Swine influenza viruses a North American perspective

Population susceptibility to North American and Eurasian swine influenza viruses in England, at three time points between 2004 and 2011

Age-stratified sera collected in 2004, 2008 and 2010 in England were evaluated for antibody to swine influenza A(H3N2) and A(H1N1) viruses from the United States or Europe as a measure of population susceptibility to the emergence of novel viruses. Children under 11 years of age had little or no measurable antibody to recent swine H3N2 viruses despite their high levels of antibody to recent H3N2 seasonal human strains. Adolescents and young adults (born 1968–1999) had higher antibody levels to swine H3N2 viruses. Antibody levels to swine H3N2 influenza show little correlation with exposure to recent seasonal H3N2 (A/Perth/16/2009) strains, but with antibody to older H3N2 strains represented by A/Wuhan/359/1995. Children had the highest seropositivity to influenza A(H1N1)pdm09 virus, and young adults had the lowest antibody levels to A/Perth/16/2009. No age group showed substantial antibody levels to A/Aragon/RR3218/2008, a European swine H1N1 virus belonging to the Eurasian lineage. After vaccination with contemporary trivalent vaccine we observed evidence of boosted reactivity to swine H3N2 viruses in children and adults, while only a limited boosting effect on antibody levels to A/Aragon/RR3218/2008 was observed in both groups. Overall, our results suggest that different vaccination strategies may be necessary according to age if swine viruses emerge as a significant pandemic threat.

Genetic characterization of influenza virus circulating in Brazilian pigs during 2009 and 2010 reveals a high prevalence of the pandemic H1N1 subtype

BACKGROUND

Influenza A viruses circulating in pigs in Brazil are still not characterized, and only limited data are available about swine influenza epidemiology in the country. Therefore, we characterized the hemagglutinin (HA) and neuraminidase (NA) genes of influenza viruses isolated from Brazilian pigs. We also evaluated one case of probable swine-to-human transmission.

METHODS

Twenty influenza viruses isolated from pigs during 2009-2010 in five Brazilian states (Minas Gerais, Sao Paulo, Parana, Rio Grande do Sul, and Mato Grosso) were used. One human isolate, from a technician who became ill after visiting a swineherd going through a respiratory disease outbreak, was also used in the study. Phylogenetic analysis for the HA and NA genes and hemagglutinin amino acid sequence alignment were performed.

RESULTS

All isolates clustered with pandemic H1N1 2009 (pH1N1) viruses and appeared to have a common ancestor. Genetic diversity was higher in the HA than in the NA gene, and the amino acid substitution S203T in one of HA's antigenic sites was found in most of the samples. The human isolate was more related to swine isolates from the same herd visited by the technician than to other human isolates, suggesting swine-to-human transmission.

CONCLUSION

Our results show that pH1N1 was disseminated and the predominant subtype in Brazilian pigs in 2009-2010.

Antiviral responses by Swine primary bronchoepithelial cells are limited compared to human bronchoepithelial cells following influenza virus infection

Swine generate reassortant influenza viruses because they can be simultaneously infected with avian and human influenza; however, the features that restrict influenza reassortment in swine and human hosts are not fully understood. Type I and III interferons (IFNs) act as the first line of defense against influenza virus infection of respiratory epithelium. To determine if human and swine have different capacities to mount an antiviral response the expression of IFN and IFN-stimulated genes (ISG) in normal human bronchial epithelial (NHBE) cells and normal swine bronchial epithelial (NSBE) cells was evaluated following infection with human (H3N2), swine (H1N1), and avian (H5N3, H5N2, H5N1) influenza A viruses. Expression of IFNλ and ISGs were substantially higher in NHBE cells compared to NSBE cells following H5 avian influenza virus infection compared to human or swine influenza virus infection. This effect was associated with reduced H5 avian influenza virus replication in human cells at late times post infection. Further, RIG-I expression was lower in NSBE cells compared to NHBE cells suggesting reduced virus sensing. Together, these studies identify key differences in the antiviral response between human and swine respiratory epithelium alluding to differences that may govern influenza reassortment.

Efficacy in pigs of inactivated and live attenuated influenza virus vaccines against infection and transmission of an emerging H3N2 similar to the 2011-2012 H3N2v

Vaccines provide a primary means to limit disease but may not be effective at blocking infection and pathogen transmission. The objective of the present study was to evaluate the efficacy of commercial inactivated swine influenza A virus (IAV) vaccines and experimental live attenuated influenza virus (LAIV) vaccines against infection with H3N2 virus and subsequent indirect transmission to naive pigs. The H3N2 virus evaluated was similar to the H3N2v detected in humans during 2011-2012, which was associated with swine contact at agricultural fairs. One commercial vaccine provided partial protection measured by reduced nasal shedding; however, indirect contacts became infected, indicating that the reduction in nasal shedding did not prevent aerosol transmission. One LAIV vaccine provided complete protection, and none of the indirect-contact pigs became infected. Clinical disease was not observed in any group, including nonvaccinated animals, a consistent observation in pigs infected with contemporary reassortant H3N2 swine viruses. Serum hemagglutination inhibition antibody titers against the challenge virus were not predictive of efficacy; titers following vaccination with a LAIV that provided sterilizing immunity were below the level considered protective, yet titers in a commercial vaccine group that was not protected were above that level. While vaccination with currently approved commercial inactivated products did not fully prevent transmission, certain vaccines may provide a benefit by limitating shedding, transmission, and zoonotic spillover of antigenically similar H3N2 viruses at agriculture fairs when administered appropriately and used in conjunction with additional control measures.

Antigenic characterization of H3N2 influenza A viruses from Ohio agricultural fairs

The demonstrated link between the emergence of H3N2 variant (H3N2v) influenza A viruses (IAVs) and swine exposure at agricultural fairs has raised concerns about the human health risk posed by IAV-infected swine. Understanding the antigenic profiles of IAVs circulating in pigs at agricultural fairs is critical to developing effective prevention and control strategies. Here, 68 H3N2 IAV isolates recovered from pigs at Ohio fairs (2009 to 2011) were antigenically characterized. These isolates were compared with other H3 IAVs recovered from commercial swine, wild birds, and canines, along with human seasonal and variant H3N2 IAVs. Antigenic cartography demonstrated that H3N2 IAV isolates from Ohio fairs could be divided into two antigenic groups: (i) the 2009 fair isolates and (ii) the 2010 and 2011 fair isolates. These same two antigenic clusters have also been observed in commercial swine populations in recent years. Human H3N2v isolates from 2010 and 2011 are antigenically clustered with swine-origin IAVs from the same time period. The isolates recovered from pigs at fairs did not cross-react with ferret antisera produced against the human seasonal H3N2 IAVs circulating during the past decade, raising the question of the degree of immunity that the human population has to swine-origin H3N2 IAVs. Our results demonstrate that H3N2 IAVs infecting pigs at fairs and H3N2v isolates were antigenically similar to the IAVs circulating in commercial swine, demonstrating that exhibition swine can function as a bridge between commercial swine and the human population.

The need to include animal protection in public health policies

Many critical public health issues require non-traditional approaches. Although many novel strategies are used, one approach not widely applied involves improving the treatment of animals. Emerging infectious diseases are pressing public health challenges that could benefit from improving the treatment of animals. Other human health issues, that overlap with animal treatment issues, and that warrant further exploration, are medical research and domestic violence. The diverse nature of these health issues and their connection with animal treatment suggest that there may be other similar intersections. Public health would benefit by including the treatment of animals as a topic of study and policy development.

Influenza virus reassortment occurs with high frequency in the absence of segment mismatch

Reassortment is fundamental to the evolution of influenza viruses and plays a key role in the generation of epidemiologically significant strains. Previous studies indicate that reassortment is restricted by segment mismatch, arising from functional incompatibilities among components of two viruses. Additional factors that dictate the efficiency of reassortment remain poorly characterized. Thus, it is unclear what conditions are favorable for reassortment and therefore under what circumstances novel influenza A viruses might arise in nature. Herein, we describe a system for studying reassortment in the absence of segment mismatch and exploit this system to determine the baseline efficiency of reassortment and the effects of infection dose and timing. Silent mutations were introduced into A/Panama/2007/99 virus such that high-resolution melt analysis could be used to differentiate all eight segments of the wild-type and the silently mutated variant virus. The use of phenotypically identical parent viruses ensured that all progeny were equally fit, allowing reassortment to be measured without selection bias. Using this system, we found that reassortment occurred efficiently (88.4%) following high multiplicity infection, suggesting the process is not appreciably limited by intracellular compartmentalization. That co-infection is the major determinant of reassortment efficiency in the absence of segment mismatch was confirmed with the observation that the proportion of viruses with reassortant genotypes increased exponentially with the proportion of cells co-infected. The number of reassortants shed from co-infected guinea pigs was likewise dependent on dose. With 10⁶ PFU inocula, 46%–86% of viruses isolated from guinea pigs were reassortants. The introduction of a delay between infections also had a strong impact on reassortment and allowed definition of time windows during which super-infection led to reassortment in culture and in vivo. Overall, our results indicate that reassortment between two like influenza viruses is efficient but also strongly dependent on dose and timing of the infections.

Reassortment is the process by which influenza viruses, which carry RNA genomes comprising eight segments, exchange genetic material. Reassortment of the genome segments of two differing influenza strains has the potential to vastly increase the diversity of circulating influenza viruses. Despite its importance to influenza virus evolution, the frequency with which reassortment occurs in a cell or an animal infected with two or more variant viruses is unclear. Toward determining how readily reassortment can occur, we assessed the incidence of reassortment during experimental infection in cultured cells and in guinea pigs. We found that reassortment can occur with high efficiency in both systems, but that that efficiency is dependent on i) the dose of each virus added to the cells or taken up by the host and ii) the relative timing with which each virus infects. These results suggest that influenza A virus reassortment may be more prevalent in nature than one might expect based on the results of surveillance studies.

Genetic characterization of Thai swine influenza viruses after the introduction of pandemic H1N1 2009

Pandemic H1N1 2009 (pH1N1), influenza virus containing triple reassortant internal genes (TRIG) from avian, human, and swine influenza viruses emerged in 2009 as a highly infectious virus that was able to be transmitted from humans to pigs. During June 2010-May 2012, influenza virus surveillance was conducted in Thai pig population. Twenty-three samples (1.75%) were successfully isolated from total of 1,335 samples. Interestingly, pH1N1 (7 isolates, 30.34%), reassortant pH1N1 (rH1N1) (1 isolate, 4.35%), Thai endemic H1N1 (enH1N1) (3 isolates, 13.04%), reassortant H3N2 with pH1N1 internal genes (rH3N2) (9 isolates, 39.13%), and reassortant H1N2 with pH1N1 internal genes (rH1N2) (3 isolates, 13.04%) were found. It should be noted that rH1N1, rH1N2, and rH3N2 viruses contained the internal genes of pH1N1 virus having a TRIG cassette descendant from the North American swine lineage. Although all isolates in this study were obtained from mild clinically sick pigs, the viruses were still highly infective and possibly may play an important role in human-animal interfacing transmission. In addition, the TRIG cassette may have an influence on antigenic shift resulting in emergence of novel viruses, as seen in this study. Continuing surveillance of influenza A natural hosts, particularly in pigs is necessary.

Subclinical influenza virus A infections in pigs exhibited at agricultural fairs, Ohio, USA, 2009-2011

Close contact between pigs and humans could result in zoonotic transmission.

Agricultural fairs are associated with bidirectional, interspecies transmission of influenza virus A between humans and pigs. We examined pigs exhibited at agricultural fairs in Ohio during 2009–2011 for signs of influenza-like illness and collected nasal swab specimens from a representative subset of these animals. Influenza virus A was recovered from pigs at 12/53 (22.6%) fairs during the 3-year sampling period. Pigs at 10/12 (83.3%) fairs from which influenza virus A was recovered did not show signs of influenza-like illness. Hemagglutinin, neuraminidase, and matrix gene combinations of the isolates were consistent with influenza virus A concurrently circulating among swine herds in the United States. Subclinical influenza virus A infections in pigs at agricultural fairs may pose a risk to human health and create challenges for passive surveillance programs for influenza virus A in swine herds.

Vaccine development for protecting swine against influenza virus

Influenza virus infects a wide variety of species including humans, pigs, horses, sea mammals and birds. Weight loss caused by influenza infection and/or co-infection with other infectious agents results in significant financial loss in swine herds. The emergence of pandemic H1N1 (A/CA/04/2009/H1N1) and H3N2 variant (H3N2v) viruses, which cause disease in both humans and livestock constitutes a concerning public health threat. Influenza virus contains eight single-stranded, negative-sense RNA genome segments. This genetic structure allows the virus to evolve rapidly by antigenic drift and shift. Antigen-specific antibodies induced by current vaccines provide limited cross protection to heterologous challenge. In pigs, this presents a major obstacle for vaccine development. Different strategies are under development to produce vaccines that provide better cross-protection for swine. Moreover, overriding interfering maternal antibodies is another goal for influenza vaccines in order to permit effective immunization of piglets at an early age. Herein, we present a review of influenza virus infection in swine, including a discussion of current vaccine approaches and techniques used for novel vaccine development.

Experimental infection with a Thai reassortant swine influenza virus of pandemic H1N1 origin induced disease

BACKGROUND

Following the emergence of the pandemic H1N1 influenza A virus in 2009 in humans, this novel virus spread into the swine population. Pigs represent a potential host for this virus and can serve as a mixing vessel for genetic mutations of the influenza virus. Reassortant viruses eventually emerged from the 2009 pandemic and were reported in swine populations worldwide including Thailand. As a result of the discovery of this emergent disease, pathogenesis studies of this novel virus were conducted in order that future disease protection and control measures in swine and human populations could be enacted.

METHODS

The pandemic H1N1 2009 virus (pH1N1) and its reassortant virus (rH1N1) isolated from pigs in Thailand were inoculated into 2 separate cohorts of 9, 3-week-old pigs. Cohorts were consisted of one group experimentally infected with pH1N1 and one group with rH1N1. A negative control group consisting of 3 pigs was also included. Clinical signs, viral shedding and pathological lesions were investigated and compared. Later, 3 pigs from viral inoculated groups and 1 pig from the control group were necropsied at 2, 4, and 12 days post inoculation (DPI).

RESULTS

The results indicated that pigs infected with both viruses demonstrated typical flu-like clinical signs and histopathological lesions of varying severity. Influenza infected-pigs of both groups had mild to moderate pulmonary signs on 1-4 DPI. Interestingly, pigs in both groups demonstrated viral RNA detection in the nasal swabs until the end of the experiment (12 DPI).

CONCLUSION

The present study demonstrated that both the pH1N1 and rH1N1 influenza viruses, isolated from naturally infected pigs, induced acute respiratory disease in experimentally inoculated nursery pigs. Although animals in the rH1N1-infected cohort demonstrated more severe clinical signs, had higher numbers of pigs shedding the virus, were noted to have increased histopathological severity of lung lesions and increased viral antigen in lung tissue, the findings were not statistically significant in comparison with the pH1N1-infected group. Interestingly, viral genetic material of both viruses could be detected from the nasal swabs until the end of the experiment. Similar to other swine influenza viruses, the clinical signs and pathological lesions in both rH1N1 and pH1N1 were limited to the respiratory tract.

Influenza A(H1N1)pdm09 virus among healthy show pigs, United States

Because animals can transmit some diseases to people, it is wise to be cautious around animals that carry these diseases. But how do you know which animals are carrying disease? Sometimes they appear perfectly healthy. A study of 57 apparently healthy show pigs at a 2009 US state fair found that almost 20% were carrying influenza virus and at least 4 were carrying the 2009 pandemic virus. Of concern is the possibility that different types of influenza virus—pandemic, swine, avian—could combine in pigs and emerge as new viruses that then spread to humans. Swine workers, veterinarians, and other persons with pig contact may be at high risk for infection with pig influenza and should receive seasonal influenza vaccines, use personal protective equipment when working with healthy pigs, and limit their contact with sick pigs. Regular monitoring of influenza virus among pigs and testing of sick persons who have been exposed to pigs are needed.

Within 5 months after the earliest detection of human influenza A(H1N1)pdm09 virus, we found molecular and culture evidence of the virus in healthy US show pigs. The mixing of humans and pigs at swine shows possibly could further the geographic and cross-species spread of influenza A viruses.

Orientações para o diagnóstico de influenza em suínos

Este trabalho descreve a colheita adequada de amostras, as técnicas/procedimentos disponíveis para o diagnóstico de influenza A em suínos, assim como os resultados e suas respectivas interpretações, para auxiliar médicos veterinários de campo na identificação dessa doença. Em suínos vivos, as amostras adequadas são: secreção nasal, fluido oral e sangue (soro). Para suínos mortos, colher preferencialmente amostras de pulmão com consolidação cranioventral. Secreção nasal e fragmentos de pulmão refrigerado são utilizados para detectar partícula viral viável (isolamento viral - IV) ou ácido nucleico viral (RT-PCR convencional e RT-PCR em tempo real). As amostras não devem ser congeladas, pois o vírus é inativado a -20°C. A caracterização molecular dos isolados é feita pela análise filogenética obtida pelo sequenciamento de DNA. O soro é utilizado para a detecção de anticorpos (Acs) por meio do teste da inibição da hemaglutinação e ELISA. O fluido oral pode ser utilizado para detecção de anticorpo (ELISA) ou de vírus. Fragmentos de pulmão fixados em formol a 10% são examinados microscopicamente para identificar pneumonia broncointersticial e para detecção de antígeno viral pela imuno-histoquímica (IHQ). Para o sucesso do diagnóstico, as amostras devem ser colhidas de suínos que estão preferencialmente na fase aguda da doença, para aumentar as chances de detecção viral. As melhores opções para o diagnóstico de influenza A em suínos vivos são RT-PCR e isolamento viral de amostras de swab nasal ou fluido oral. Pulmão para análise por RT-PCR, isolamento viral ou IHQ é a amostra de escolha em suínos mortos. Testes sorológicos têm valor diagnóstico limitado e são utilizados apenas para determinar o estado imune do rebanho, não indicando doença clínica, pois os Acs são detectados 7-10 dias pós-infecção (fase subaguda). O diagnóstico de influenza é importante para avaliar o envolvimento desse agente no complexo de doença respiratória suína. Além disso, o isolamento do vírus influenza é essencial para o monitoramento dos principais subtipos circulantes em uma determinada região ou país, assim como para a detecção de novos rearranjos virais, já que influenza é considerada uma zoonose.

Diagnostics and surveillance for Swine influenza

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

Estimating reassortment rates in co-circulating Eurasian swine influenza viruses

Swine have often been considered as a mixing vessel for different influenza strains. In order to assess their role in more detail, we undertook a retrospective sequencing study to detect and characterize the reassortants present in European swine and to estimate the rate of reassortment between H1N1, H1N2 and H3N2 subtypes with Eurasian (avian-like) internal protein-coding segments. We analysed 69 newly obtained whole genome sequences of subtypes H1N1-H3N2 from swine influenza viruses sampled between 1982 and 2008, using Illumina and 454 platforms. Analyses of these genomes, together with previously published genomes, revealed a large monophyletic clade of Eurasian swine-lineage polymerase segments containing H1N1, H1N2 and H3N2 subtypes. We subsequently examined reassortments between the haemagglutinin and neuraminidase segments and estimated the reassortment rates between lineages using a recently developed evolutionary analysis method. High rates of reassortment between H1N2 and H1N1 Eurasian swine lineages were detected in European strains, with an average of one reassortment every 2-3 years. This rapid reassortment results from co-circulating lineages in swine, and in consequence we should expect further reassortments between currently circulating swine strains and the recent swine-origin H1N1v pandemic strain.

Histopathological and immunohistochemical findings of swine with spontaneous influenza A infection in Brazil, 2009-2010

Swine influenza (SI) is caused by the type A swine influenza virus (SIV). It is a highly contagious disease with a rapid course and recovery. The major clinical signs and symptoms are cough, fever, anorexia and poor performance. The disease has been associated with other co-infections in many countries, but not in Brazil, where, however, the first outbreak has been reported in 2011. The main aim of this study was to characterize the histological features in association with the immunohistochemical (IHC) results for influenza A (IA), porcine circovirus type 2 (PCV2) and porcine reproductive and respiratory syndrome virus (PRRSV) in lung samples from 60 pigs submitted to Setor de Patologia Veterinária at the Universidade Federal do Rio Grande do Sul (SPV-UFRGS), Brazil, during 2009-2010. All of these lung samples had changes characterized by interstitial pneumonia with necrotizing bronchiolitis, never observed previously in the evaluation of swine lungs in our laboratory routine. Pigs in this study had showed clinical signs of a respiratory infection. Swine samples originated from Rio Grande do Sul 31 (52%), Santa Catarina 14 (23%), Paraná 11 (18%), and Mato Grosso do Sul 4 (7%). Positive anti-IA IHC labelling was observed in 45% of the cases, which were associated with necrotizing bronchiolitis, atelectasis, purulent bronchopneumonia and hyperemia. Moreover, type II pneumocyte hyperplasia, alveolar and bronchiolar polyp-like structures, bronchus-associated lymphoid tissue (BALT) hyperplasia and pleuritis were the significant features in negative anti-IA IHC, which were also associated with chronic lesions. There were only two cases with positive anti-PCV2 IHC and none to PRRSV. Therefore, SIV was the predominant infectious agent in the lung samples studied. The viral antigen is often absent due to the rapid progress of SI, which may explain the negative IHC results for IA (55%); therefore, IHC should be performed at the beginning of the disease. This study has shown how important a careful histological evaluation is for the diagnosis. Since 2009, a new histological feature of swine pneumonia in animals with respiratory clinical signs has been observed in samples from pigs with clinical respiratory disease submitted to SPV-UFRGS. In addition, the results proved the importance of histological evaluation for swine herd health management.

Seasonal trivalent inactivated influenza vaccine does not protect against newly emerging variants of influenza A (H3N2v) virus in ferrets

The recent increase in human cases of influenza A H3N2 variant virus [A(H3N2)v] highlights the need to assess whether seasonal influenza vaccination provides cross-protection against A(H3N2)v virus. Our data demonstrate that the 2011-2012 trivalent inactivated influenza vaccine (TIV) protected ferrets against homologous H3N2 virus challenge but provided minimal to no protection against A(H3N2)v virus. The complete absence of specific hemagglutination inhibition antibody response to A(H3N2)v is consistent with the poor cross-protection observed among TIV-immune animals.

Comparison of Human-Like H1 (δ-Cluster) Influenza A Viruses in the Swine Host

Influenza A viruses cause acute respiratory disease in swine. Viruses with H1 hemagglutinin genes from the human seasonal lineage (δ-cluster) have been isolated from North American swine since 2003. The objective of this work was to study the pathogenesis and transmission of δ-cluster H1 influenza viruses in swine, comparing three isolates from different phylogenetic subclusters, geographic locations, and years of isolation. Two isolates from the δ2 subcluster, A/sw/MN/07002083/07 H1N1 (MN07) and A/sw/IL/00685/05 H1N1 (IL05), and A/sw/TX/01976/08 H1N2 (TX08) from the δ1 sub-cluster were evaluated. All isolates caused disease and were transmitted to contact pigs. Respiratory disease was apparent in pigs infected with MN07 and IL05 viruses; however, clinical signs and lung lesions were reduced in severity as compared to TX08. On day 5 following infection MN07-infected pigs had lower virus titers than the TX08 pigs, suggesting that although this H1N1 was successfully transmitted, it may not replicate as efficiently in the upper or lower respiratory tract. MN07 and IL05 H1N1 induced higher serum antibody titers than TX08. Greater serological cross-reactivity was observed for viruses from the same HA phylogenetic sub-cluster; however, antigenic differences between the sub-clusters may have implications for disease control strategies for pigs.

Neutralizing DNA aptamers against swine influenza H3N2 viruses

Triple reassortant influenza A viruses (IAVs) of swine, particularly the North American H3N2 subtype, circulate in swine herds and may reassort and result in the emergence of novel zoonotic strains. Current diagnostic tools rely on isolation of the viruses, followed by serotyping by hemagglutination or genome sequencing, both of which can be expensive and time-consuming. Thus, novel subtype-specific ligands and methods are needed for rapid testing and subtyping of IAVs in the field. To address this need, we selected DNA aptamers against the recombinant HA protein from swine IAV H3 cluster IV using systematic evolution of ligands by exponential enrichment (SELEX). Four candidate aptamers (HA68, HA7, HA2a, and HA2b) were identified and characterized. The dissociation constants (K(d)) of aptamers HA68, HA7, HA2a, and HA2b against recombinant H3 protein were 7.1, 22.3, 16.0, and 3.7 nM, respectively. The binding site of HA68 to H3 was identified to be between nucleotide residues 8 and 40. All aptamers inhibited H3 hemagglutination. HA68 was highly specific to all four lineages within the North American H3N2 subtype. Further, the other three aptamers specifically identified live viruses belonging to the phylogenetic clusters I, II/III, and IV especially the virus that closely related to the recent H3N2 variant (H3N2v). Aptamer HA68 was also able to bind and detect H3N2v isolated from recent human cases. In conclusion, we provide subtype-specific aptamers against H3N2 IAVs of swine that can now be used in rapid detection and typing protocols for field applications.

Full genomic analysis of an influenza A (H1N2) virus identified during 2009 pandemic in Eastern India: evidence of reassortment event between co-circulating A(H1N1)pdm09 and A/Brisbane/10/2007-like H3N2 strains

BACKGROUND

During the pandemic [Influenza A(H1N1)pdm09] period in 2009-2010, an influenza A (Inf-A) virus with H1N2 subtype (designated as A/Eastern India/N-1289/2009) was detected from a 25 years old male from Mizoram (North-eastern India).

OBJECTIVE

To characterize full genome of the H1N2 influenza virus.

METHODS

For initial detection of Influenza viruses, amplification of matrix protein (M) gene of Inf-A and B viruses was carried out by real time RT-PCR. Influenza A positive viruses are then further subtyped with HA and NA gene specific primers. Sequencing and the phylogenetic analysis was performed for the H1N2 strain to understand its origin.

RESULTS

The outcome of this full genome study revealed a unique reassortment event where the N-1289 virus acquired it's HA gene from a 2009 pandemic H1N1 virus with swine origin and the other genes from H3N2-like viruses of human origin.

CONCLUSIONS

This study provides information on possibility of occurrence of reassortment events during influenza season when infectivity is high and two different subtypes of Inf-A viruses co-circulate in same geographical location.

Genomic reassortment of influenza A virus in North American swine, 1998-2011

Revealing the frequency and determinants of reassortment among RNA genome segments is fundamental to understanding basic aspects of the biology and evolution of the influenza virus. To estimate the extent of genomic reassortment in influenza viruses circulating in North American swine, we performed a phylogenetic analysis of 139 whole-genome viral sequences sampled during 1998-2011 and representing seven antigenically distinct viral lineages. The highest amounts of reassortment were detected between the H3 and the internal gene segments (PB2, PB1, PA, NP, M and NS), while the lowest reassortment frequencies were observed among the H1γ, H1pdm and neuraminidase segments, particularly N1. Less reassortment was observed among specific haemagglutinin-neuraminidase combinations that were more prevalent in swine, suggesting that some genome constellations may be evolutionarily more stable.

6th International Conference on Emerging Zoonoses

The 6th International Conference on Emerging Zoonoses, held at Cancun, Mexico, 24-27 February 2011, offered 84 participants from 18 countries, a snapshot of current research in numerous zoonoses caused by viruses, bacteria or prions. Co-chaired by Professors Heinz Feldmann and Jürgen Richt, the conference explored 10 topics: (i) The ecology of emerging zoonotic diseases; (ii) The role of wildlife in emerging zoonoses; (iii) Cross-species transmission of zoonotic pathogens; (iv) Emerging and neglected influenza viruses; (v) Haemorrhagic fever viruses; (vi) Emerging bacterial diseases; (vii) Outbreak responses to zoonotic diseases; (viii) Food-borne zoonotic diseases; (ix) Prion diseases; and (x) Modelling and prediction of emergence of zoonoses. Human medicine, veterinary medicine and environmental challenges are viewed as a unity, which must be considered under the umbrella of 'One Health'. Several presentations attempted to integrate the insights gained from field data with mathematical models in the search for effective control measures of specific zoonoses. The overriding objective of the research presentations was to create, improve and use the tools essential to address the risk of contagions in a globalized society. In seeking to fulfil this objective, a three-step approach has often been applied: (i) use cultured cells, model and natural animal hosts and human clinical models to study infection; (ii) combine traditional histopathological and biochemical approaches with functional genomics, proteomics and computational biology; and (iii) obtain signatures of virulence and insights into mechanisms of host defense response, immune evasion and pathogenesis. This meeting review summarizes 39 of the conference presentations and mentions briefly the 16 articles in this Special Supplement, most of which were presented at the conference in earlier versions. The full affiliations of all presenters and many colleagues have been included to facilitate further inquiries from readers.

Pandemism of swine flu and its prospective drug therapy

Swine flu is a respiratory disease caused by influenza A H1N1 virus. The current pandemic of swine flu is most probably due to a mutation-more specifically, a re-assortment of four known strains of influenza A virus subtype H1N1. Antigenic variation of influenza viruses while circulating in the population is an important factor leading to difficulties in controlling influenza by vaccination. Due to the global effect of swine flu and its effect on humans, extensive investigations are being undertaken. In this context, Tamiflu is the only available drug used in the prophylaxis of this disease and is made from the compound shikimic acid. Due to the sudden increase in the demand of shikimic acid, its price has increased greatly. Thus, it is necessary to find an alternative approach for the treatment of swine flu. This review presents the overall information of swine flu, beginning from its emergence to the prevention and treatment of the disease, with a major emphasis on the alternative approach (bacterial fermentation process) for the treatment of swine flu. The alternative approach for the treatment of swine flu includes the production of shikimic acid from a fermentation process and it can be produced in large quantities without any time limitations.

Isolation of the pandemic (H1N1) 2009 virus and its reassortant with an H3N2 swine influenza virus from healthy weaning pigs in Thailand in 2011

A total of 300 nasal swabs were collected from 5 pig farms in two provinces in the Eastern part of Thailand in February 2011 and were subjected to viral isolation of influenza A viruses. Two H3N2 and 6 H1N1 influenza A viruses were isolated from swabs collected from clinically healthy weaning pigs on farms in Chonburi and Chachoengsao provinces, respectively. The H3N2 isolates consisted of the hemagglutinin (HA) and neuraminidase (NA) genes closely related to Thai SIVs and derived from a cluster of human seasonal H3N2 strains circulating around 1996-1997. The remaining gene segments of the isolates originated from the Pandemic (H1N1) 2009 (A (H1N1) pdm09) virus. Antigenicity of the H3N2 isolates was distinguishable from a human seasonal vaccine strain in the 1996-1998 seasons that represented antigenicity of the seasonal strains around 1996-1998. Nasal swabs from a Chachoengsao farm yielded A (H1N1) pdm09 viruses in chicken embryonated eggs and MDCK cells. A (H1N1) pdm09 viruses isolated in this study grew poorly in MDCK cells. Deduced amino acid sequences of the HA1 region of the HA protein of egg isolated viruses were identical to the sequences directly amplified from original swab samples. Our result demonstrated that the A (H1N1) pdm09 virus has been established in the Thai pig population and this has resulted in genetic reassortment with Thai SIV that previously circulated among pigs.

Heightened adaptive immune responses following vaccination with a temperature-sensitive, live-attenuated influenza virus compared to adjuvanted, whole-inactivated virus in pigs

In the United States there are currently two influenza vaccine platforms approved for use in humans-conventional inactivated virus and live-attenuated influenza virus (LAIV). One of the major challenges for influenza A virus (IAV) vaccination is designing a platform that provides protection across strains. Pandemic H1N1 (pH1N1) IAV swept the globe in 2009 and crossed the species barrier, infecting swine in several countries. Pigs are a natural host for IAV and serve as a model for evaluating immune responses following vaccination and challenge. Recently, a temperature-sensitive (ts) LAIV was developed by introducing modifications in the polymerase genes of a swine-like triple reassortant (tr) virus and when paired with pandemic HA and NA, provided sterilizing immunity upon intratracheal challenge with virulent pH1N1 virus. The utility of a ts LAIV is expanded in this report to show vaccination of pigs induced a cell-mediated immune response characterized by an increased number of antigen-specific IFN-secreting cells and expanded T cell populations when compared to pigs vaccinated with a whole inactivated virus (WIV) vaccine. Following challenge, there was a significant increase in the percentage of proliferating lymphocytes in the LAIV group compared to the WIV group following restimulation with pH1N1 in vitro. Also, there was an increase in the percentage of CD4/CD8 double-positive memory T cells in LAIV vaccinated pigs compared to WIV vaccinated pigs. Hemagglutination inhibition and serum neutralization titers were significantly higher in the LAIV-vaccinated pigs compared to the WIV vaccinated pigs following the initial dose of vaccine. Taken together, these results indicate the ts LAIV vaccine, generated from a triple reassortant IAV, elicits greater cell-mediated and humoral immune responses in pigs.

DNA-vaccine platform development against H1N1 subtype of swine influenza A viruses

Swine influenza virus (SIV) is an important viral pathogen in pig populations. However, commercial vaccines cannot provide complete protection with induced humoral immunity only, and require frequent updates to fight against current isolates. DNA vaccination is an effective means of eliciting both arms of the immune system, the humoral and cellular immune responses. In this study, DNA vector pcDNA3.1 was inserted with a chimeric intron downstream of the CMV promoter region followed by a Kozak sequence to enhance the expression of gene inserts. The C-terminal of the VP22 gene (VP22c), encoding the tegument protein of bovine herpesvirus-1, was fused separately to the N-terminal of four quadruplicated epitopes: two B-cell epitopes (HA91-108 and M2e), and two T-cell epitopes (NP366-374 and NP380-393), which were conserved, at least among the three SIV subtypes prevailing in pig populations in North America. Linker -KK- was used to space between each copy of the two B-cell epitopes, and -RVKR- was used for the two T-cell epitopes, in order to enhance the presentation of epitopes to the immune system. The expression of epitopes was confirmed in in vitro transfection of 293FT cells, and higher percentages of epitope-positive cells were achieved from the plasmids containing VP22c than those without. After the DNA plasmids were administered to mice intramuscularly in combination or separately, or boosted with recombinant proteins of quadruplicated epitopes fused to VP22c, the vaccine stimulated the desired epitope-specific humoral immunity to the two B-cell epitopes, and cellular immunity to the epitope NP380-393. Our results indicate that plasmids with quadruplicated epitopes fused to the VP22c may be a potential vehicle in developing epitopes as vaccines against SIV.

Pathogenicity and transmissibility of North American triple reassortant swine influenza A viruses in ferrets

North American triple reassortant swine (TRS) influenza A viruses have caused sporadic human infections since 2005, but human-to-human transmission has not been documented. These viruses have six gene segments (PB2, PB1, PA, HA, NP, and NS) closely related to those of the 2009 H1N1 pandemic viruses. Therefore, understanding of these viruses' pathogenicity and transmissibility may help to identify determinants of virulence of the 2009 H1N1 pandemic viruses and to elucidate potential human health threats posed by the TRS viruses. Here we evaluated in a ferret model the pathogenicity and transmissibility of three groups of North American TRS viruses containing swine-like and/or human-like HA and NA gene segments. The study was designed only to detect informative and significant patterns in the transmissibility and pathogenicity of these three groups of viruses. We observed that irrespective of their HA and NA lineages, the TRS viruses were moderately pathogenic in ferrets and grew efficiently in both the upper and lower respiratory tracts. All North American TRS viruses studied were transmitted between ferrets via direct contact. However, their transmissibility by respiratory droplets was related to their HA and NA lineages: TRS viruses with human-like HA and NA were transmitted most efficiently, those with swine-like HA and NA were transmitted minimally or not transmitted, and those with swine-like HA and human-like NA (N2) showed intermediate transmissibility. We conclude that the lineages of HA and NA may play a crucial role in the respiratory droplet transmissibility of these viruses. These findings have important implications for pandemic planning and warrant confirmation.

Strain-dependent effects of PB1-F2 of triple-reassortant H3N2 influenza viruses in swine

The PB1-F2 protein of the influenza A viruses (IAVs) can act as a virulence factor in mice. Its contribution to the virulence of IAV in swine, however, remains largely unexplored. In this study, we chose two genetically related H3N2 triple-reassortant IAVs to assess the impact of PB1-F2 in virus replication and virulence in pigs. Using reverse genetics, we disrupted the PB1-F2 ORF of A/swine/Wisconsin/14094/99 (H3N2) (Sw/99) and A/turkey/Ohio/313053/04 (H3N2) (Ty/04). Removing the PB1-F2 ORF led to increased expression of PB1-N40 in a strain-dependent manner. Ablation of the PB1-F2 ORF (or incorporation of the N66S mutation in the PB1-F2 ORF, Sw/99 N66S) affected the replication in porcine alveolar macrophages of only the Sw/99 KO (PB1-F2 knockout) and Sw/99 N66S variants. The Ty/04 KO strain showed decreased virus replication in swine respiratory explants, whereas no such effect was observed in Sw/99 KO, compared with the wild-type (WT) counterparts. In pigs, PB1-F2 did not affect virus shedding or viral load in the lungs for any of these strains. Upon necropsy, PB1-F2 had no effect on the lung pathology caused by Sw/99 variants. Interestingly, the Ty/04 KO-infected pigs showed significantly increased lung pathology at 3 days post-infection compared with pigs infected with the Ty/04 WT strain. In addition, the pulmonary levels of interleukin (IL)-6, IL-8 and gamma interferon were regulated differentially by the expression of PB1-F2. Taken together, these results indicate that PB1-F2 modulates virus replication, virulence and innate immune responses in pigs in a strain-dependent fashion.

Conserved synthetic peptides from the hemagglutinin of influenza viruses induce broad humoral and T-cell responses in a pig model

Outbreaks involving either H5N1 or H1N1 influenza viruses (IV) have recently become an increasing threat to cause potential pandemics. Pigs have an important role in this aspect. As reflected in the 2009 human H1N1 pandemia, they may act as a vehicle for mixing and generating new assortments of viruses potentially pathogenic to animals and humans. Lack of universal vaccines against the highly variable influenza virus forces scientists to continuously design vaccines à la carte, which is an expensive and risky practice overall when dealing with virulent strains. Therefore, we focused our efforts on developing a broadly protective influenza vaccine based on the Informational Spectrum Method (ISM). This theoretical prediction allows the selection of highly conserved peptide sequences from within the hemagglutinin subunit 1 protein (HA1) from either H5 or H1 viruses which are located in the flanking region of the HA binding site and with the potential to elicit broader immune responses than conventional vaccines. Confirming the theoretical predictions, immunization of conventional farm pigs with the synthetic peptides induced humoral responses in every single pig. The fact that the induced antibodies were able to recognize in vitro heterologous influenza viruses such as the pandemic H1N1 virus (pH1N1), two swine influenza field isolates (SwH1N1 and SwH3N2) and a H5N1 highly pathogenic avian virus, confirm the broad recognition of the antibodies induced. Unexpectedly, all pigs also showed T-cell responses that not only recognized the specific peptides, but also the pH1N1 virus. Finally, a partial effect on the kinetics of virus clearance was observed after the intranasal infection with the pH1N1 virus, setting forth the groundwork for the design of peptide-based vaccines against influenza viruses. Further insights into the understanding of the mechanisms involved in the protection afforded will be necessary to optimize future vaccine formulations.

Identification of swine H1N2/pandemic H1N1 reassortant influenza virus in pigs, United States

In October and November 2010, novel H1N2 reassortant influenza viruses were identified from pigs showing mild respiratory signs that included cough and depression. Sequence and phylogenetic analysis showed that the novel H1N2 reassortants possesses HA and NA genes derived from recent H1N2 swine isolates similar to those isolated from Midwest. Compared to the majority of reported reassortants, both viruses preserved human-like host restrictive and putative antigenic sites in their HA and NA genes. The four internal genes, PB2, PB1, PA, and NS were similar to the contemporary swine triple reassortant viruses' internal genes (TRIG). Interestingly, NP and M genes of the novel reassortants were derived from the 2009 pandemic H1N1. The NP and M proteins of the two isolates demonstrated one (E16G) and four (G34A, D53E, I109T, and V313I) amino acid changes in the M2 and NP proteins, respectively. Similar amino acid changes were also noticed upon incorporation of the 2009 pandemic H1N1 NP in other reassortant viruses reported in the U.S. Thus the role of those amino acids in relation to host adaptation need to be further investigated. The reassortments of pandemic H1N1 with swine influenza viruses and the potential of interspecies transmission of these reassortants from swine to other species including human indicate the importance of systematic surveillance of swine population to determine the origin, the prevalence of similar reassortants in the U.S. and their impact on both swine production and public health.

Evolution of novel reassortant A/H3N2 influenza viruses in North American swine and humans, 2009-2011

Novel H3N2 influenza viruses (H3N2v) containing seven genome segments from swine lineage triple-reassortant H3N2 viruses and a 2009 pandemic H1N1 (H1N1pdm09) matrix protein segment (pM) were isolated from 12 humans in the United States between August and December 2011. To understand the evolution of these novel H3N2 viruses in swine and humans, we undertook a phylogenetic analysis of 674 M sequences and 388 HA and NA sequences from influenza viruses isolated from North American swine during 2009-2011, as well as HA, NA, and M sequences from eight H3N2v viruses isolated from humans. We identified 34 swine influenza viruses (termed rH3N2p) with the same combination of H3, N2, and pM segments as the H3N2v viruses isolated from humans. Notably, these rH3N2p viruses were generated in swine via reassortment events between H3N2 viruses and the pM segment approximately 4 to 10 times since 2009. The pM segment has also reassorted with multiple distinct lineages of H1 virus, especially H1δ viruses. Importantly, the N2 segment of all H3N2v viruses isolated from humans is derived from a genetically distinct N2 lineage that has circulated in swine since being acquired by reassortment with seasonal human H3N2 viruses in 2001-2002, rather than from the N2 that is associated with the 1998 H3N2 swine lineage. The identification of this N2 variant may have implications for influenza vaccine design and the potential pandemic threat of H3N2v to human age groups with differing levels of prior exposure and immunity.

Quail as a potential mixing vessel for the generation of new reassortant influenza A viruses

Quail has been proposed as one of the intermediate hosts supporting the generation of newly reassortant influenza A viruses (IAVs) with the potential to infect humans. To evaluate the role of quail as an intermediate host of IAVs, co-infections of quail with swine-origin pandemic H1N1 2009 (pH1N1) and low pathogenic avian influenza (LPAI) duck H3N2 (dkH3N2) viruses (n=10) or endemic Thai swine H1N1 (swH1N1) and dkH3N2 viruses (n=10) were conducted. Three additional groups of five quail were each inoculated with pH1N1, swH1N1 and dkH3N2 as control groups to verify that each virus can infect quail. Our result showed that co-infected quail shed higher viral titers from the respiratory tract than single virus infected quail. This study confirmed that reassortant viruses could be readily generated in the respiratory tract of quail from both the pH1N1/dkH3N2 co-infected group (100% of quail generating reassortant viruses) and the swH1N1/dkH3N2 (33% of quail generating reassortant viruses) co-infected group without discernible clinical signs. The reassortment efficacy between the two combination of viruses was different in that the frequency of reassortant viruses was significantly higher in pH1N1/dkH3N2 co-infected quail (21.4%) compared to swH1N1/dkH3N2 co-infected quail (0.8%), indicating that gene combinations in pH1N1 have a higher potential to reassort with dkH3N2 compared to swH1N1. In summary, our result confirmed that quail could be an intermediate host of IAVs for generating new reassortant viruses. Our finding highlights the importance of monitoring IAVs especially pH1N1 in quail.

Pathogenicity and transmission in pigs of the novel A(H3N2)v influenza virus isolated from humans and characterization of swine H3N2 viruses isolated in 2010-2011

Swine influenza virus (SIV) H3N2 with triple reassorted internal genes (TRIG) has been enzootic in Unites States since 1998. Transmission of the 2009 pandemic H1N1 (pH1N1) virus to pigs in the United States was followed by reassortment with endemic SIV, resulting in reassorted viruses that include novel H3N2 genotypes (rH3N2p). Between July and December 2011, 12 cases of human infections with swine-lineage H3N2 viruses containing the pandemic matrix (pM) gene [A(H3N2)v] were detected. Whole-genome analysis of H3N2 viruses isolated from pigs from 2009 to 2011 sequenced in this study and other available H3N2 sequences showed six different rH3N2p genotypes present in the U.S. swine population since 2009. The presence of the pM gene was a common feature among all rH3N2p genotypes, but no specific genotype appeared to predominate in the swine population. We compared the pathogenic, transmission, genetic, and antigenic properties of a human A(H3N2)v isolate and two swine H3N2 isolates, H3N2-TRIG and rH3N2p. Our in vivo study detected no increased virulence in A(H3N2)v or rH3N2p viruses compared to endemic H3N2-TRIG virus. Antibodies to cluster IV H3N2-TRIG and rH3N2p viruses had reduced cross-reactivity to A(H3N2)v compared to other cluster IV H3N2-TRIG and rH3N2p viruses. Genetic analysis of the hemagglutinin gene indicated that although rH3N2p and A(H3N2)v are related to cluster IV of H3N2-TRIG, some recent rH3N2p isolates appeared to be forming a separate cluster along with the human isolates of A(H3N2)v. Continued monitoring of these H3N2 viruses is necessary to evaluate the evolution and potential loss of population immunity in swine and humans.

Serological evidence of swine influenza in Brazil

The aim of this work was to detect serum antibodies specific to influenza viruses in swine in Brazil. Serum samples of 355 pigs from 17 herds in Minas Gerais state were tested by hemagglutination inhibition (HI) for antibodies against H1N1 swine (SIV) and human influenza viruses, and H3N2 SIV. HI revealed that 158 animals (44·5%) and 11 herds (64·7%) were positive for H1N1 SIV, 36 animals (10·1%) and four herds (23·5%) were positive for H3N2 SIV, and 136 animals (38·3%) and 10 herds (58·8%) were positive for H1N1 human. This study indicates that swine influenza is disseminated throughout Minas Gerais state, Brazil.

Phylodynamic inference of infectious diseases caused by HIV, enterovirus 71 and the 2009 swine-origin human influenza virus

The rapidly evolving nature of viruses, particularly RNA viruses, exhibit complex phylodynamic behaviors, accumulating mutations across the genome on a timescale observable by humans. Phylodynamic investigations provide unique and quantitative evolutionary data on the origin, progression in terms of time and space, and epidemic and transmission history of pathogens. This review describes recent phylodynamic conjectures on three distinct categories of human viruses, including HIV, enterovirus 71 and the swine-origin pandemic influenza virus H1N1, which are of enormous importance to public health.

Kinetics of lung lesion development and pro-inflammatory cytokine response in pigs with vaccine-associated enhanced respiratory disease induced by challenge with pandemic (2009) A/H1N1 influenza virus

The objective of this report was to characterize the enhanced clinical disease and lung lesions observed in pigs vaccinated with inactivated H1N2 swine δ-cluster influenza A virus and challenged with pandemic 2009 A/H1N1 human influenza virus. Eighty-four, 6-week-old, cross-bred pigs were randomly allocated into 3 groups of 28 pigs to represent vaccinated/challenged (V/C), non-vaccinated/challenged (NV/C), and non-vaccinated/non-challenged (NV/NC) control groups. Pigs were intratracheally inoculated with pH1N1 and euthanized at 1, 2, 5, and 21 days post inoculation (dpi). Macroscopically, V/C pigs demonstrated greater percentages of pneumonia compared to NV/C pigs. Histologically, V/C pigs demonstrated severe bronchointerstitial pneumonia with necrotizing bronchiolitis accompanied by interlobular and alveolar edema and hemorrhage at 1 and 2 dpi. The magnitude of peribronchiolar lymphocytic cuffing was greater in V/C pigs by 5 dpi. Microscopic lung lesion scores were significantly higher in the V/C pigs at 2 and 5 dpi compared to NV/C and NV/NC pigs. Elevated TNF-α, IL-1β, IL-6, and IL-8 were detected in bronchoalveolar lavage fluid at all time points in V/C pigs compared to NV/C pigs. These data suggest H1 inactivated vaccines followed by heterologous challenge resulted in potentiated clinical signs and enhanced pulmonary lesions and correlated with an elevated proinflammatory cytokine response in the lung. The lung alterations and host immune response are consistent with the vaccine-associated enhanced respiratory disease (VAERD) clinical outcome observed reproducibly in this swine model.

Multiple reassortment between pandemic (H1N1) 2009 and endemic influenza viruses in pigs, United States

As a result of human-to-pig transmission, pandemic influenza A (H1N1) 2009 virus was detected in pigs soon after it emerged in humans. In the United States, this transmission was quickly followed by multiple reassortment between the pandemic virus and endemic swine viruses. Nine reassortant viruses representing 7 genotypes were detected in commercial pig farms in the United States. Field observations suggested that the newly described reassortant viruses did not differ substantially from pandemic (H1N1) 2009 or endemic strains in their ability to cause disease. Comparable growth properties of reassortant and endemic viruses in vitro supported these observations; similarly, a representative reassortant virus replicated in ferrets to the same extent as did pandemic (H1N1) 2009 and endemic swine virus. These novel reassortant viruses highlight the increasing complexity of influenza viruses within pig populations and the frequency at which viral diversification occurs in this ecologically important viral reservoir.

Multiple reassortment between pandemic (H1N1) 2009 and endemic influenza viruses in pigs, United States

TOC Summary: Viruses belonging to these novel genotypes are indistinguishable phenotypically from endemic swine viruses.

As a result of human-to-pig transmission, pandemic influenza A (H1N1) 2009 virus was detected in pigs soon after it emerged in humans. In the United States, this transmission was quickly followed by multiple reassortment between the pandemic virus and endemic swine viruses. Nine reassortant viruses representing 7 genotypes were detected in commercial pig farms in the United States. Field observations suggested that the newly described reassortant viruses did not differ substantially from pandemic (H1N1) 2009 or endemic strains in their ability to cause disease. Comparable growth properties of reassortant and endemic viruses in vitro supported these observations; similarly, a representative reassortant virus replicated in ferrets to the same extent as did pandemic (H1N1) 2009 and endemic swine virus. These novel reassortant viruses highlight the increasing complexity of influenza viruses within pig populations and the frequency at which viral diversification occurs in this ecologically important viral reservoir.

The neuraminidase and matrix genes of the 2009 pandemic influenza H1N1 virus cooperate functionally to facilitate efficient replication and transmissibility in pigs

The 2009 pandemic H1N1 virus (pH1N1) contains neuraminidase (NA) and matrix (M) genes from Eurasian avian-like swine influenza viruses (SIVs), with the remaining six genes from North American triple-reassortant SIVs. To characterize the role of the pH1N1 NA and M genes in pathogenesis and transmission, their impact was evaluated in the background of an H1N1 triple-reassortant (tr1930) SIV in which the HA (H3) and NA (N2) of influenza A/swine/Texas/4199-2/98 virus were replaced with those from the classical H1N1 A/swine/Iowa/15/30 (1930) virus. The laboratory-adapted 1930 virus did not shed nor transmit in pigs, but tr1930 was able to shed in infected pigs. The NA, M or both genes of the tr1930 virus were then substituted by those of pH1N1. The resulting virus with both NA and M from pH1N1 grew to significantly higher titre in cell cultures than the viruses with single NA or M from pH1N1. In a pig model, only the virus containing both NA and M from pH1N1 was transmitted to and infected sentinels, whereas the viruses with single NA or M from pH1N1 did not. These results demonstrate that the right combination of NA and M genes is critical for the replication and transmissibility of influenza viruses in pigs.

Safety, immunogenicity, and efficacy of an alphavirus replicon-based swine influenza virus hemagglutinin vaccine

A single-cycle, propagation-defective replicon particle (RP) vaccine expressing a swine influenza virus hemagglutinin (HA) gene was constructed and evaluated in several different animal studies. Studies done in both the intended host (pigs) and non-host (mice) species demonstrated that the RP vaccine is not shed or spread by vaccinated animals to comingled cohorts, nor does it revert to virulence following vaccination. In addition, vaccinated pigs develop both specific humoral and IFN-γ immune responses, and young pigs are protected against homologous influenza virus challenge.

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.

Swine influenza virus vaccines: to change or not to change-that's the question

Commercial vaccines currently available against swine influenza virus (SIV) are inactivated, adjuvanted, whole virus vaccines, based on H1N1 and/or H3N2 and/or H1N2 SIVs. In keeping with the antigenic and genetic differences between SIVs circulating in Europe and the US, the vaccines for each region are produced locally and contain different strains. Even within a continent, there is no standardization of vaccine strains, and the antigen mass and adjuvants can also differ between different commercial products. Recombinant protein vaccines against SIV, vector, and DNA vaccines, and vaccines attenuated by reverse genetics have been tested in experimental studies, but they have not yet reached the market. In this review, we aim to present a critical analysis of the performance of commercial inactivated and novel generation SIV vaccines in experimental vaccination challenge studies in pigs. We pay special attention to the differences between commercial SIV vaccines and vaccination attitudes in Europe and in North America, to the issue of vaccine strain selection and changes, and to the potential advantages of novel generation vaccines over the traditional killed SIV vaccines.

Pandemic H1N1 influenza virus-like particles are immunogenic and provide protective immunity to pigs

The outbreak of the 2009 influenza pandemic underscored the important role of swine in influenza virus evolution and the emergence of novel viruses with pandemic potential. Vaccination is the most common practice to control swine influenza in swine industry. Influenza virus-like particle (VLP) vaccines are an alternative approach and have been demonstrated to be immunogenic and confer protection against influenza virus challenge in chickens, mice and ferrets. In this study, we generated VLPs consisting of HA, NA and M1 proteins derived from pandemic virus A/California/04/2009 in insect cells. The immunogenicity and efficacy following vaccination of VLPs were evaluated in swine. Our data showed that vaccination using VLPs elicited robust levels of serum IgG, mucosal IgA, and viral neutralizing antibodies against A/Sw/Manitoba/MAFRI32/2009 H1N1. Following challenge with pandemic H1N1 2009, vaccinated pigs were protected, displaying reduced lung lesions, virus shedding and inhibition of virus replication in the lungs compared to non-vaccinated control pigs. Thus, VLPs can serve as a promising vaccination strategy to control influenza in swine.