary characteristics of H1N1 SIVs, to identify and unify the lineages and genetic characteristics of the H1N1 SIVs isolated in mainland China.

Results

Most of the strains were isolated during the period of 2008 to 2010 from Guangdong and Shandong provinces, China. Based on the phylogenetic and genotypic analyses, all of the H1N1 SIV strains can be classified into 8 lineages and 10 genotypes. All strains were of the characteristics of low pathogenic influenza viruses. The viruses of different lineage are characterized with different amino acid residues at the receptor-binding sites. Viruses containing PB2 genes of the classical swine, early seasonal human and recent seasonal human lineage might be more infectious to human. Some genotypes were directly related with human influenza viruses, which include strains that harbored genes derived from human influenza viruses.

Conclusions

Phylogenetic diversity and complexity existed in H1N1 SIVs isolated in mainland China. These H1N1 SIV strains were closely related to other subtype influenza viruses, especially to human influenza viruses. Moreover, it was shown that, novel lineages and genotypes of H1N1 SIVs emerged recently in mainland China. These findings provided new and essential information for further understanding of the genetic and evolutionary characteristics and monitoring the H1N1 SIVs in mainland China.

Seroepidemiological evidence of avian influenza A virus transmission to pigs in southern China

Recently, three novel avian-origin swine influenza viruses (SIVs) were first isolated from pigs in Guangdong Province, southern China, yet little is known about the seroprevalence of avian influenza viruses among pigs in southern China. Here, we report for the first time the seroprevalence of avian H3, H4, and H6 influenza viruses in swine populations and the lack of seroepidemiological evidence of avian H5 influenza virus transmission to pigs in China.

Complete genome sequence of an avian-like H4N8 swine influenza virus discovered in southern China

We report here the complete genomic sequence of an avian-like H4N8 swine influenza virus containing an H5N1 avian influenza virus segment from swine in southern China. Phylogenetic analyses of the sequences of all eight viral RNA segments demonstrated that these are wholly avian influenza viruses of the Asia lineage. To our knowledge, this is the first report of interspecies transmission of an avian H4N8 influenza virus to domestic pigs under natural conditions.

Complete Genome Sequence of a Novel Avian-Like H3N2 Swine Influenza Virus Discovered in Southern China

We report here the complete genomic sequence of a novel avian-like H3N2 swine influenza virus containing an H5N1 highly pathogenic avian influenza virus segment that was obtained from swine in southern China. Phylogenetic analysis indicated that this virus might originate from domestic aquatic birds. The sequence information provided herein suggests that continuing study is required to determine if this virus can be established in the swine population and pose potential threats to public health.

Antigenic and genetic characterization of a European avian-like H1N1 swine influenza virus from a boy in China in 2011

Cross-species transmission of influenza A viruses from swine to human occurs occasionally. In 2011, an influenza A H1N1 virus, A/Jiangsu/ALS1/2011 (JS/ALS1/2011), was isolated from a boy who suffered from severe pneumonia in China. The virus is closely related antigenically and genetically to avian-like swine H1N1 viruses that have recently been circulating in pigs in China and that were initially detected in European pig populations in 1979. The isolation of JS/ALS1/2011 provides additional evidence that swine influenza viruses can occasionally infect humans and emphasizes the importance of reinforcing influenza virus surveillance in both pigs and humans.

Pathogenicity and transmissibility of reassortant H9 influenza viruses with genes from pandemic H1N1 virus

Both H9N2 avian influenza and 2009 pandemic H1N1 viruses (pH1N1) are able to infect humans and swine, which has raised concerns that novel reassortant H9 viruses with pH1N1 genes might be generated in these hosts by reassortment. Although previous studies have demonstrated that reassortant H9 viruses with pH1N1 genes show increased virulence in mice and transmissibility in ferrets, the virulence and transmissibility of reassortant H9 viruses in natural hosts such as chickens and swine remain unknown. This study generated two reassortant H9 viruses (H9N2/CA09 and H9N1/CA09) in the background of the pH1N1 A/California/04/2009 (CA09) virus by replacing either both the haemagglutinin (HA) and neuraminidase (NA) genes or only the HA gene with the respective genes from the A/quail/Hong Kong/G1/1997 (H9N2) virus and evaluated their replication, pathogenicity and transmission in chickens and pigs compared with the parental viruses. Chickens that were infected with the parental H9N2 and reassortant H9 viruses seroconverted. The parental H9N2 and reassortant H9N2/CA09 viruses were transmitted to sentinel chickens, but H9N1/CA09 virus was not. The parental H9N2 replicated poorly and was not transmitted in pigs, whereas both H9N2/CA09 and H9N1/CA09 viruses replicated and were transmitted efficiently in pigs, similar to the pH1N1 virus. These results demonstrated that reassortant H9 viruses with pH1N1 genes show enhanced replication and transmissibility in pigs compared with the parental H9N2 virus, indicating that they may pose a threat for humans if such reassortants arise in swine.

Mouse-adapted H9N2 influenza A virus PB2 protein M147L and E627K mutations are critical for high virulence

H9N2 influenza viruses have been circulating worldwide in multiple avian species and have repeatedly infected humans to cause typical disease. The continued avian-to-human interspecies transmission of H9N2 viruses raises concerns about the possibility of viral adaption with increased virulence for humans. To investigate the genetic basis of H9N2 influenza virus host range and pathogenicity in mammals, we generated a mouse-adapted H9N2 virus (SD16-MA) that possessed significantly higher virulence than wide-type virus (SD16). Increased virulence was detectable after 8 sequential lung passages in mice. Five amino acid substitutions were found in the genome of SD16-MA compared with SD16 virus: PB2 (M147L, V250G and E627K), HA (L226Q) and M1 (R210K). Assessments of replication in mice showed that all of the SD16-MA PB2, HA and M1 genome segments increased virus replication; however, only the mouse-adapted PB2 significantly increased virulence. Although the PB2 E627K amino acid substitution enhanced viral polymerase activity and replication, none of the single mutations of mouse adapted PB2 could confer increased virulence on the SD16 backbone. The combination of M147L and E627K significantly enhanced viral replication ability and virulence in mice. Thus, our results show that the combination of PB2 amino acids at position 147 and 627 is critical for the increased pathogenicity of H9N2 influenza virus in mammalian host.

H9N2 influenza A virus circulates in H5N1 endemically infected poultry population in Egypt

We describe the identification and characterization of the H9N2 influenza subtype reported in Egyptian broiler and broiler breeder farms for the first time. Circulation of this subtype in a highly pathogenic H5N1 influenza virus endemic population provides an opportunity for genetic reassortment and emergence of novel viruses.

Molecular basis of efficient replication and pathogenicity of H9N2 avian influenza viruses in mice

H9N2 subtype avian influenza viruses (AIVs) have shown expanded host range and can infect mammals, such as humans and swine. To date the mechanisms of mammalian adaptation and interspecies transmission of H9N2 AIVs remain poorly understood. To explore the molecular basis determining mammalian adaptation of H9N2 AIVs, we compared two avian field H9N2 isolates in a mouse model: one (A/chicken/Guangdong/TS/2004, TS) is nonpathogenic, another one (A/chicken/Guangdong/V/2008, V) is lethal with efficient replication in mouse brains. In order to determine the basis of the differences in pathogenicity and brain tropism between these two viruses, recombinants with a single gene from the TS (or V) virus in the background of the V (or TS) virus were generated using reverse genetics and evaluated in a mouse model. The results showed that the PB2 gene is the major factor determining the virulence in the mouse model although other genes also have variable impacts on virus replication and pathogenicity. Further studies using PB2 chimeric viruses and mutated viruses with a single amino acid substitution at position 627 [glutamic acid (E) to lysine, (K)] in PB2 revealed that PB2 627K is critical for pathogenicity and viral replication of H9N2 viruses in mouse brains. All together, these results indicate that the PB2 gene and especially position 627 determine virus replication and pathogenicity in mice. This study provides insights into the molecular basis of mammalian adaptation and interspecies transmission of H9N2 AIVs.

Protective efficacy of a broadly cross-reactive swine influenza DNA vaccine encoding M2e, cytotoxic T lymphocyte epitope and consensus H3 hemagglutinin

BACKGROUND

Pigs have been implicated as mixing reservoir for the generation of new pandemic influenza strains, control of swine influenza has both veterinary and public health significance. Unlike human influenza vaccines, strains used for commercially available swine influenza vaccines are not regularly replaced, making the vaccines provide limited protection against antigenically diverse viruses. It is therefore necessary to develop broadly protective swine influenza vaccines that are efficacious to both homologous and heterologous virus infections. In this study, two forms of DNA vaccines were constructed, one was made by fusing M2e to consensus H3HA (MHa), which represents the majority of the HA sequences of H3N2 swine influenza viruses. Another was made by fusing M2e and a conserved CTL epitope (NP147-155) to consensus H3HA (MNHa). Their protective efficacies against homologous and heterologous challenges were tested.

RESULTS

BALB/c mice were immunized twice by particle-mediated epidermal delivery (gene gun) with the two DNA vaccines. It was shown that the two vaccines elicited substantial antibody responses, and MNHa induced more significant T cell-mediated immune response than MHa did. Then two H3N2 strains representative of different evolutional and antigenic clusters were used to challenge the vaccine-immunized mice (homosubtypic challenge). Results indicated that both of the DNA vaccines prevented homosubtypic virus infections completely. The vaccines' heterologous protective efficacies were further tested by challenging with a H1N1 swine influenza virus and a reassortant 2009 pandemic strain. It was found that MNHa reduced the lung viral titers significantly in both challenge groups, histopathological observation showed obvious reduction of lung pathogenesis as compared to MHa and control groups.

CONCLUSIONS

The combined utility of the consensus HA and the conserved M2e and CTL epitope can confer complete and partial protection against homologous and heterologous challenges, respectively, in mouse model. This may provide a basis for the development of universal swine influenza vaccines.

Isolation and characterization of an H9N2 influenza virus isolated in Argentina

As part of our ongoing efforts on animal influenza surveillance in Argentina, an H9N2 virus was isolated from a wild aquatic bird (Netta peposaca), A/rosy-billed pochard/Argentina/CIP051-559/2007 (H9N2) - herein referred to as 559/H9N2. Due to the important role that H9N2 viruses play in the ecology of influenza in nature, the 559/H9N2 isolate was characterized molecularly and biologically. Phylogenetic analysis of the HA gene revealed that the 559/H9N2 virus maintained an independent evolutionary pathway and shared a sister-group relationship with North American viruses, suggesting a common ancestor. The rest of the genome segments clustered with viruses from South America. Experimental inoculation of the 559/H9N2 in chickens and quail revealed efficient replication and transmission only in quail. Our results add to the notion of the unique evolutionary trend of avian influenza viruses in South America. Our study increases our understanding of H9N2 viruses in nature and emphasizes the importance of expanding animal influenza surveillance efforts to better define the ecology of influenza viruses at a global scale.

Epidemiological survey and genetic evolution of H9 subtype influenza viruses in Shanghai, China, from 2006 to 2010

The H9N2 influenza virus is endemic in poultry. We report its occurrence in live-poultry markets, fair-trade markets and poultry farms in the Shanghai region between September 2006 and December 2010. An analysis of partial sequences of the HA, NA, PB1, PB2 and NP genes of eleven distinct H9N2 isolates revealed that all carried an RSSR motif at the cleavage site of HA, diagnostic of low pathogenicity in chickens. A phylogenetic analysis indicated that these isolates are derived from the lineage represented by Duck/HK/Y280/97, but they have evolved a range of reassortments. Their PB1 and NP sequences resembled those of H5N1 strains, indicating a hybrid origin involving both H9 and H5 strains. The HA and NA sequences present in all eleven isolates resembled those of the Duck/HK/Y280/97-like lineage. Infection by H9N2 is commonplace in Shanghai live-poultry markets, allowing the viruses to have evolved rapidly.

Isolation, identification, and phylogenetic analysis of reassortant low-pathogenic avian influenza virus H3N1 from Pakistan

During routine avian influenza surveillance in Pakistan, a low-pathogenic avian influenza virus (LPAI) subtype H3N1 was isolated for the first time from domestic chickens. The higher seroprevalence of H3N1 was recorded in both commercial and domestic poultry in ecological zones of Pakistan where the geographical proximity with neighboring countries and attractive birding sites provide better opportunities for frequent movements of wild and migratory birds, and their intermingling with the local domestic and commercial poultry. Subsequent whole genome sequencing of this virus revealed a new introduction of a reassortant Eurasian avian strain, which was distinguishable from corresponding human and swine strains isolated elsewhere. Phylogenetically, the HA gene was mostly clustered with Nordic (Scandinavian) strains of influenza viruses, whereas the NA and PB1 genes showed a maximum nucleotide sequence homology with the Indian H11N1, and the PB2 gene was found to be closely related to the Altai H5N2. The Matrix and NP genes of H3N1 mostly clustered with the European avian influenza viruses (AIV), whereas its NS and PA genes showed maximum nucleotide homologies with the African (Egypt) AIV strains. A sequence and amino acid analysis revealed an LP motif, avian-like receptor specificity, potential glycosylation sites, and sensitivities to oseltamivir, zanamivir, and amantadine. Some point mutations possessed by this Pakistani AIV H3N1 were also found in human, equine, and swine H3 influenza viruses. This H3N1 isolate showed less nucleotide sequence homology with the previously known Pakistani AIV as compared with other Eurasian AIV strains.

Deletions in the neuraminidase stalk region of H2N2 and H9N2 avian influenza virus subtypes do not affect postinfluenza secondary bacterial pneumonia

We investigated the synergism between influenza virus and Streptococcus pneumoniae, particularly the role of deletions in the stalk region of the neuraminidase (NA) of H2N2 and H9N2 avian influenza viruses. Deletions in the NA stalk (ΔNA) had no effect on NA activity or on the adherence of S. pneumoniae to virus-infected human alveolar epithelial (A549) and mouse lung adenoma (LA-4) cells, although it delayed virus elution from turkey red blood cells. Sequential S. pneumoniae infection of mice previously inoculated with isogenic recombinant H2N2 and H9N2 influenza viruses displayed severe pneumonia, elevated levels of intrapulmonary proinflammatory responses, and death. No differences between the WT and ΔNA mutant viruses were detected with respect to effects on postinfluenza pneumococcal pneumonia as measured by bacterial growth, lung inflammation, morbidity, mortality, and cytokine/chemokine concentrations. Differences were observed, however, in influenza virus-infected mice that were treated with oseltamivir prior to a challenge with S. pneumoniae. Under these circumstances, mice infected with ΔNA viruses were associated with a better prognosis following a secondary bacterial challenge. These data suggest that the H2N2 and H9N2 subtypes of avian influenza A viruses can contribute to secondary bacterial pneumonia and deletions in the NA stalk may modulate its outcome in the context of antiviral therapy.

Contemporary epidemiology of North American lineage triple reassortant influenza A viruses in pigs

The 2009 pandemic H1N1 infection in humans has been one of the greatest concerns for public health in recent years. However, influenza in pigs is a zoonotic viral disease well-known to virologists for almost one century with the classical H1N1 subtype the only responsible agent for swine influenza in the United States for many decades. Swine influenza was first recognized clinically in pigs in the Midwestern U.S. in 1918 and since that time it has remained important to the swine industry throughout the world. Since 1988, however, the epidemiology of swine influenza changed dramatically. A number of emerging subtypes and genotypes have become established in the U.S. swine population. The ability of multiple influenza virus lineages to infect pigs is associated with the emergence of reassortant viruses with new genomic arrangements, and the introduction of the 2009 pandemic H1N1 from humans to swine represents a well-known example. The recent epidemiological data regarding the current state of influenza A virus subtypes circulating in the Canadian and American swine population is discussed in this review.