Influenza Viruses In Pigs Research Articles

Isolation and characterization of genotype 4 Eurasian avian-like H1N1 influenza virus in pigs suffering from pneumonia

Swine influenza viruses pose ongoing threat to pork industry throughout the world. In 2023, fattening pigs from a swine farm in Inner Mongolia of China experienced influenza-like symptoms. Co-infection of influenza A virus with Pasteurella multocida was diagnosed in lung tissues of diseased pigs and a genotype 4 (G4) Eurasian avian-like (EA) H1N1 virus was isolated, which was named as A/swine/Neimenggu/0326/2023. We demonstrated the virus preferentially bound human-like SAα2,6Gal receptor. It was noteworthy that the virus possessed multiple genetic markers for mammalian adaptation in the internal genes. Animal studies showed that compared with genotype 1 (G1) EA H1N1 virus and early prevalent G4 EA H1N1 virus, A/swine/Neimenggu/0326/2023 virus exhibited increased virus shedding, enhanced replication in lungs, and caused more severe lung lesions in pigs. These findings indicate that the G4 EA H1N1 virus poses increased threat to pork industry, controlling the prevailing viruses in pigs should be promptly implemented.

Quantifying the relationship between within-host dynamics and transmission for viral diseases of livestock.

Understanding the population dynamics of an infectious disease requires linking within-host dynamics and between-host transmission in a quantitative manner, but this is seldom done in practice. Here a simple phenomenological model for viral dynamics within a host is linked to between-host transmission by assuming that the probability of transmission is related to log viral titre. Data from transmission experiments for two viral diseases of livestock, foot-and-mouth disease virus in cattle and swine influenza virus in pigs, are used to parametrize the model and, importantly, test the underlying assumptions. The model allows the relationship between within-host parameters and transmission to be determined explicitly through their influence on the reproduction number and generation time. Furthermore, these critical within-host parameters (time and level of peak titre, viral growth and clearance rates) can be computed from more complex within-host models, raising the possibility of assessing the impact of within-host processes on between-host transmission in a more detailed quantitative manner.

Infectivity and transmissibility of an avian H3N1 influenza virus in pigs

In 2019 a low pathogenic H3N1 avian influenza virus (AIV) caused an outbreak in Belgian poultry farms, characterized by an unusually high mortality in chickens. Influenza A viruses of the H1 and H3 subtype can infect pigs and become established in swine populations. Therefore, the H3N1 epizootic raised concern about AIV transmission to pigs and from pigs to humans. Here, we assessed the replication efficiency of this virus in explants of the porcine respiratory tract and in pigs, using virus titration and/or RT-qPCR. We also examined transmission from directly, intranasally inoculated pigs to contact pigs. The H3N1 AIV replicated to moderate titers in explants of the bronchioles and lungs, but not in the nasal mucosa or trachea. In the pig infection study, infectious virus was only detected in a few lung samples collected between 1 and 3 days post-inoculation. Virus titers were between 1.7 and 4.8 log10 TCID50. In line with the ex vivo experiment, no virus was isolated from the upper respiratory tract of pigs. In the transmission experiment, we could not detect virus transmission from directly inoculated to contact pigs. An increase in serum antibody titers was observed only in the inoculated pigs. We conclude that the porcine respiratory tract tissue explants can be a useful tool to assess the replication efficiency of AIVs in pigs. The H3N1 AIV examined here is unlikely to pose a risk to swine populations. However, continuous risk assessment studies of emerging AIVs in pigs are necessary, since different virus strains will have different genotypic and phenotypic traits.

Natural Reassortment of Eurasian Avian-Like Swine H1N1 and Avian H9N2 Influenza Viruses in Pigs, China.

Several zoonotic influenza A viruses detected in humans contain genes derived from avian H9N2 subtypes. We uncovered a Eurasian avian-like H1N1 swine influenza virus with polymerase basic 1 and matrix gene segments derived from the H9N2 subtype, suggesting that H9N2 viruses are infecting pigs and reassorting with swine influenza viruses in China.

Pathogenicity of novel reassortant Eurasian avian-like H1N1 influenza virus in pigs

Reassortant Eurasian avian-like (EA) H1N1 virus, possessing 2009 pandemic (pdm/09) and triple-reassortant (TR)-derived internal genes, namely G4 genotype, has replaced the G1 genotype EA H1N1 virus (all the genes were of EA origin) and become predominant in swine populations in China. Understanding the pathogenicity of G4 viruses in pigs is of great importance for disease control. Here, we conducted comprehensive analyses of replication and pathogenicity of G4 and G1 EA H1N1 viruses in pigs. G4 virus exhibited enhanced replication, increased duration of virus shedding, and caused more severe respiratory lesions in pigs compared with G1 virus. G4 virus, with viral ribonucleoprotein (vRNP) complex genes of pdm/09 origin, exhibited higher levels of nuclear accumulation and higher polymerase activity, which is essential for improved replication of G4 virus. These findings indicate that G4 virus poses a great threat to both swine industry and public health, and control measures should be urgently implemented.

Reverse zoonotic transmission of influenza virus in Lagos State Nigeria

Background: Influenza virus is a major cause of infections in animals and human resulting in an annual 3–5 million cases of severe illness, and 290,000–650,000 deaths worldwide. It can spread from pigs to man and from man to pigs. Pigs have receptors that can allow human and avian influenza viruses to mix and produce new subtypes capable of causing pandemic disease. But there are limited information on the transmission of human influenza virus into pig population in Nigeria and sub-Saharan Africa. We therefore aimed to investigate the pattern of transmission of influenza virus in pigs and pig-handlers in Lagos State, Nigeria. Methods and materials: Between April, 2013 and May, 2017, 25 pig farms were visited in different divisions of Lagos State. Nasal and oropharyngeal swabs collected from pigs (n = 1972) and human (n = 432) were extracted and investigated for the presence of influenza virus matrix gene by one-step RT-PCR. Further molecular characterisation was performed. Phylogenetic analysis was done in MEGA 6.0 using BioNJ method with 1000 times bootstrapping replicates of the alignment. Chi-square and Fisher's exact tests were used to calculate p values at 95% confidence interval using GraphPad Prism, USA. Results: Our findings revealed influenza A virus of 1.3% in pigs and 0.005% in pig-handlers. Interestingly, human influenza A (H1N1) was isolated in pig. Phylogenetic analyses showed close relatedness to influenza viruses from several African countries such as Ethiopia and Egypt with 99% homology. Conclusion: In conclusion, this study provided for the first time, molecular evidence of reverse zoonotic transmission of human influenza A (H1N1) to pigs in Nigeria. A condition referred to as anthroponosis. The introduction of human influenza virus into pigs can lead to the emergence of new strains previously unknown to human and animal population thereby posing a great threat that can result in unpredictable fatal consequences. A nationwide surveillance to evaluate and determine the actual burden of influenza reverse zoonosis in Nigeria is hereby advocated. This will assist in the development of policies to prevent the continuous transmission of human influenza virus to pigs in order to avoid a similar occurrence of the last influenza pandemic.

Prevalent Eurasian avian-like H1N1 swine influenza virus with 2009 pandemic viral genes facilitating human infection

Pigs are considered as important hosts or "mixing vessels" for the generation of pandemic influenza viruses. Systematic surveillance of influenza viruses in pigs is essential for early warning and preparedness for the next potential pandemic. Here, we report on an influenza virus surveillance of pigs from 2011 to 2018 in China, and identify a recently emerged genotype 4 (G4) reassortant Eurasian avian-like (EA) H1N1 virus, which bears 2009 pandemic (pdm/09) and triple-reassortant (TR)-derived internal genes and has been predominant in swine populations since 2016. Similar to pdm/09 virus, G4 viruses bind to human-type receptors, produce much higher progeny virus in human airway epithelial cells, and show efficient infectivity and aerosol transmission in ferrets. Moreover, low antigenic cross-reactivity of human influenza vaccine strains with G4 reassortant EA H1N1 virus indicates that preexisting population immunity does not provide protection against G4 viruses. Further serological surveillance among occupational exposure population showed that 10.4% (35/338) of swine workers were positive for G4 EA H1N1 virus, especially for participants 18 y to 35 y old, who had 20.5% (9/44) seropositive rates, indicating that the predominant G4 EA H1N1 virus has acquired increased human infectivity. Such infectivity greatly enhances the opportunity for virus adaptation in humans and raises concerns for the possible generation of pandemic viruses.

A Single Amino Acid at Position 431 of the PB2 Protein Determines the Virulence of H1N1 Swine Influenza Viruses in Mice.

Genetic reassortments occurred continuously among multiple subtypes or genotypes of influenza viruses prevalent in pigs. Of note, some reassortant viruses bearing the internal genes of the 2009 pandemic H1N1 (2009/H1N1) virus sporadically caused human infection, which highlights their potential threats to human public health. In this study, we performed phylogenetic analysis on swine influenza viruses (SIVs) circulating in Liaoning Province, China. A total of 22 viruses, including 18 H1N1 and 4 H1N2 viruses, were isolated from 5,750 nasal swabs collected from pigs in slaughterhouses from 2014 to 2016. H1N1 viruses formed four genotypes, which included Eurasian avian-like H1N1 (EA H1N1) and double/triple reassortant H1N1 derived from EA H1N1, 2009/H1N1, and triple reassortant H1N2 (TR H1N2) viruses. H1N1 SIVs with different genotypes and even those within the same genotypes represented different pathogenicities in mice. We further characterized two naturally isolated H1N1 SIVs that had similar viral genomes but differed substantially in their virulence in mice and found that a single amino acid at position 431 in the basic polymerase 2 (PB2) protein significantly affected the viral replication capacity and virulence of these two viruses. Taken together, our findings revealed the diverse genomic origins and virulence of the SIVs prevalent in Liaoning Province during 2014 to 2016, which highlights that continuous surveillance is essential to monitor the evolution of SIVs. We identified a naturally occurring amino acid mutation in the PB2 protein of H1N1 SIVs that impacts the viral replication and virulence in mice by altering the viral polymerase activity.IMPORTANCE The frequent reassortment among different influenza viruses in pigs adds complexity to the epidemiology of swine influenza. The diverse viral virulence phenotypes underline the need to investigate the possible genetic determinants for evaluating the pandemic potential to human public health. Here, we found that multiple genotypes of influenza viruses cocirculate in the swine population in Liaoning Province, China. Furthermore, we pinpointed a single amino acid at position 431 in the PB2 protein which plays a critical role in the virulence of H1N1 viruses in mice and found that the alteration of viral polymerase activities is the cause of the different virulence. Our study further indicated that the virulence of influenza virus is a polygenic trait, and the newly identified virulence-related residue in the PB2 provides important information for broadening knowledge on the genetic basis of viral virulence of influenza viruses.

Interferon as a Mucosal Adjuvant for an Influenza Vaccine in Pigs.

Interferon, a natural protein that is produced by a variety of cells during viral infection, activates the transcription of multiple functional genes in cells, regulates synergy among various signaling pathways, and mediates many biological functions such as antiviral activity, immune regulation, and cell growth. However, clinical research on interferon in livestock is lacking. In this study, recombinant porcine interferon (PoIFNα) was used as an adjuvant, in combination with inactivated influenza virus, to vaccinate 6-week-old pigs via nasal infusion. The transcription of target genes was then monitored and the functions of PoIFNα were determined with respect to the activation of mucosal immunity. We found that a combination of low-dose PoIFNα and inactivated influenza virus could significantly up-regulate the expression of immunoregulatory cytokines such as IL-2, IL-18, IFN-γ, IL-6, and IL-10 by real-time PCR, suggesting the induction of a strong mucosal innate immune response after administration. In addition, low-dose PoIFNα can significant enhancing the transcription of genes encoding homing factors including CCR9 and CCR10 (P < 0.001), thereby resulting in the induction of higher levels of HA-specific antibodies (P < 0.05), which can be determined by ELISA and IFA. Post-immunization challenges with H1N1 virus demonstrated that PoIFNα, combined with inactivated influenza virus, could alleviate clinical signs in pigs during the early stages of viral infection. These studies reveal low-dose PoIFNα as a potential mucosal adjuvant for influenza virus in pigs.

T and B Cell Immune Responses to Influenza Viruses in Pigs.

Influenza viruses are an ongoing threat to humans and are endemic in pigs, causing considerable economic losses to farmers. Pigs are also a source of new viruses potentially capable of initiating human pandemics. Many tools including monoclonal antibodies, recombinant cytokines and chemokines, gene probes, tetramers, and inbred pigs allow refined analysis of immune responses against influenza. Recent advances in understanding of the pig innate system indicate that it shares many features with that of humans, although there is a larger gamma delta component. The fine specificity and mechanisms of cross-protective T cell immunity have yet to be fully defined, although it is clear that the local immune response is important. The repertoire of pig antibody response to influenza has not been thoroughly explored. Here we review current understanding of adaptive immune responses against influenza in pigs and the use of the pig as a model to study human disease.

Spatiotemporal Distribution and Evolution of the A/H1N1 2009 Pandemic Influenza Virus in Pigs in France from 2009 to 2017: Identification of a Potential Swine-Specific Lineage

The H1N1 influenza virus responsible for the most recent pandemic in 2009 (H1N1pdm) has spread to swine populations worldwide while it replaced the previous seasonal H1N1 virus in humans. In France, surveillance of swine influenza A viruses in pig herds with respiratory outbreaks led to the detection of 44 H1N1pdm strains between 2009 and 2017, regardless of the season, and findings were not correlated with pig density. From these isolates, 17 whole-genome sequences were obtained, as were 6 additional hemagglutinin (HA)/neuraminidase (NA) sequences, in order to perform spatial and temporal analyses of genetic diversity and to compare evolutionary patterns of H1N1pdm in pigs to patterns for human strains. Following mutation accumulation and fixation over time, phylogenetic analyses revealed for the first time the divergence of a swine-specific genogroup within the H1N1pdm lineage. The divergence is thought to have occurred around 2011, although this was demonstrated only through strains isolated in 2015 to 2016 in the southern half of France. To date, these H1N1pdm swine strains have not been related to any increased virulence in swine herds and have not exhibited any antigenic drift compared to seasonal human strains. However, further monitoring is encouraged, as diverging evolutionary patterns in these two species, i.e., swine and humans, may lead to the emergence of viruses with a potentially higher risk to both animal and human health.IMPORTANCE Pigs are a "mixing vessel" for influenza A viruses (IAVs) because of their ability to be infected by avian and human IAVs and their propensity to facilitate viral genomic reassortment events. Also, as IAVs may evolve differently in swine and humans, pigs can become a reservoir for old human strains against which the human population has become immunologically naive. Thus, viruses from the novel swine-specific H1N1pdm genogroup may continue to diverge from seasonal H1N1pdm strains and/or from other H1N1pdm viruses infecting pigs and lead to the emergence of viruses that would not be covered by human vaccines and/or swine vaccines based on antigens closely related to the original H1N1pdm virus. This discovery confirms the importance of encouraging swine IAV monitoring because H1N1pdm swine viruses could carry an increased risk to both human and swine health in the future as a whole H1N1pdm virus or gene provider in subsequent reassortant viruses.

Histamine contributes to severe pneumonia in pigs infected with 2009 pandemic H1N1 influenza virus.

Histamine is a biogenic amine that influences many immune cells. In this study, we investigated the effect of histamine on the pathogenesis of 2009 pandemic H1N1 influenza virus in pigs. Histamine was not detected in the tracheal tissues of infected pigs, and no difference was found in the pathological damage found in infected pigs with and without treatment with a histamine antagonist. Lung tissues from untreated infected pigs showed severe interstitial pneumonia with accumulation of histamine, in contrast to those from infected pigs that were treated with the histamine antagonist. The expression of inflammatory cytokines was much higher in the lungs of untreated infected pigs than in infected pigs treated with the histamine antagonist. These data suggest that histamine necessary for the development of the severe pneumonia in infected pigs.

Novel triple-reassortant influenza viruses in pigs, Guangxi, China

Considered a “mixing vessel” for influenza viruses, pigs can give rise to new influenza virus reassortants that can threaten humans. During our surveillance of pigs in Guangxi, China from 2013 to 2015, we isolated 11 H1N1 and three H3N2 influenza A viruses of swine origin (IAVs-S). Out of the 14, we detected ten novel triple-reassortant viruses, which contained surface genes (hemagglutinin and neuraminidase) from Eurasian avian-like (EA) H1N1 or seasonal human-like H3N2, matrix (M) genes from H1N1/2009 pandemic or EA H1N1, nonstructural (NS) genes from classical swine, and the remaining genes from H1N1/2009 pandemic. Mouse studies indicate that these IAVs-S replicate efficiently without prior adaptation, with some isolates demonstrating lethality. Notably, the reassortant EA H1N1 viruses with EA-like M gene have been reported in human infections. Further investigations will help to assess the potential risk of these novel triple-reassortant viruses to humans.

Intradermal immunization with inactivated swine influenza virus and adjuvant polydi(sodium carboxylatoethylphenoxy)phosphazene (PCEP) induced humoral and cell-mediated immunity and reduced lung viral titres in pigs

Swine influenza virus is endemic worldwide and it is responsible for significant economic losses to the swine industry. A vaccine that stimulates a rapid and long-lasting protective immune response to prevent this infection is highly sought. Poly[di(sodium carboxylatoethylphenoxy)-phosphazene (PCEP) has demonstrated adjuvant activity when formulated as part of multiple vaccines in mice and pigs. In this study we examined the magnitude and type of immune response induced in pigs vaccinated via the intramuscular or intradermal routes with inactivated swine influenza virus (SIV) H1N1 vaccine formulated with PCEP. Intradermal administration of PCEP-adjuvanted inactivated SIV vaccine stimulated significant anti-SIV antibody titres, increased neutralizing antibodies, and significantly reduced lung virus load with limited reduction of gross lung lesions after challenge with virulent H1N1 relative to control animals. These results indicate that PCEP may be effective as a vaccine adjuvant against swine influenza viruses in pigs and should be considered a potential candidate adjuvant for future swine intradermal influenza vaccines.

Evidence of infection with avian, human, and swine influenza viruses in pigs in Cairo, Egypt.

The majority of the Egyptian swine population was culled in the aftermath of the 2009 H1N1 pandemic, but small-scale growing remains. We sampled pigs from piggeries and an abattoir in Cairo. We found virological evidence of infection with avian H9N2 and H5N1 viruses as well as human pandemic H1N1 influenza virus. Serological evidence suggested previous exposure to avian H5N1 and H9N2, human pandemic H1N1, and swine avian-like and human-like viruses. This raises concern about potential reassortment of influenza viruses in pigs and highlights the need for better control and prevention of influenza virus infection in pigs.

Polyanhydride nanovaccine against swine influenza virus in pigs

We have recently demonstrated the effectiveness of an influenza A virus (IAV) subunit vaccine based on biodegradable polyanhydride nanoparticles delivery in mice. In the present study, we evaluated the efficacy of ∼200nm polyanhydride nanoparticles encapsulating inactivated swine influenza A virus (SwIAV) as a vaccine to induce protective immunity against a heterologous IAV challenge in pigs. Nursery pigs were vaccinated intranasally twice with inactivated SwIAV H1N2 (KAg) or polyanhydride nanoparticle-encapsulated KAg (KAg nanovaccine), and efficacy was evaluated against a heterologous zoonotic virulent SwIAV H1N1 challenge. Pigs were monitored for fever daily. Local and systemic antibody responses, antigen-specific proliferation of peripheral blood mononuclear cells, gross and microscopic lung lesions, and virus load in the respiratory tract were compared among the groups of animals. Our pre-challenge results indicated that KAg nanovaccine induced virus-specific lymphocyte proliferation and increased the frequency of CD4+CD8αα+ T helper and CD8+ cytotoxic T cells in peripheral blood mononuclear cells. KAg nanovaccine-immunized pigs were protected from fever following SwIAV challenge. In addition, pigs immunized with the KAg nanovaccine presented with lower viral antigens in lung sections and had 6 to 8-fold reduction in nasal shedding of SwIAV four days post-challenge compared to control animals. Immunologically, increased IFN-γ secreting T lymphocyte populations against both the vaccine and challenge viruses were detected in KAg nanovaccine-immunized pigs compared to the animals immunized with KAg alone. However, in the KAg nanovaccine-immunized pigs, hemagglutination inhibition, IgG and IgA antibody responses, and virus neutralization titers were comparable to that in the animals immunized with KAg alone. Overall, our data indicated that intranasal delivery of polyanhydride-based SwIAV nanovaccine augmented antigen-specific cellular immune response in pigs, with promise to induce cross-protective immunity.

Truncation of C-terminal 20 amino acids in PA-X contributes to adaptation of swine influenza virus in pigs.

The PA-X protein is a fusion protein incorporating the N-terminal 191 amino acids of the PA protein with a short C-terminal sequence encoded by an overlapping ORF (X-ORF) in segment 3 that is accessed by + 1 ribosomal frameshifting, and this X-ORF exists in either full length or a truncated form (either 61-or 41-condons). Genetic evolution analysis indicates that all swine influenza viruses (SIVs) possessed full-length PA-X prior to 1985, but since then SIVs with truncated PA-X have gradually increased and become dominant, implying that truncation of this protein may contribute to the adaptation of influenza virus in pigs. To verify this hypothesis, we constructed PA-X extended viruses in the background of a “triple-reassortment” H1N2 SIV with truncated PA-X, and evaluated their biological characteristics in vitro and in vivo. Compared with full-length PA-X, SIV with truncated PA-X had increased viral replication in porcine cells and swine respiratory tissues, along with enhanced pathogenicity, replication and transmissibility in pigs. Furthermore, we found that truncation of PA-X improved the inhibition of IFN-I mRNA expression. Hereby, our results imply that truncation of PA-X may contribute to the adaptation of SIV in pigs.

Infectivity and Transmissibility of Avian H9N2 Influenza Viruses in Pigs.

The H9N2 influenza viruses that are enzootic in terrestrial poultry in China pose a persistent pandemic threat to humans. To investigate whether the continuous circulation and adaptation of these viruses in terrestrial poultry increased their infectivity to pigs, we conducted a serological survey in pig herds with H9N2 viruses selected from the aquatic avian gene pool (Y439 lineage) and the enzootic terrestrial poultry viruses (G1 and Y280 lineages). We also compared the infectivity and transmissibility of these viruses in pigs. It was found that more than 15% of the pigs sampled from 2010 to 2012 in southern China were seropositive to either G1 or Y280 lineage viruses, but none of the sera were positive to the H9 viruses from the Y439 lineage. Viruses of the G1 and Y280 lineages were able to infect experimental pigs, with detectable nasal shedding of the viruses and seroconversion, whereas viruses of the Y439 lineage did not cause a productive infection in pigs. Thus, adaptation and prevalence in terrestrial poultry could lead to interspecies transmission of H9N2 viruses from birds to pigs. Although H9N2 viruses do not appear to be continuously transmissible among pigs, repeated introductions of H9 viruses to pigs naturally increase the risk of generating mammalian-adapted or reassorted variants that are potentially infectious to humans. This study highlights the importance of monitoring the activity of H9N2 viruses in terrestrial poultry and pigs. H9N2 subtype of influenza viruses has repeatedly been introduced into mammalian hosts, including humans and pigs, so awareness of their activity and evolution is important for influenza pandemic preparedness. However, since H9N2 viruses usually cause mild or even asymptomatic infections in mammalian hosts, they may be overlooked in influenza surveillance. Here, we found that the H9N2 viruses established in terrestrial poultry had higher infectivity in pigs than those from aquatic birds, which suggests that adaptation of the H9N2 viruses in terrestrial poultry might have increased the infectivity of the virus to mammals. Therefore, monitoring the prevalence and evolution of H9 viruses prevalent in terrestrial birds and conducting risk assessment of their threat to mammals are critical for evaluating the pandemic potential of this virus.

Co-infection of classic swine H1N1 influenza virus in pigs persistently infected with porcine rubulavirus

Porcine rubulavirus (PorPV) and swine influenza virus infection causes respiratory disease in pigs. PorPV persistent infection could facilitate the establishment of secondary infections. The aim of this study was to analyse the pathogenicity of classic swine H1N1 influenza virus (swH1N1) in growing pigs persistently infected with porcine rubulavirus. Conventional six-week-old pigs were intranasally inoculated with PorPV, swH1N1, or PorPV/swH1N1. A mock-infected group was included. The co-infection with swH1N1 was at 44 days post-infection (DPI), right after clinical signs of PorPV infection had stopped. The pigs of the co-infection group presented an increase of clinical signs compared to the simple infection groups. In all infected groups, the most recurrent lung lesion was hyperplasia of the bronchiolar-associated lymphoid tissue and interstitial pneumonia. By means of immunohistochemical evaluation it was possible to demonstrate the presence of the two viral agents infecting simultaneously the bronchiolar epithelium. Viral excretion of PorPV in nasal and oral fluid was recorded at 28 and 52 DPI, respectively. PorPV persisted in several samples from respiratory tissues (RT), secondary lymphoid organs (SLO), and bronchoalveolar lavage fluid (BALF). For swH1N1, the viral excretion in nasal fluids was significantly higher in single-infected swH1N1 pigs than in the co-infected group. However, the co-infection group exhibited an increase in the presence of swH1N1 in RT, SLO, and BALF at two days after co-infection. In conclusion, the results obtained confirm an increase in the clinical signs of infection, and PorPV was observed to impact the spread of swH1N1 in analysed tissues in the early stage of co-infection, although viral shedding was not enhanced. In the present study, the interaction of swH1N1 infection is demonstrated in pigs persistently infected with PorPV.

Novel triple-reassortant H1N1 swine influenza viruses in pigs in Tianjin, Northern China

Pigs are susceptible to both human and avian influenza viruses and therefore have been proposed to be mixing vessels for the generation of pandemic influenza viruses through reassortment. In this study, for the first time, we report the isolation and genetic analyses of three novel triple-reassortant H1N1 swine influenza viruses from pigs in Tianjin, Northern China. Phylogenetic analysis showed that these novel viruses contained genes from the 2009 pandemic H1N1 (PB2, PB1, PA and NP), Eurasian swine (HA, NA and M) and triple-reassortant swine (NS) lineages. This indicated that the reassortment among the 2009 pandemic H1N1, Eurasian swine and triple-reassortant swine influenza viruses had taken place in pigs in Tianjin and resulted in the generation of new viruses. Furthermore, three human-like H1N1, two classical swine H1N1 and two Eurasian swine H1N1 viruses were also isolated during the swine influenza virus surveillance from 2009 to 2013, which indicated that multiple genetic lineages of swine H1N1 viruses were co-circulating in the swine population in Tianjin, China. The emergence of novel triple-reassortant H1N1 swine influenza viruses may be a potential threat to human health and emphasizes the importance of further continuous surveillance.