Isolation and genetic analysis of human origin H1N1 and H3N2 influenza viruses from pigs in China

Infection dynamics of subtype H9N2 low pathogenic avian influenza a virus in turkeys

While mammals can be infected by influenza A virus either sporadically or with well adapted lineages, aquatic birds are the natural reservoir of the pathogen. So far most of the knowledge on influenza virus dynamics was however gained on mammalian models. In this study, we infected turkeys using a low pathogenic avian influenza virus and determined the infection dynamics with a target-cell limited model. Results showed that turkeys had a different set of infection characteristics, compared with humans and ponies. The viral clearance rates were similar between turkeys and ponies but higher than that in humans. The cell death rates and cell to cell transmission rates were similar between turkeys and humans but higher than those in ponies. Overall, this study indicated the variations of within-host dynamics of influenza infection in avian, humans, and other mammalian systems.

The epidemiology of swine influenza

Globally swine influenza is one of the most important diseases of the pig industry, with various subtypes of swine influenza virus co-circulating in the field. Swine influenza can not only cause large economic losses for the pig industry but can also lead to epidemics or pandemics in the human population. We provide an overview of the pathogenic characteristics of the disease, diagnosis, risk factors for the occurrence on pig farms, impact on pigs and humans and methods to control it. This review is designed to promote understanding of the epidemiology of swine influenza which will benefit the control of the disease in both pigs and humans.

A Systematic Review Analyzing the Prevalence and Circulation of Influenza Viruses in Swine Population Worldwide

The global anxiety and a significant threat to public health due to the current COVID-19 pandemic reiterate the need for active surveillance for the zoonotic virus diseases of pandemic potential. Influenza virus due to its wide host range and zoonotic potential poses such a significant threat to public health. Swine serve as a “mixing vessel” for influenza virus reassortment and evolution which as a result may facilitate the emergence of new strains or subtypes of zoonotic potential. In this context, the currently available scientific data hold a high significance to unravel influenza virus epidemiology and evolution. With this objective, the current systematic review summarizes the original research articles and case reports of all the four types of influenza viruses reported in swine populations worldwide. A total of 281 articles were found eligible through screening of PubMed and Google Scholar databases and hence were included in this systematic review. The highest number of research articles (n = 107) were reported from Asia, followed by Americas (n = 97), Europe (n = 55), Africa (n = 18), and Australia (n = 4). The H1N1, H1N2, H3N2, and A(H1N1)pdm09 viruses were the most common influenza A virus subtypes reported in swine in most countries across the globe, however, few strains of influenza B, C, and D viruses were also reported in certain countries. Multiple reports of the avian influenza virus strains documented in the last two decades in swine in China, the United States, Canada, South Korea, Nigeria, and Egypt provided the evidence of interspecies transmission of influenza viruses from birds to swine. Inter-species transmission of equine influenza virus H3N8 from horse to swine in China expanded the genetic diversity of swine influenza viruses. Additionally, numerous reports of the double and triple-reassortant strains which emerged due to reassortments among avian, human, and swine strains within swine further increased the genetic diversity of swine influenza viruses. These findings are alarming hence active surveillance should be in place to prevent future influenza pandemics.

Continuous evolution of influenza A viruses of swine from 2013 to 2015 in Guangdong, China

Southern China is considered an important source of influenza virus pandemics because of the large, diverse viral reservoirs in poultry and swine. To examine the trend in influenza A virus of swine (IAV-S), an active surveillance program has been conducted from 2013 to 2015 in Guangdong, China. The phylogenetic analyses showed that the external genes of the isolates were assigned to the Eurasian avian-like swine (EA) H1N1 and/or human-like H3N2 lineages with multiple substitutions, indicating a notable genetic shift. Moreover, the internal genes derived from different origins (PB2, PB1, PA, NP: pdm/09 (pandemic influenza virus 2009)-origin, M: pdm/09- or EA-origin, NS: North American Triple Reassortant (TR)-origin have become the dominant backbone of IAV-S in southern China. According to the origins of the eight gene segments, the isolates can be categorized into five genotypes. The results of mice experiment showed that the YJ4 (genotype 1) and DG2 (genotype 4) are the most pathogenic to mice, and the viruses are observed in kidneys and brains, indicating the systemic infection. The alterations of the IAV-S gene composition supported the continued implementation of the intensive surveillance of IAV-S and the greater attention focused on potential shifts toward transmission to humans.

Prevalence, distribution and risk factors of farmer reported swine influenza infection in Guangdong Province, China

A cross-sectional study was undertaken to better understand the husbandry, management and biosecurity practices of pig farms in Guangdong Province (GD), China to identify risk factors for farmer reported swine influenza (SI) on their farms. Questionnaires were administered to 153 owners/managers of piggeries (average of 7 from each of the 21 prefectures in GD). Univariable and multivariable logistic regression analyses were used to identify risk factors for farmer reported SI in piggeries during the six months preceding the questionnaire administration. The ability of wild birds to enter piggeries (OR 2.50, 95% CI: 1.01-6.16), the presence of poultry on a pig-farm (OR 3.24, 95% CI: 1.52-6.94) and no biosecurity measures applied to workers before entry to the piggery (OR 2.65, 95% CI: 1.04-6.78) were found to increase the likelihood of SI being reported by farmers in a multivariable logistic regression model. The findings of this study highlight the importance of understanding the local pig industry and the practices adopted when developing control measures to reduce the risk of SI to pig farms.

Swine and Avian Influenza Outbreaks in Recent Times

Influenza A is a zoonotic virus and wild waterfowls are the main reservoir of avian influenza viruses, which are precursors of human influenza A viruses. Through mutations and gene reassortment, some strains of avian influenza viruses establish stable lineages in poultry species, pigs, horses, and humans. The first zoonotic influenza pandemic of the twenty-first century, the swine H1N1 pandemic of 2009, originated from Mexico, and fortunately the virus was only of modest virulence. However, lessons have been learned on the shortcomings of the global preparedness for influenza pandemic, and this should be considered as a valuable experience for the preparation of the next major outbreak. Of more concern is the emergence of the highly pathogenic avian influenza A [H5N1], ongoing since 1996, and the low pathogenic avian influenza A [H7N9], since 2013, which have crossed the species barrier to humans in China. Risks of a H5N1 pandemic appear to be receding with declining human cases, and the H7N9 influenza virus is now the leading candidate as the next pandemic influenza virus. However, influenza pandemics are unpredictable in their timing, specific strain of virus, and origin. Most experts predict that the next influenza pandemic will arise from Asia, especially China, and will be directly of avian origin. Continued influenza surveillance in animals and humans globally with prompt reporting to the WHO and the World Animal Health Organization with sharing of data promptly between countries is essential. Long-term solutions to prevent cross-species transmission of zoonotic influenza viruses to humans and development of more effective, longer-lasting vaccines against emerging avian influenza viruses are needed. Currently there is no evidence of an impending zoonotic or avian influenza pandemic, and the viruses of interest, H5N1 and H7N9 avian influenza A viruses, have not mutated to allow for easy transmission to humans nor human to human.

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.

Novel reassortant influenza viruses between pandemic (H1N1) 2009 and other influenza viruses pose a risk to public health

Influenza A virus (IAV) is characterized by eight single-stranded, negative sense RNA segments, which allows for gene reassortment among different IAV subtypes when they co-infect a single host cell simultaneously. Genetic reassortment is an important way to favor the evolution of influenza virus. Novel reassortant virus may pose a pandemic among humans. In history, three human pandemic influenza viruses were caused by genetic reassortment between avian, human and swine influenza viruses. Since 2009, pandemic (H1N1) 2009 (pdm/09 H1N1) influenza virus composed of two swine influenza virus genes highlighted the genetic reassortment again. Due to wide host species and high transmission of the pdm/09 H1N1 influenza virus, many different avian, human or swine influenza virus subtypes may reassert with it to generate novel reassortant viruses, which may result in a next pandemic among humans. So, it is necessary to understand the potential threat of current reassortant viruses between the pdm/09 H1N1 and other influenza viruses to public health. This study summarized the status of the reassortant viruses between the pdm/09 H1N1 and other influenza viruses of different species origins in natural and experimental conditions. The aim of this summarization is to facilitate us to further understand the potential threats of novel reassortant influenza viruses to public health and to make effective prevention and control strategies for these pathogens.

Reconstruction of the Evolutionary Dynamics of A(H3N2) Influenza Viruses Circulating in Italy from 2004 to 2012

BACKGROUND

Influenza A viruses are characterised by their rapid evolution, and the appearance of point mutations in the viral hemagglutinin (HA) domain causes seasonal epidemics. The A(H3N2) virus has higher mutation rate than the A(H1N1) virus. The aim of this study was to reconstruct the evolutionary dynamics of the A(H3N2) viruses circulating in Italy between 2004 and 2012 in the light of the forces driving viral evolution.

METHODS

Phylodinamic analyses were made using a Bayesian method, and codon-specific positive selection acting on the HA coding sequence was evaluated.

RESULTS

Global and local phylogenetic analyses showed that the Italian strains collected between 2004 and 2012 grouped into five significant Italian clades that included viral sequences circulating in different epidemic seasons. The time of the most recent common ancestor (tMRCA) of the tree root was between May and December 2003. The tMRCA estimates of the major clades suggest that the origin of a new viral strain precedes the effective circulation of the strain in the Italian population by 6-31 months, thus supporting a central role of global migration in seeding the epidemics in Italy. The study of selection pressure showed that four codons were under positive selection, three of which were located in antigenic sites. Analysis of population dynamics showed the alternation of periods of exponential growth followed by a decrease in the effective number of infections corresponding to epidemic and inter-epidemic seasons.

CONCLUSIONS

Our analyses suggest that a complex interaction between the immune status of the population, migrations, and a few selective sweeps drive the influenza A(H3N2) virus evolution. Our findings suggest the possibility of the year-round survival of local strains even in temperate zones, a hypothesis that warrants further investigation.

Antigenic Detection of Human Strain of Influenza Virus A (H3N2) in Swine Populations at Three Locations in Nigeria and Ghana during the Dry Early Months of 2014

Since the first detection of human H3N2 influenza virus in Taiwanese pigs in 1970, infection of pigs with wholly human viruses has been known to occur in other parts of the world. These viruses, referred to as human-like H3N2 viruses, have been known to cause clinical and subclinical infections of swine populations. Due to the paucity and complete unavailability of information on transmission of influenza viruses from other species, especially humans, to swine in Nigeria and Ghana, respectively, this study was designed to investigate the presence and prevalence of a human strain of influenza A (H3N2) in swine populations at three locations in two cities within these two West African countries in January and February, 2014. Using stratified random technique, nasal swab specimens were collected from seventy-five (75) pigs at two locations in Ibadan, Nigeria and from fifty (50) pigs in Kumasi, Ghana. These specimens were tested directly by a sensitive Quantitative Solid Phase Antigen-detection Sandwich ELISA using anti-A/Brisbane/10/2007 haemagglutinin monoclonal antibody. Influenza virus A/Brisbane/10/2007 (H3N2) was detected among pigs at the three study locations, with an aggregate prevalence of 4.0% for the two locations in Ibadan, Nigeria and also 4.0% for Kumasi, Ghana. Transmission of influenza viruses from other species to swine portends serious sinister prospects for genetic reassortment and evolvement of novel viruses. We therefore recommend that further studies should be carried out to investigate the presence of other circulating human and avian influenza viruses in swine populations in West Africa and also determine the extent of genetic reassortment of strains circulating among these pigs. This would provide an early warning system for detection of novel influenza viruses, which could have pandemic potentials.

Emergence of human-like H3N2 influenza viruses in pet dogs in Guangxi, China

Background

After the 1968 H3N2 pandemic emerged in humans, H3N2 influenza viruses continuously circulated and evolved in nature. An H3N2 variant was circulating in humans in the 1990s and subsequently introduced into the pig population in the 2000s. This virus gradually became the main subtype of swine influenza virus worldwide. However, there were no reports of infections in dogs with this virus.

Findings

In 2013, 35 nasal swabs from pet dogs were positive for Influenza A virus by RT-PCR. Two viruses were isolated and genetically characterized. In the phylogenetic trees of all gene segments, two H3N2 canine isolates clustered with Moscow/10/99 and most H3N2 swine influenza viruses. These results indicated that two H3N2 CIVs possessed high homology with human/swine influenza viruses, which at the same time exhibited some amino acid substitutions in NA, polymerase basic protein 1 (PB1), and nucleoprotein (NP), which probably were related to the interspecies transmission.

Conclusions

These two viruses share the highest homology with swine H3N2, Moscow/99-like viruses, which indicated that these viruses might originate from swine viruses.

Characterization of porcine P58IPK gene and its up-regulation after H1N1 or H3N2 influenza virus infection

BACKGROUND

The 58-kDa inhibitor of the interferon-induced double-stranded RNA-activated protein kinase (P58IPK) is a cellular protein that is activated during influenza virus infection. Although the function of human P58IPK has been studied for a long time, porcine P58IPK (pP58IPK) has little been studied except for its cloning.

OBJECTIVE

In this study, we aimed to investigate the characteristics of the pP58IPK gene, determine its subcellular localization, and find its expression change during H1N1 or H3N2 infection.

STUDY DESIGN

First, the sequence and structure of pP58IPK were analyzed. Second, pP58IPK gene was cloned into pEGFP-N1 and pEGFP-C1 vectors, respectively, which were transfected into cells to determine its subcellular localization. Third, Lung tissues of piglets from H1N1 infected, H3N2 infected and control groups were analyzed using histopathology, real-time PCR, and immunohistochemistry.

RESULTS

The sequence and structure of pP58IPK was highly similar to the counterpart of human. pP58IPK protein distributed only in the cytoplasm. Lung tissues of piglets infected by H1N1 or H3N2 appeared obvious pathological changes, and the expression of pP58IPK in both mRNA and protein level was up-regulated by approximate 1.5-fold in piglets infected by H1N1 or H3N2 comparing with control piglets.

CONCLUSIONS

We analyzed the characteristics of the pP58IPK gene, constructed a phylogenetic tree, determined its subcellular localization, and investigated its expression changes during H1N1 or H3N2 infection. The fundamental data accumulated in this study provides a potential medical model for investigating the function of P58IPK during influenza A viruses infection.

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.

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.

Genetics, evolution, and the zoonotic capacity of European Swine influenza viruses

The European swine influenza virus lineage differs genetically from the classical swine influenza viruses and the triple reassortants found in North America and Asia. The avian-like swine H1N1 viruses emerged in 1979 after an avian-to-swine transmission and spread to all major European pig-producing countries. Reassortment of these viruses with seasonal H3N2 viruses led to human-like swine H3N2 viruses which appeared in 1984. Finally, human-like swine H1N2 viruses emerged in 1994. These are triple reassortants comprising genes of avian-like H1N1, seasonal H1N1, and seasonal H3N2 viruses. All three subtypes established persistent infection chains and became prevalent in the European pig population. They successively replaced the circulating classical swine H1N1 viruses of that time and gave rise to a number of reassortant viruses including the pandemic (H1N1) 2009 virus. All three European lineages have the capacity to infect humans but zoonotic infections are benign.

Review: micronutrient selenium deficiency influences evolution of some viral infectious diseases

Recently emerged viral infectious diseases (VIDs) include HIV/AIDS, influenzas H5N1 and 2009 H1N1, SARS, and Ebola hemorrhagic fevers. Earlier research determined metabolic oxidative stress in hosts deficient in antioxidant selenium (Se) (<1 μMol Se/L of blood) induces both impaired human host immunocompetence and rapidly mutated benign variants of RNA viruses to virulence. These viral mutations are consistent, rather than stochastic, and long-lived. When Se-deficient virus-infected hosts were supplemented with dietary Se, viral mutation rates diminished and immunocompetence improved. Herein is described the role of micronutrient Se deficiency on the evolution of some contemporary RNA viruses and their subsequent VIDs. Distinguishing cellular and biomolecular evidence for several VIDs suggests that environmental conditions conducive to chronic dietary Se deprivation could be monitored for bioindicators of incipient viral virulence and subsequent pathogenesis.

Swine influenza surveillance in East and Southeast Asia: a systematic review

East and Southeast Asia are important pig- and poultry-producing areas, where the majority of production takes place on small-scale farms with low biosecurity levels. This systematic review synthesizes data on swine influenza virology, serology and epidemiology in East and Southeast Asia. A total of 77 research articles, literature reviews and conference papers were selected and analyzed from 510 references retrieved from PubMed and ISI Web of KnowledgeSM. The number of published articles increased in the last 3 years, which may be attributed to improvement in monitoring and/or a better promotion of surveillance data. Nevertheless, large inequalities in surveillance and research among countries are underlined. Virological results represent the largest part of published data, while the serological and epidemiological features of swine influenza in East and Southeast Asia remain poorly described. The literature shows that there have been several emergences of swine influenza in the region, and also considerable evidence of multiple introductions of North American and avian-like European strains. Furthermore, several avian-origin strains are isolated from pigs, including H5 and H9 subtypes. However, their low seroprevalence in swine also shows that pigs remain poorly infected by these subtypes. We conclude that sero-epidemioligical investigations have been neglected, and that they may help to improve virological surveillance. Inter- and intra-continental surveillance of gene flows will benefit the region. Greater investment is needed in swine influenza surveillance, to improve our knowledge of circulating strains as well as the epidemiology and disease burden in the region.

Outbreak of swine influenza in Argentina reveals a non-contemporary human H3N2 virus highly transmissible among pigs

Sporadic outbreaks of human H3N2 influenza A virus (IAV) infections in swine populations have been reported in Asia, Europe and North America since 1970. In South America, serological surveys in pigs indicate that IAVs of the H3 and H1 subtypes are currently in circulation; however, neither virus isolation nor characterization has been reported. In November 2008, an outbreak of respiratory disease in pigs consistent with swine influenza virus (SIV) infection was detected in Argentina. The current study describes the clinical epidemiology, pathology, and molecular and biological characteristics of the virus. Phylogenetic analysis revealed that the virus isolate shared nucleotide identities of 96-98 % with H3N2 IAVs that circulated in humans from 2000 to 2003. Antigenically, sera from experimentally inoculated animals cross-reacted mainly with non-contemporary human-origin H3N2 influenza viruses. In an experimental infection in a commercial swine breed, the virus was of low virulence but was transmitted efficiently to contact pigs and caused severe disease when an infected animal acquired a secondary bacterial infection. This is the first report of a wholly human H3N2 IAV associated with clinical disease in pigs in South America. These studies highlight the importance of two-way transmission of IAVs and SIVs between pigs and humans, and call for enhanced influenza surveillance in the pig population worldwide.

Diversity of influenza viruses in swine and the emergence of a novel human pandemic influenza A (H1N1)

Abstract  The novel H1N1 influenza virus that emerged in humans in Mexico in early 2009 and transmitted efficiently in the human population with global spread has been declared a pandemic strain. Here we review influenza infections in swine since 1918 and the introduction of different avian and human influenza virus genes into swine influenza viruses of North America and Eurasia. These introductions often result in viruses of increased fitness for pigs that occasionally transmit to humans. The novel virus affecting humans is derived from a North American swine influenza virus that has acquired two gene segments [Neuraminidase (NA) and Matrix (M)] from the European swine lineages. This reassortant appears to have increased fitness in humans. The potential for increased virulence in humans and of further reassortment between the novel H1N1 influenza virus and oseltamivir resistant seasonal H1N1 or with highly pathogenic H5N1 influenza stresses the need for urgent pandemic planning.

Analysis of influenza A viruses of subtype H1 from wild birds, turkeys and pigs in Germany reveals interspecies transmission events

BACKGROUND

Despite considerable host species barriers, interspecies transmissions of influenza A viruses between wild birds, poultry and pigs have been demonstrated repeatedly. In particular, viruses of the subtypes H1 and H3 were transmitted between pigs and poultry, predominantly turkeys, in regions with a high population density of both species. The recovery of a swine influenza H1N1 virus from a turkey flock in Germany in 2009 prompted us to investigate molecularly the subtype H1 viruses recently detected in wild birds, pigs and poultry.

OBJECTIVES

The goal of this study was to investigate the relationship between H1N1 viruses originating from wild and domestic animals of Germany and to identify potential trans-species transmission or reassortment events.

METHODS

Hemagglutinin and neuraminidase gene or full-length genome sequences were generated from selected, current H1N1 viruses from wild birds, pigs and turkeys. Phylogenetic analyses were combined with genotyping and analyses of the deduced amino acid sequences with respect to biologically active sites. Antigenic relationships were assessed by hemagglutination inhibition reactions.

RESULTS

Phylogenetic analysis of the hemagglutinin sequences showed that viruses from distinct H1 subgroups co-circulate among domestic animals and wild birds. In addition, these viruses comprised different genotypes and were distinguishable antigenically. An H1N1 virus isolated from a turkey farm in northern Germany in 2009 showed the highest similarity with the avian-like porcine H1N1 influenza viruses circulating in Europe since the late 1970s.

CONCLUSIONS

The data demonstrate the genetic and antigenic heterogeneity of H1 viruses currently circulating in domestic and wild animals in Germany and points to turkeys as a possible bridge between avian and mammalian hosts.

The role of swine in the generation of novel influenza viruses

The ecology of influenza A viruses is very complicated involving multiple host species and viral genes. Avian species have variable susceptibility to influenza A viruses with wild aquatic birds being the reservoir for this group of pathogens. Occasionally, influenza A viruses are transmitted to mammals from avian species, which can lead to the development of human pandemic strains by direct or indirect transmission to man. Because swine are also susceptible to infection with avian and human influenza viruses, genetic reassortment between these viruses and/or swine influenza viruses can occur. The potential to generate novel influenza viruses has resulted in swine being labelled 'mixing vessels'. The mixing vessel theory is one mechanism by which unique viruses can be transmitted from an avian reservoir to man. Although swine can generate novel influenza viruses capable of infecting man, at present, it is difficult to predict which viruses, if any, will cause a human pandemic. Clearly, the ecology of influenza A viruses is dynamic and can impact human health, companion animals, as well as the health of livestock and poultry for production of valuable protein commodities. For these reasons, influenza is, and will continue to be, a serious threat to the wellbeing of mankind.

Serological and virological surveillance of swine H1N1 and H3N2 influenza virus infection in two farms located in Hubei province, central China

Swine influenza viruses H1N1 and H3N2 have been reported in the swine population worldwide. From June 2008 to June 2009, we carried out serological and virological surveillance of swine influenza in the Hubei province in central China. The serological results indicated that antibodies to H1N1 swine influenza virus in the swine population were high with a 42.5% (204/480) positive rate, whereas antibodies to H3N2 swine influenza virus were low with a 7.9% (38/480) positive rate. Virological surveillance showed that only one sample from weanling pigs was positive by RT-PCR. Phylogenetic analysis of the hemagglutinin and neuraminidase genes revealed that the A/Sw/HB/S1/2009 isolate was closely related to avian-like H1N1 viruses and seemed to be derived from the European swine H1N1 viruses. In conclusion, H1N1 influenza viruses were more dominant in the pig population than H3N2 influenza viruses in central China, and infection with avian-like H1N1 viruses persistently emerged in the swine population in the area.

Genetic diversity of H9N2 influenza viruses from pigs in China: a potential threat to human health?

Pandemic strains of influenza A virus might arise by genetic reassortment between viruses from different hosts. Pigs are susceptible to both human and avian influenza viruses and have been proposed to be intermediate hosts or mixing vessels, for the generation of pandemic influenza viruses through reassortment or adaptation to the mammalian host. In this study, we summarize and report for the first time the coexistence of 10 (A-J) genotypes in pigs in China by analyzing the eight genes of 28 swine H9N2 viruses isolated in China from 1998 to 2007. Swine H9N2 viruses in genotype A and B were completely derived from Y280-like and Shanghai/F/98-like viruses, respectively, which indicated avian-to-pig interspecies transmission of H9N2 viruses did exist in China. The other eight genotype (C-J) viruses might be double-reassortant viruses, in which six genotype (E-J) viruses possessed 1-4 H5-like gene segments indicating they were reassortants of H9 and H5 viruses. In conclusion, genetic diversity of H9N2 influenza viruses from pigs in China provides further evidence that avian to pig interspecies transmission of H9N2 viruses did occur and might result in the generation of new reassortant viruses by genetic reassortment with swine H1N1, H1N2 and H3N2 influenza viruses, therefore, these swine H9N2 influenza viruses might be a potential threat to human health and continuing to carry out swine influenza virus surveillance in China is of great significance.

Novel swine influenza virus reassortants in pigs, China

During swine influenza virus surveillance in pigs in China during 2006-2009, we isolated subtypes H1N1, H1N2, and H3N2 and found novel reassortment between contemporary swine and avian panzootic viruses. These reassortment events raise concern about generation of novel viruses in pigs, which could have pandemic potential.

Isolation and genetic analysis of a novel triple-reassortant H1N1 influenza virus from a pig in China

Influenza A viruses of subtype H1N1 have been reported widely in pigs in China, associated with clinical disease. These mainly include classical swine H1N1, avian-like H1N1, and human-like H1N1 viruses. In this study, we reported a novel triple-reassortant H1N1 virus (A/swine/Guangdong/1/2010) containing genes from the classical swine (NP, NS), human (PB1) and avian (HA, NA, M, PB2, PA) lineages, which was for the first time reported in China. Also, phylogenetic analysis further confirmed that five genes segments (NS, NP, PB2, PB1, PA) of the isolate were closely related to the novel reassortant H1N2 viruses isolated in China in 2006, while the other three (HA, NA, M) were closely related to avian-like H1N1 viruses in China. The isolation of triple-reassortant H1N1 influenza virus provides further evidence that pigs serve as emergence hosts or "mixing vessels", and swine influenza virus (SIV) surveillance in China should be given a high priority.

Serological surveillance of influenza A virus infection in swine populations in Fujian province, China: no evidence of naturally occurring H5N1 infection in pigs

Several highly pathogenic H5N1 avian influenza viruses were isolated from swine populations in Fujian Province, China, since 2001. Because it is thought that H5N1 infection in pigs might result in virus adaptation to humans, we surveyed swine populations in Fujian Province in 2004 and 2007 for serological evidence of the infection. Twenty-five pig farms covering all nine administrative districts of Fujian Province were sampled and a total of 1407 serum specimens were collected. The haemagglutination inhibition (HI) tests revealed no evidence of H5 infection and only a few cases of H9 infection. The negative results for H5 infection were further verified by micro-neutralization tests. By contrast, H1 influenza virus infections were prevalent in swine in both surveys according to the results of enzyme-linked immunosorbent assay (ELISA). The H3 infection rate was reduced dramatically in 2007 compared with 2004, when examined by HI and ELISA. In summary, the results imply that the swine populations in Fujian Province had not been affected greatly by the H5N1 avian influenza virus, given that there is no serological evidence that H5N1 influenza virus has infected the pig populations. The reported isolates represent only sporadic cases.

Experimental inoculation of pigs with pandemic H1N1 2009 virus and HI cross-reactivity with contemporary swine influenza virus antisera

BACKGROUND

A novel A/H1N1 was identified in the human population in North America in April 2009. The gene constellation of the virus was a combination from swine influenza A viruses (SIV) of North American and Eurasian lineages that had never before been identified in swine or other species.

OBJECTIVES

The objectives were to (i) evaluate the clinical response of swine following experimental inoculation with pandemic H1N1 2009; (ii) assess serologic cross-reactivity between H1N1 2009 and contemporary SIV antisera; and (iii) develop a molecular assay to differentiate North American-lineage SIV from H1N1 2009.

METHODS

Experiment 1: Weaned pigs were experimentally infected with A/California/04/2009 (H1N1). Experiment 2: The cross-reactivity of a panel of US SIV H1N1 or H1N2 antisera with three isolates of pandemic A/H1N1 was evaluated. Experiment 3: A polymerase chain reaction (PCR)-based diagnostic test was developed and validated on samples from experimentally infected pigs.

RESULTS AND CONCLUSIONS

In experiment 1, all inoculated pigs demonstrated clinical signs and lesions similar to those induced by endemic SIV. Viable virus and antigen were only detected in the respiratory tract. In experiment 2, serologic cross-reactivity was limited against H1N1 2009 isolates, notably among virus antisera from the same HA phylogenetic cluster. The limited cross-reactivity suggests North American pigs may not be fully protected against H1N1 2009 from previous exposure or vaccination and novel tests are needed to rapidly diagnose the introduction of H1N1 2009. In experiment 3, an RT-PCR test that discriminates between H1N1 2009 and endemic North American SIV was developed and validated on clinical samples.

Emergence of European avian influenza virus-like H1N1 swine influenza A viruses in China

During swine influenza surveillance from 2007 to 2008, 10 H1N1 viruses were isolated and analyzed for their antigenic and phylogenetic properties. Our study revealed the emergence of avian-origin European H1N1 swine influenza virus in China, which highlights the necessity of swine influenza surveillance for potential pandemic preparedness.

High genetic and antigenic similarity between a swine H3N2 influenza A virus and a prior human influenza vaccine virus: a possible immune pressure-driven cross-species transmission

In late April of 2009, a global outbreak of human influenza was reported. The causative agent is a highly unusual reassortant H1N1 influenza virus carrying genetic segments derived from swine, human and avian influenza viruses. In this study, we compared the HA, NA and other gene segments of a swine H3N2 influenza A virus, A/Swine/Guangdong/z5/2003, which was isolated from pigs in 2003 in Guangdong Province, China, to the predominant human and swine H3N2 viruses. We found that the similarity of gene segments of A/Swine/Guangdong/z5/2003 was closer to Moscow/99-like human H3N2 virus than Europe swine H3N2 viruses during 1999-2002. These results suggest that A/Swine/Guangdong/z5/2003 may be porcine in origin, possibly being driven by human immune pressure induced by either natural H3N2 virus infection or use of A/Moscow/10/99 (H3N2)-based human influenza vaccine. The results further confirm that swine may play a dual role as a "shelter" for hosting influenza virus from humans or birds and as a "mixing vessel" for generating reassortant influenza viruses, such as the one causing current influenza pandemic.

Isolation and genetic characterization of avian-like H1N1 and novel ressortant H1N2 influenza viruses from pigs in China

As pigs are susceptible to both human and avian influenza viruses, they have been proposed to be intermediate hosts or mixing vessels for the generation of pandemic influenza viruses through reassortment or adaptation to the mammalian host. In this study, we reported avian-like H1N1 and novel ressortant H1N2 influenza viruses from pigs in China. Homology and phylogenetic analyses showed that the H1N1 virus (A/swine/Zhejiang/1/07) was closely to avian-like H1N1 viruses and seemed to be derived from the European swine H1N1 viruses, which was for the first time reported in China; and the two H1N2 viruses (A/swine/Shanghai/1/07 and A/swine/Guangxi/13/06) were novel ressortant H1N2 influenza viruses containing genes from the classical swine (HA, NP, M and NS), human (NA and PB1) and avian (PB2 and PA) lineages, which indicted that the reassortment among human, avian, and swine influenza viruses had taken place in pigs in China and resulted in the generation of new viruses. The isolation of avian-like H1N1 influenza virus originated from the European swine H1N1 viruses, especially the emergence of two novel ressortant H1N2 influenza viruses provides further evidence that pigs serve as intermediate hosts or "mixing vessels", and swine influenza virus surveillance in China should be given a high priority.

Fusion of C3d with hemagglutinin enhances protective immunity against swine influenza virus

H1N1 and H3N2 are the dominant subtypes causing swine influenza in China and other countries. It is important to develop effective vaccines against both H1N1 and H3N2 subtypes of swine influenza virus (SIV). We examined the effects of a DNA vaccine expressing an influenza HA fused to three copies of murine complement C3d in mice. Plasmids encoding soluble HA (sHA), complete HA (tmHA), or a soluble fused form of HA (sHA-mC3d3) were constructed from the H3N2 subtype of SIV. The immune response was monitored by an enzyme-linked immunosorbent assay (ELISA), hemagglutination inhibition (HI) assays, and virus neutralization tests. Analysis of antibody titers indicated that immunization with HA-mC3d3 resulted in higher titers of anti-HA antibodies and higher antibody affinities, compared with serum from mice immunized with sHA or tmHA. Furthermore, the C3d fusion increased the Th2-biased immune response, by inducing IL-4 production. Splenocytes from mice immunized with sHA-mC3d3 produced about three-fold more IL-4 than did splenocytes from mice immunized with sHA or tmHA. Seven days post-challenge with homologous virus (H3N2), no virus was isolated from the mice immunized with HA-expressing plasmids. However, 10 days post-challenge with heterologous virus (H1N1), only mice immunized with sHA-mC3d3 had no virus or microscopic lesions in the kidneys and cerebrum. In conclusion, C3d enhanced antibody responses to hemagglutinin and protective immunity against SIV of different subtypes.

Isolation and genetic characterization of avian origin H9N2 influenza viruses from pigs in China

As pigs are susceptible to infection with both avian and human influenza A viruses, they have been proposed to be an intermediate host for the adaptation of avian influenza viruses to humans. In April 2006, a disease caused by highly pathogenic porcine reproductive and respiratory syndrome virus (PRRSV) occurred in several pig farms and subsequently overwhelmed almost half of China with more than 2,000,000 cases of pig infection. Here we report a case in which four swine H9N2 influenza viruses were isolated from pigs infected by highly pathogenic PRRSVs in Guangxi province in China. All the eight gene segments of the four swine H9N2 viruses are highly homologous to A/Pigeon/Nanchang/2-0461/00 (H9N2) or A/Wild Duck/Nanchang/2-0480/00 (H9N2). Phylogenetic analyses of eight genes show that the swine H9N2 influenza viruses are of avian origin and may be the descendants of A/Duck/Hong Kong/Y280/97-like viruses. Molecular analysis of the HA gene indicates that our H9N2 isolates might have high-affinity binding to the alpha2,6-NeuAcGal receptor found in human cells. In conclusion, our finding provides further evidence about the interspecies transmission of avian influenza viruses to pigs and emphasizes the importance of reinforcing swine influenza virus (SIV) surveillance, especially after the emergence of highly pathogenic PRRSVs in pigs in China.

Genetic evolution of swine influenza A (H3N2) viruses in China from 1970 to 2006

Pigs are susceptible to both human and avian influenza viruses and have been proposed to be intermediate hosts, or mixing vessels, for the generation of pandemic influenza viruses through reassortment or adaptation to the mammalian host. In this study, we summarize and report for the first time the coexistence of wholly human-like H3N2 viruses, double-reassortant H3N2 viruses, and triple-reassortant H3N2 viruses in pigs in China by analyzing the eight genes of swine influenza A (H3N2) viruses found in China from 1970 to 2006. In 1970, the first wholly human-like H3N2 (Hong Kong/68-like) viruses were isolated from pigs in Taiwan, and then in the next years Victoria/75-like, Sydney/97-like, New York/99-like, and Moscow/99-like swine H3N2 viruses were regularly isolated in China. In the 1980s, two triple-reassortant viruses were isolated from pigs. Recently, the double-reassortant viruses containing genes from the human (HA and NA) and avian (PB2, PB1, PA, NP, M, and NS) lineages and the triple-reassortant viruses containing genes from the human (HA and NA), classical swine (NP), and avian (PB2, PB1, PA, M, and NS) lineages emerged in pigs in China. The coexistence of wholly human-like and reassortant viruses provides further evidence that pigs serve as intermediate hosts, or mixing vessels, and emphasizes the importance of reinforcing swine influenza virus surveillance in China.