Analysis of the influenza virus gene pool of avian species from southern China

Preferential Selection and Contribution of Non-Structural Protein 1 (NS1) to the Efficient Transmission of Panzootic Avian Influenza H5N8 Virus Clades 2.3.4.4A and B in Chickens and Ducks

Highly pathogenic avian influenza virus H5N8 clade 2.3.4.4 caused outbreaks in poultry at an unprecedented global scale. The virus was spread by wild birds in Asia in two waves: clade 2.3.4.4A in 2014/2015 and clade 2.3.4.4B from 2016 up to today. Both clades were highly virulent in chickens, but only clade B viruses exhibited high virulence in ducks. Viral factors which contribute to virulence and transmission of these panzootic H5N8 2.3.4.4 viruses are largely unknown. The NS1 protein, typically composed of 230 amino acids (aa), is a multifunctional protein which is also a pathogenicity factor. Here, we studied the evolutionary trajectory of H5N8 NS1 proteins from 2013 to 2019 and their role in the fitness of H5N8 viruses in chickens and ducks. Sequence analysis and in vitro experiments indicated that clade 2.3.4.4A and clade 2.3.4.4B viruses have a preference for NS1 of 237 aa and 217 aa, respectively, over NS1 of 230 aa. NS217 was exclusively seen in domestic and wild birds in Europe. The extension of the NS1 C terminus (CTE) of clade B virus reduced virus transmission and replication in chickens and ducks and partially impaired the systemic tropism to the endothelium in ducks. Conversely, lower impact on fitness of clade A virus was observed. Remarkably, the NS1 of clade A and clade B, regardless of length, was efficient in blocking interferon (IFN) induction in infected chickens, and changes in the NS1 C terminus reduced the efficiency for interferon antagonism. Together, the NS1 C terminus contributes to the efficient transmission and high fitness of H5N8 viruses in chickens and ducks. IMPORTANCE The panzootic H5N8 highly pathogenic avian influenza viruses of clade 2.3.4.4A and 2.3.4.4B devastated the poultry industry globally. Clade 2.3.4.4A was predominant in 2014/2015 while clade 2.3.4.4B was widely spread in 2016/2017. The two clades exhibited different pathotypes in ducks. Virus factors contributing to virulence and transmission are largely unknown. The NS1 protein is typically composed of 230 amino acids (aa) and is an essential interferon (IFN) antagonist. Here, we found that the NS1 protein of clade 2.3.4.4A preferentially evolved toward long NS1 with 237 aa, while clade 2.3.4.4B evolved toward shorter NS1 with 217 aa (exclusively found in Europe) due to stop codons in the C terminus (CTE). We showed that the NS1 CTE of H5N8 is required for efficient virus replication, transmission, and endotheliotropism in ducks. In chickens, H5N8 NS1 evolved toward higher efficiency to block IFN response. These findings may explain the preferential pattern for short NS1 and high fitness of the panzootic H5N8 in birds.

Virus-like Particle Vaccines: A Prospective Panacea Against an Avian Influenza Panzootic

Epizootics of highly pathogenic avian influenza (HPAI) have resulted in the deaths of millions of birds leading to huge financial losses to the poultry industry worldwide. The roles of migratory wild birds in the harbouring, mutation, and transmission of avian influenza viruses (AIVs), and the lack of broad-spectrum prophylactic vaccines present imminent threats of a global panzootic. To prevent this, control measures that include effective AIV surveillance programmes, treatment regimens, and universal vaccines are being developed and analysed for their effectiveness. We reviewed the epidemiology of AIVs with regards to past avian influenza (AI) outbreaks in birds. The AIV surveillance programmes in wild and domestic birds, as well as their roles in AI control were also evaluated. We discussed the limitations of the currently used AI vaccines, which necessitated the development of a universal vaccine. We evaluated the current development of AI vaccines based upon virus-like particles (VLPs), particularly those displaying the matrix-2 ectodomain (M2e) peptide. Finally, we highlighted the prospects of these VLP vaccines as universal vaccines with the potential of preventing an AI panzootic.

The Pandemic Threat of Emerging H5 and H7 Avian Influenza Viruses

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

Prevalence of the C-terminal truncations of NS1 in avian influenza A viruses and effect on virulence and replication of a highly pathogenic H7N1 virus in chickens

Highly pathogenic (HP) avian influenza viruses (AIV) evolve from low pathogenic (LP) precursors after circulation in poultry by reassortment and/or single mutations in different gene segments including that encoding NS1. The carboxyl terminal end (CTE) of NS1 exhibits deletions between amino acid 202 and 230 with still unknown impact on virulence of AIV in chickens. In this study, NS1 protein sequences of all AIV subtypes in birds from 1902 to 2015 were analyzed to study the prevalence and distribution of CTE truncation (ΔCTE). Thirteen different ΔCTE forms were observed in NS1 proteins from 11 HA and 8 NA subtypes with high prevalences in H9, H7, H6 and H10 and N9, N2, N6 and N1 subtypes particularly in chickens and minor poultry species. With 88% NS217 lacking amino acids 218-230 was the most common ΔCTE form followed by NS224 (3.6%). NS217 was found in 10 and 8 different HA and NA subtypes, respectively, whereas NS224 was detected exclusively in the Italian HPAIV H7N1 suggesting relevance for virulence. To test this assumption, 3 recombinant HPAIV H7N1 were constructed carrying wild-type HP NS1 (Hp-NS224), NS1 with extended CTE (Hp-NS230) or NS1 from LPAIV H7N1 (Hp-NSLp), and tested in-vitro and in-vivo. Extension of CTE in Hp NS1 significantly decreased virus replication in chicken embryo kidney cells. Truncation in the NS1 decreased the tropism of Hp-NS224 to the endothelium, central nervous system and respiratory tract epithelium without significant difference in virulence in chickens. This study described the variable forms of ΔCTE in NS1 and indicated that CTE is not an essential virulence determinant particularly for the Italian HPAIV H7N1 but may be a host-adaptation marker required for efficient virus replication.

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.

The genetic match between vaccine strains and circulating seasonal influenza A viruses in Vietnam, 2001-2009

Please cite this paper as: Vuong et al. (2013). The genetic match between vaccine strains and circulating seasonal influenza A viruses in Vietnam, 2001–2009. Influenza and Other Respiratory Viruses 7(6), 1151–1157.

Background  Vietnam is currently developing domestic capability to manufacture influenza vaccines but information on the genetic and antigenic characteristics of locally circulating seasonal influenza viruses is limited. To assess the relevance of WHO recommended vaccine strains to the situation in Vietnam, we analyzed the genetic relatedness of the hemagglutinin (HA) gene of seasonal influenza A viruses circulating in Vietnam from 2001 to 2009 to WHO recommended vaccine strains over the same period.

Methods and Principal findings  We sequenced the HA gene of 32 H1N1 and 44 H3N2 seasonal influenza A isolates from laboratory‐based sentinel surveillance sites in Hanoi from 2001 to 2005 and from a national influenza surveillance system from 2005 to 2009. H1 and H3 HA phylogenetic trees rooted to vaccine strains A/Beijing/295/1995 (H1N1) and A/Moscow/10/1999 (H3N2), respectively, were constructed with contemporary HA sequences of isolates from neighboring countries. We found some genetic differences between seasonal influenza H3N2 viruses and three WHO influenza vaccine strains recommended for use in the Northern and Southern Hemispheres for the 2001–2004 and 2007–2008 seasons and close genetic identity of circulating H3N2 strains with the recommended WHO Southern Hemisphere vaccine strains for 2004 and 2009 seasons. The genetic similarity of circulating H1N1 strains with the WHO recommended vaccine strains are described for the study period 2001–2009.

Conclusions  The HA gene of seasonal influenza virus strains in Vietnam (especially influenza A/H3N2) showed varying degrees of genetic identity compared with those of the Northern or Southern Hemisphere vaccine strains recommended by WHO. The close relatedness of the HA of Vietnamese strains and contemporary strains from nearby countries indicate a good genetic match of circulating strains during study period. Greater representation of virus isolates from South East Asia in the vaccine strain selection process is desirable of influenza vaccine development in Vietnam.

Epidemiological update on swine influenza (H1N1) in pigs

The 2009 H1N1 pandemic has slowed down its spread after initial speed of transmission. The conventional swine influenza H1N1 virus (SIV) in pig populations worldwide needs to be differentiated from pandemic H1N1 influenza virus, however it is also essential to know about the exact role of pigs in the spread and mutations taking place in pig-to-pig transmission. The present paper reviews epidemiological features of classical SIV and its differentiation with pandemic influenza.

Influenza in migratory birds and evidence of limited intercontinental virus exchange

Migratory waterfowl of the world are the natural reservoirs of influenza viruses of all known subtypes. However, it is unknown whether these waterfowl perpetuate highly pathogenic (HP) H5 and H7 avian influenza viruses. Here we report influenza virus surveillance from 2001 to 2006 in wild ducks in Alberta, Canada, and in shorebirds and gulls at Delaware Bay (New Jersey), United States, and examine the frequency of exchange of influenza viruses between the Eurasian and American virus clades, or superfamilies. Influenza viruses belonging to each of the subtypes H1 through H13 and N1 through N9 were detected in these waterfowl, but H14 and H15 were not found. Viruses of the HP Asian H5N1 subtypes were not detected, and serologic studies in adult mallard ducks provided no evidence of their circulation. The recently described H16 subtype of influenza viruses was detected in American shorebirds and gulls but not in ducks. We also found an unusual cluster of H7N3 influenza viruses in shorebirds and gulls that was able to replicate well in chickens and kill chicken embryos. Genetic analysis of 6,767 avian influenza gene segments and 248 complete avian influenza viruses supported the notion that the exchange of entire influenza viruses between the Eurasian and American clades does not occur frequently. Overall, the available evidence does not support the perpetuation of HP H5N1 influenza in migratory birds and suggests that the introduction of HP Asian H5N1 to the Americas by migratory birds is likely to be a rare event.

Influenza surveillance in wild migratory birds has been done at two sites in North America: 1) in Alberta, Canada, for the past 31 years, and 2) along Delaware Bay, United States, for the past 22 years. These studies support the concept that wild migratory birds are the reservoirs of all influenza A viruses and that the influenza viruses in the world can be divided into two distinct superfamilies, one in Eurasia and the other in the Americas. From time to time these viruses spread to domestic poultry and to humans and cause pandemics of disease. Many investigators have expanded these studies particularly in Europe, Asia, and the Americas. The emergence of highly pathogenic H5N1 in Asia a decade ago and the continuing evolution and spread of these H5N1 viruses to the whole of Eurasia is a continuing problem for veterinary and human public health. The available evidence from Eurasia is that migratory birds can be infected and may be involved in local spread of the highly pathogenic H5N1 virus. The question addressed in the present study is why the highly pathogenic H5N1 influenza virus has not yet reached the Americas despite the overlap in migratory bird pathways, particularly in Alaska. Genomic analysis of influenza viruses from our repository failed to provide evidence of influenza viruses with their whole genome originating from Eurasia. However, we found occasional influenza viruses from North America with single or multiple genes that originated in Eurasia. Our interpretation is that while influenza viruses do exchange between the two hemispheres, this is a rare occurrence. Regardless, enhanced surveillance should be continued in the Americas in case this rare event occurs.

Reliable universal RT-PCR assays for studying influenza polymerase subunit gene sequences from all 16 haemagglutinin subtypes

Realizable one-step RT-PCR assays specific for influenza PB2, PB1 and PA segments are described in this report. The designs of the consensus primers were based on more than five thousands polymerase genes derived from avian or mammalian viral strains. All the viral RNA tested in this study could be consistently amplified by the assays. The reaction products were specific and could be used for direct DNA sequencing. These assays might be useful tools to study the sequences of these genes.

Genetic and Pathogenic Characterization of H6N1 Avian Influenza Viruses Isolated in Taiwan Between 1972 and 2005

This article reports the genetic and pathogenic characteristics of 34 isolates of H6N1 avian influenza viruses isolated in Taiwan between 1972 and 2005. Genetic analyses showed that a unique lineage of H6N1 viruses has been established in domestic chickens in Taiwan since 1997, and this lineage of viruses differs from the H6N1 viruses circulating in Hong Kong and Southeastern China. Pathogenicity tests showed that all Taiwanese H6N1 viruses were of low pathogenicity but might lead to economic loss when associated with other diseases. Hemagglutination inhibition tests showed that antigenic drift has occurred in Taiwanese H6N1 viruses, and sequence comparison has identified a total of five possible antigenic sites on the hemagglutinin molecule of the H6N1 viruses. Some Taiwanese H6N 1 viruses could replicate in mice without preadaptation, indicating that these viruses have the potential to cause cross-species infection into mammals.

Amplification of the entire genome of influenza A virus H1N1 and H3N2 subtypes by reverse-transcription polymerase chain reaction

This study describes the development of a simple RT-PCR method to amplify the whole genome of the influenza A virus based on the amplification of full-length gene segments. Primers were designed based on the conserved regions of both the 5'-end and the 3'-end of each gene segment. After optimizing the duration and temperature of denaturing, annealing, and extension, these primers could amplify all of the full-length gene segments. To test the accuracy of the method, all amplicons were subjected to DNA sequencing with an autosequencer. Eighteen strains of influenza A virus which belonged to H1N1 or H3N2 subtypes were tested. All eight segments of both subtypes were successfully amplified in all tested strains. Using a newly developed reverse-transcriptase (RT), primers and PCR running conditions, this study established a protocol to amplify the entire genome of the influenza A virus. This method provides a tool which can be used for the amplification of all genes of the H1N1 and H3N2 subtypes of influenza A virus prior to analysis of their sequences, and to construct expression plasmids for further study.

Influenza A viruses in feral Canadian ducks: extensive reassortment in nature

The current dogma of influenza accepts that feral aquatic birds are the reservoir for influenza A viruses. Although the genomic information of human influenza A viruses is increasing, little of this type of data is available for viruses circulating in feral waterfowl. This study presents the genetic characterization of 35 viruses isolated from wild Canadian ducks from 1983 to 2000, as the first attempt at a comprehensive genotypic analysis of influenza viruses isolated from feral ducks. This study demonstrates that influenza virus genes circulating in Canadian ducks have achieved evolutionary stasis. The majority of these duck virus genes are clustered in distinct North American clades; however, some H6 and H9 genes are clustered with those from Eurasian viruses. Genes appeared to reassort in a random fashion. None of the genotypes identified remained present throughout all of the years examined and most PA and PB2 genes that crossed over into swine were clustered in one phylogenetic grouping. Additionally, matrix genes were identified that branch very early in the evolutionary tree. These findings demonstrate the diversity of the influenza virus gene pool in Canadian ducks, and suggest that genes which cluster in specific phylogenetic groupings in the PB2 and PA genes can be used for markers of viruses with the potential for crossing the species barrier. A more comprehensive study of this important reservoir is needed to provide further insight into the genomic composition of viruses that crossover the species barrier, which would be a useful component to pandemic planning.

Multiple Genotypes of Nonpathogenic H6N2 Influenza Viruses Isolated from Chickens in California

From February 2000 through September 2001, a limited number of H6N2 influenza viruses were isolated from chickens in California. This report describes the genetic characterization of nine of these H6N2 viruses. All of the viruses analyzed had phylogenetically similar hemagglutinin (HA) and neuraminidase molecules that suggested the viruses shared a recent common ancestor. The analysis of the HA sequence of these viruses with all available H6 viruses from different hosts and locations showed that these genes do not separate into well-defined North American and Eurasian lineages. The neuraminidase genes of the California viruses contain an 18 amino acid deletion, a possible adaptation to growth in chickens. Analysis of the remaining gene segments of the California viruses revealed that three distinct genotypes of H6N2 viruses were present.

Reassortment and interspecies transmission of North American H6N2 influenza viruses

H6N2 influenza viruses were isolated from California chickens in 2000 and 2001. Here we report the characterization of these H6N2 viruses, one of the few descriptions of non-H5, non-H7 subtype influenza viruses in this host. The H6N2 viruses were nonpathogenic in experimentally infected chickens and could be divided into three genotypes. All three genotypes of virus had similar surface glycoproteins and all contained an 18 amino acid deletion in the neuraminidase, a characteristic of other chicken influenza viruses. Differences were apparent, however, in the complement of replicative protein genes between the genotypes. The presence of multiple H6N2 genotypes suggests that independent transmission and/or reassortment events may have taken place between aquatic bird and chicken influenza viruses.

H3N2 influenza viruses from domestic chickens in Italy: an increasing role for chickens in the ecology of influenza?

In Italy, multiple H3N2 influenza viruses were isolated from chickens with mild respiratory disease and were shown to replicate in the respiratory tracts of experimentally infected chickens; this finding is the first to show that H3N2 influenza viruses can replicate and cause disease in chickens. H3N2 influenza viruses in pigs on nearby farms seemed a likely source of the virus; however, antigenic and molecular analyses revealed that the gene segments of the viruses in chickens were mainly of Eurasian avian origin and were distinguishable from those isolated from pigs and wild aquatic birds in Italy. Thus, several different H3 influenza viruses were circulating in Italy, but we failed to identify the source of the chicken H3N2 influenza viruses that have disappeared subsequently from Italian poultry. Until recently, the transmission of influenza viruses (other than the H5 and H7 subtypes) from their reservoir in aquatic birds to chickens was rarely detected and highly pathogenic and non-pathogenic viruses were considered to be restricted to poultry species. However, the recent reports of the transmission of H9N2 and H5N1 influenza viruses to chickens in Hong Kong and, subsequently, to humans and our findings of the transmission of H3N2 influenza viruses to domestic chickens in Italy suggest an increased role for chickens as an intermediate host in the ecology of influenza.

Characterization of the influenza A virus gene pool in avian species in southern China: was H6N1 a derivative or a precursor of H5N1?

In 1997, an H5N1 influenza virus outbreak occurred in chickens in Hong Kong, and the virus was transmitted directly to humans. Because there is limited information about the avian influenza virus reservoir in that region, we genetically characterized virus strains isolated in Hong Kong during the 1997 outbreak. We sequenced the gene segments of a heterogeneous group of viruses of seven different serotypes (H3N8, H4N8, H6N1, H6N9, H11N1, H11N9, and H11N8) isolated from various bird species. The phylogenetic relationships divided these viruses into several subgroups. An H6N1 virus isolated from teal (A/teal/Hong Kong/W312/97 [H6N1]) showed very high (>98%) nucleotide homology to the human influenza virus A/Hong Kong/156/97 (H5N1) in the six internal genes. The N1 neuraminidase sequence showed 97% nucleotide homology to that of the human H5N1 virus, and the N1 protein of both viruses had the same 19-amino-acid deletion in the stalk region. The deduced hemagglutinin amino acid sequence of the H6N1 virus was most similar to that of A/shearwater/Australia/1/72 (H6N5). The H6N1 virus is the first known isolate with seven H5N1-like segments and may have been the donor of the neuraminidase and the internal genes of the H5N1 viruses. The high homology between the internal genes of H9N2, H6N1, and the H5N1 isolates indicates that these subtypes are able to exchange their internal genes and are therefore a potential source of new pathogenic influenza virus strains. Our analysis suggests that surveillance for influenza A viruses should be conducted for wild aquatic birds as well as for poultry, pigs, and humans and that H6 isolates should be further characterized.

Immunology of avian influenza virus: a review

Avian influenza virus can cause serious disease in a wide variety of birds and mammals, but its natural host range is in wild ducks, gulls, and shorebirds. Infections in poultry can be inapparent or cause respiratory disease, decreases in production, or a rapidly fatal systemic disease known as highly pathogenic avian influenza (HPAI). For the protection of poultry, neutralizing antibody to the hemagglutinin and neuraminidase proteins provide the primary protection against disease. A variety of vaccines elicit neutralizing antibody, including killed whole virus vaccines and fowl-pox recombinant vaccines. Antigenic drift of influenza viruses appears to be less important in causing vaccine failures in poultry as compared to humans. The cytotoxic T lymphocyte response can reduce viral shedding in mildly pathogenic avian influenza viruses, but provides questionable protection against HPAI. Influenza viruses can directly affect the immune response of infected birds, and the role of the Mx gene, interferons, and other cytokines in protection from disease remains unknown.

Natural and 'in vitro' selected antigenic variants of influenza A virus (H2N2)

We provide data on the prevalence of SRH antibody to influenza A/Singapore/1/57 (H2N2). Approximately 10.3% of sera had antibody to the influenza A (H2N2) subtype virus in comparison to the 36.9% of positive sera to a representative influenza A (H3N2) and 31.5% to influenza A (H1N1) viruses. The percentage of subjects with antibody constantly decreased from the older to the younger age groups. Persons born after 1968 were essentially seronegative, whereas subjects born before 1900, and in the decade 1950-1959, showed the highest antibody levels to influenza A (H2N2) viruses. These age groups also appeared to have 'protective' levels of anti-HA antibody to influenza A (H2N2) virus. An antigenic variant of A/Singapore/1/57 virus was selected in the laboratory using a monoclonal antibody to HA. Serological comparison of the new in vitro variant with the parental virus and two naturally occurring viruses, namely A/England/12/64 and Tokyo/3/67, showed that certain human sera were able to distinguish the variant, indicating a restricted antibody repertoire in these adult and children's sera, providing an explanation of how such variants could actually arise in nature.

Continued evolution of H1N1 and H3N2 influenza viruses in pigs in Italy

Swine influenza viruses possessing avian genes were first detected in Europe in 1979 (Scholtissek et al., 1983, Virology, 129, 521-523) and continue to circulate in pigs in that region of the world. To characterize the molecular epidemiology of swine influenza viruses currently circulating in Europe, we used dot-blot hybridization and sequence analysis to determine the origin of the genes encoding the nonsurface proteins ("internal" genes) of 10 H1N1 and 11 H3N2 swine influenza viruses isolated in Italy between 1992 and 1995. All of the 126 genes examined were of avian origin; thus the currently circulating H3N2 strains which possess A/Port Chalmers/1/73-like surface glycoproteins appear to be descendants of the reassortant human-avian viruses that emerged between 1983 and 1985 in Italy. Sequence analysis of matrix (M), nonstructural, and nucleoprotein genes, as well as phylogenetic analysis of M gene showed that the H1N1 and H3N2 viruses from the pigs were closely related to recent isolates of the avian-like swine H1N1 influenza strain currently circulating in northern Europe and were distinguishable from the genes of viruses isolated from European swine in 1979. To evaluate the frequency of transmission of swine H1N1 and H3N2 viruses to man, we tested 123 human sera for hemagglutination-inhibiting antibodies against avian and mammalian H1N1 and H3N2 virus strains. Our findings indicate that swine influenza viruses possessing A/Port Chalmers/1/73-like hemagglutinin may have transmitted to approximately 20% of young persons under 20 years of age who had contact with pigs. Thus, H3N2 swine viruses, possibly possessing avian-derived internal genes, may be entering humans more often than was previously thought. We strongly recommend that pigs be regularly monitored as a potential early warning system for detection of future pandemic strains.

Emergence of avian H1N1 influenza viruses in pigs in China

Avian influenza A viruses from Asia are recognized as the source of genes that reassorted with human viral genes to generate the Asian/57 (H2N2) and Hong Kong/68 (H3N2) pandemic strains earlier in this century. Here we report the genetic analysis of avian influenza A H1N1 viruses recently isolated from pigs in southern China, a host suspected to generate new pandemic strains through gene reassortment events. Each of the eight gene segments was of avian origin. Phylogenetic analysis indicates that these genes form an Asian sublineage of the Eurasian avian lineage, suggesting that these viruses are an independent introduction into pigs in Asia. The presence of avian influenza viruses in pigs in China places them in an optimal position for transmission to humans and may serve as an early warning of the emergence of the next human influenza virus pandemic.

An epidemiological study of influenza viruses among Chinese farm families with household ducks and pigs

To examine the possibility of interspecies transmission and genetic reassortment of influenza viruses on farms in Southern China, we surveyed 20 farm families living outside the city of Nanchang who raised pigs and ducks in their homes. Weekly interviews of family members and virus isolation studies of throat swabs and faecal samples, collected from September 1992 to September 1993, established the seasonal pattern of respiratory tract infections in these families and identified 11 influenza viruses (6 in humans and 5 in ducks). Most of the human isolates were type A of H3N2 subtype. Serologic studies of farm pigs indicated infection by the same human viruses circulating in family members, but there was no evidence that either swine or avian viruses had been transmitted to pigs. Eight of 156 human serum samples inhibited the neuraminidase activity of two of the duck isolates, raising the possibility of interspecies transmission of these avian viruses. Genotype analysis of duck and human isolates provided no evidence for reassortment. Our finding support the concept that intermingling of humans, pigs and ducks on Chinese farms is favourable to the generation of new, potentially hazardous strains of influenza virus.