Detection of H1 Swine Influenza A Virus Antibodies in Human Serum Samples by Age Group1

Most H1 influenza A viruses (IAVs) of swine are derived from past human viruses. As human population immunity against these IAVs gradually decreases, the risk of reintroduction to humans increases. We examined 549 serum samples from persons 0-97 years of age collected in Belgium during 2017-2018 for hemagglutination inhibiting and virus neutralizing antibodies against 7 major H1 swine IAV (swIAV) clades and 3 human progenitor IAVs. Seroprevalence (titers >40) rates were >50% for classical swine and European human-like swIAVs, >24% for North American human-like δ1a and Asian avian-like swIAVs, and <10% for North American human-like δ1b and European avian-like swIAVs, but rates were age-dependent. Antibody titers against human-like swIAVs and supposed human precursor IAVs correlated with correlation coefficients of 0.30-0.86. Our serologic findings suggest that European avian-like, clade 1C.2.1, and North American human-like δ1b, clade 1B.2.2.2, H1 swIAVs pose the highest pandemic risk.

Structure and applications of novel influenza HA tri-stalk protein for evaluation of HA stem-specific immunity

Long alpha helix (LAH) from influenza virus hemagglutinin (HA) stem or stalk domain is one of the most conserved influenza virus antigens. Expression of N-terminally extended LAH in E. coli leads to assembly of α-h elical homotrimer which is structurally nearly identical to the corresponding region of post-fusion form of native HA. This novel tri-stalk protein was able to differentiate between group 1 and 2 influenza in ELISA with virus-infected mice sera. It was also successfully applied for enzyme-linked immunospot assay to estimate the number of HA stem-reactive antibody (Ab)-secreting cells in mice. An in-house indirect ELISA was developed using a HA tri-stalk protein as a coating antigen for evaluation of HA stem-specific Ab levels in human sera collected in Luxembourg from 211 persons with occupational exposure to swine before the pandemic H1N1/09 virus had spread to Western Europe. Our results show that 70% of these pre-pandemic sera are positive for HA stem-specific Abs. In addition, levels of HA stem-specific Abs have positive correlation with the corresponding IgG titers and neutralizing activities against pandemic H1N1/09 virus.

Epidemiological Risk Factors for Animal Influenza A Viruses Overcoming Species Barriers

Drivers and risk factors for Influenza A virus transmission across species barriers are poorly understood, despite the ever present threat to human and animal health potentially on a pandemic scale. Here we review the published evidence for epidemiological risk factors associated with influenza viruses transmitting between animal species and from animals to humans. A total of 39 papers were found with evidence of epidemiological risk factors for influenza virus transmission from animals to humans; 18 of which had some statistical measure associated with the transmission of a virus. Circumstantial or observational evidence of risk factors for transmission between animal species was found in 21 papers, including proximity to infected animals, ingestion of infected material and potential association with a species known to carry influenza virus. Only three publications were found which presented a statistical measure of an epidemiological risk factor for the transmission of influenza between animal species. This review has identified a significant gap in knowledge regarding epidemiological risk factors for the transmission of influenza viruses between animal species.

Weighing serological evidence of human exposure to animal influenza viruses - a literature review

Assessing influenza A virus strains circulating in animals and their potential to cross the species barrier and cause human infections is important to improve human influenza surveillance and preparedness. We reviewed studies describing serological evidence of human exposure to animal influenza viruses. Comparing serological data is difficult due to a lack of standardisation in study designs and in laboratory methods used in published reports. Therefore, we designed a scoring system to assess and weigh specificity of obtained serology results in the selected articles. Many studies report reliable evidence of antibodies to swine influenza viruses among persons occupationally exposed to pigs. Most avian influenza studies target H5, H7 and H9 subtypes and most serological evidence of human exposure to avian influenza viruses is reported for these subtypes. Avian influenza studies receiving a low grade in this review often reported higher seroprevalences in humans compared with studies with a high grade. Official surveillance systems mainly focus on avian H5 and H7 viruses. Swine influenza viruses and avian subtypes other than H5 and H7 (emphasising H9) should be additionally included in official surveillance systems. Surveillance efforts should also be directed towards understudied geographical areas, such as Africa and South America.

Increased risk of A(H1N1)pdm09 influenza infection in UK pig industry workers compared to a general population cohort

BACKGROUND

Pigs are mixing vessels for influenza viral reassortment, but the extent of influenza transmission between swine and humans is not well understood.

OBJECTIVES

To assess whether occupational exposure to pigs is a risk factor for human infection with human and swine-adapted influenza viruses.

METHODS

UK pig industry workers were frequency-matched on age, region, sampling month, and gender with a community-based comparison group from the Flu Watch study. HI assays quantified antibodies for swine and human A(H1) and A(H3) influenza viruses (titres ≥ 40 considered seropositive and indicative of infection). Virus-specific associations between seropositivity and occupational pig exposure were examined using multivariable regression models adjusted for vaccination. Pigs on the same farms were also tested for seropositivity.

RESULTS

Forty-two percent of pigs were seropositive to A(H1N1)pdm09. Pig industry workers showed evidence of increased odds of A(H1N1)pdm09 seropositivity compared to the comparison group, albeit with wide confidence intervals (CIs), adjusted odds ratio after accounting for possible cross-reactivity with other swine A(H1) viruses (aOR) 25·3, 95% CI (1·4-536·3), P = 0·028.

CONCLUSION

The results indicate that A(H1N1)pdm09 virus was common in UK pigs during the pandemic and subsequent period of human A(H1N1)pdm09 circulation, and occupational exposure to pigs was a risk factor for human infection. Influenza immunisation of pig industry workers may reduce transmission and the potential for virus reassortment.

Transmission and pathogenicity of novel reassortants derived from Eurasian avian-like and 2009 pandemic H1N1 influenza viruses in mice and guinea pigs

Given the present extensive co-circulation in pigs of Eurasian avian-like (EA) swine H1N1 and 2009 pandemic (pdm/09) H1N1 viruses, reassortment between them is highly plausible but largely uncharacterized. Here, experimentally co-infected pigs with a representative EA virus and a pdm/09 virus yielded 55 novel reassortant viruses that could be categorized into 17 genotypes from Gt1 to Gt17 based on segment segregation. Majority of novel reassortants were isolated from the lower respiratory tract. Most of reassortant viruses were more pathogenic and contagious than the parental EA viruses in mice and guinea pigs. The most transmissible reassortant genotypes demonstrated in guinea pigs (Gt2, Gt3, Gt7, Gt10 and Gt13) were also the most lethal in mice. Notably, nearly all these highly virulent reassortants (all except Gt13) were characterized with possession of EA H1 and full complement of pdm/09 ribonucleoprotein genes. Compositionally, we demonstrated that EA H1-222G contributed to virulence by its ability to bind avian-type sialic acid receptors, and that pdm/09 RNP conferred the most robust polymerase activity to reassortants. The present study revealed high reassortment compatibility between EA and pdm/09 viruses in pigs, which could give rise to progeny reassortant viruses with enhanced virulence and transmissibility in mice and guinea pig models.

Swine Influenza A(H3N2) Virus Infection in Immunocompromised Man, Italy, 2014

Because swine influenza virus infection is seldom diagnosed in humans, its frequency might be underestimated. We report a immunocompromised hematologic patient with swine influenza A(H3N2) virus in 2014 in Italy. Local pigs were the source of this human infection.

Serological Evidence and Risk Factors for Swine Influenza Infections among Chinese Swine Workers in Guangdong Province

During July to September 2014, we performed a controlled, cross-sectional, seroepidemiologic study among 203 swine workers and 115 control subjects in Guangdong Province. Sera were tested using a hemagglutination inhibition assay against locally-isolated swine H3N2 and H1N1 viruses and commercially-obtained human influenza viral antigens. We found swine workers had a greater prevalence and odds of seropositivity against the swine H3N2 virus (17.3% vs. 7.0%; adjusted OR, 3.4; 95% CI, 1.1 -10.7). Younger age, self-report of a respiratory illness during the last 12 months, and seropositivity against seasonal H3N2 virus were identified as significant risk factors for seropositivity against swine H3N2 virus. As swine workers in China may be exposed to novel influenza viruses, it seems prudent for China to conduct special surveillance for such viruses among them. It also seems wise to offer such workers seasonal influenza vaccines with a goal to reduce cross-species influenza virus transmission.

Influenza, a One Health paradigm--novel therapeutic strategies to fight a zoonotic pathogen with pandemic potential

Influenza virus is a paradigm for a pathogen that frequently crosses the species barrier from animals to humans, causing severe disease in the human population. This ranges from frequent epidemics to occasional pandemic outbreaks with millions of death. All previous pandemics in humans were caused by animal viruses or virus reassortants carrying animal virus genes, underlining that the fight against influenza requires a One Health approach integrating human and veterinary medicine. Furthermore, the fundamental question of what enables a flu pathogen to jump from animals to humans can only be tackled in a transdisciplinary approach between virologists, immunologists and cell biologists. To address this need the German FluResearchNet was established as a first nationwide influenza research network that virtually integrates all national expertise in the field of influenza to unravel viral and host determinants of pathogenicity and species transmission and to explore novel avenues of antiviral intervention. Here we focus on the various novel anti-flu approaches that were developed as part of the FluResearchNet activities.

Modified vaccinia virus Ankara expressing the hemagglutinin of pandemic (H1N1) 2009 virus induces cross-protective immunity against Eurasian 'avian-like' H1N1 swine viruses in mice

Objectives

To examine cross-reactivity between hemagglutinin (HA) derived from A/California/7/09 (CA/09) virus and that derived from representative Eurasian “avian-like” (EA) H1N1 swine viruses isolated in Italy between 1999 and 2008 during virological surveillance in pigs.

Design

Modified vaccinia virus Ankara (MVA) expressing the HA gene of CA/09 virus (MVA-HA-CA/09) was used as a vaccine to investigate cross-protective immunity against H1N1 swine viruses in mice.

Sample

Two classical swine H1N1 (CS) viruses and four representative EA-like H1N1 swine viruses previously isolated during outbreaks of respiratory disease in pigs on farms in Northern Italy were used in this study.

Setting

Female C57BL/6 mice were vaccinated with MVA/HA/CA/09 and then challenged intranasally with H1N1 swine viruses.

Main outcome measures

Cross-reactive antibody responses were determined by hemagglutination- inhibition (HI) and virus microneutralizing (MN) assays of sera from MVA-vaccinated mice. The extent of protective immunity against infection with H1N1 swine viruses was determined by measuring lung viral load on days 2 and 4 post-challenge.

Results and Conclusions

Systemic immunization of mice with CA/09-derived HA, vectored by MVA, elicited cross-protective immunity against recent EA-like swine viruses. This immune protection was related to the levels of cross-reactive HI antibodies in the sera of the immunized mice and was dependent on the similarity of the antigenic site Sa of H1 HAs. Our findings suggest that the herd immunity elicited in humans by the pandemic (H1N1) 2009 virus could limit the transmission of recent EA-like swine HA genes into the influenza A virus gene pool in humans.

Prevalence of antibodies to European porcine influenza viruses in humans living in high pig density areas of Germany

The risk of zoonotic human infection caused by European porcine influenza virus strains was estimated in German regions with a high pig density. Sera from 622 healthy volunteers were collected between April 2009 and November 2011, mainly in Westphalia and western Lower Saxony. These included 362 subjects with occupational contact to pigs and 260 blood donors without any direct exposition to pigs. Samples were analysed by the haemagglutination inhibition (HI) assay against a panel of six swine viruses of subtypes avian-like H1N1 and human-like H3N2 as well as against human H1N1 and H3N2 viruses including the pandemic H1N1 strain of 2009. Reciprocal HI titres ≥20 were quoted as seroreactive. Compared to the control group, a significantly higher proportion of subjects with direct contact to pigs exhibited seroreactivity against porcine antigens of the avian-like H1N1 (37.0 %/7.7 %), the human-like H3N2 (59.7 %/43.1 %), the pandemic H1N1 strain of 2009 (51.7 %/26.5 %) and against a historic seasonal H3N2 strain that is closely related antigenetically to currently circulating human-like H3N2 viruses of European pigs (57.5 %/36.5 %). This trend was also observed when a reciprocal HI titre ≥40 was chosen as cut-off. Particularly, in younger subjects, the differences in seroreactivity against porcine strains between the exposed and non-exposed group were significant. The data indicate a higher risk of infection in the exposed individuals.

Evidence of cross-reactive immunity to 2009 pandemic influenza A virus in workers seropositive to swine H1N1 influenza viruses circulating in Italy

Background

Pigs play a key epidemiologic role in the ecology of influenza A viruses (IAVs) emerging from animal hosts and transmitted to humans. Between 2008 and 2010, we investigated the health risk of occupational exposure to swine influenza viruses (SIVs) in Italy, during the emergence and spread of the 2009 H1N1 pandemic (H1N1pdm) virus.

Methodology/Principal Findings

Serum samples from 123 swine workers (SWs) and 379 control subjects (Cs), not exposed to pig herds, were tested by haemagglutination inhibition (HI) assay against selected SIVs belonging to H1N1 (swH1N1), H1N2 (swH1N2) and H3N2 (swH3N2) subtypes circulating in the study area. Potential cross-reactivity between swine and human IAVs was evaluated by testing sera against recent, pandemic and seasonal, human influenza viruses (H1N1 and H3N2 antigenic subtypes). Samples tested against swH1N1 and H1N1pdm viruses were categorized into sera collected before (n. 84 SWs; n. 234 Cs) and after (n. 39 SWs; n. 145 Cs) the pandemic peak. HI-antibody titers ≥10 were considered positive. In both pre-pandemic and post-pandemic peak subperiods, SWs showed significantly higher swH1N1 seroprevalences when compared with Cs (52.4% vs. 4.7% and 59% vs. 9.7%, respectively). Comparable HI results were obtained against H1N1pdm antigen (58.3% vs. 7.7% and 59% vs. 31.7%, respectively). No differences were found between HI seroreactivity detected in SWs and Cs against swH1N2 (33.3% vs. 40.4%) and swH3N2 (51.2 vs. 55.4%) viruses. These findings indicate the occurrence of swH1N1 transmission from pigs to Italian SWs.

Conclusion/Significance

A significant increase of H1N1pdm seroprevalences occurred in the post-pandemic peak subperiod in the Cs (p<0.001) whereas SWs showed no differences between the two subperiods, suggesting a possible occurrence of cross-protective immunity related to previous swH1N1 infections. These data underline the importance of risk assessment and occupational health surveillance activities aimed at early detection and control of SIVs with pandemic potential in humans.

Transmission of influenza A virus in pigs

Influenza A virus infections cause respiratory disease in pigs and are a risk to public health. The pig plays an important role in influenza ecology because of its ability to support replication of influenza viruses from avian, swine and human species. Influenza A virus is widespread in pigs worldwide, and influenza A virus interspecies transmission has been documented in many events. Influenza A virus is mostly transmitted through direct pig-to-pig contact and aerosols although other indirect routes of transmission may also exist. Several factors contribute to differences in the transmission dynamics within populations including among others vaccination, pig flow, animal movement and animal introduction which highlights the complexity of influenza A transmission in pigs. In addition, pigs can serve as a reservoir of influenza A viruses for other pigs and other species and understanding mechanisms of transmission within pigs and from pigs to other species and vice versa is crucial. In this paper, we review the current understanding of influenza virus transmission in pigs. We highlight the ubiquity of influenza A virus in the pig population and the widespread distribution of pandemic H1N1 virus worldwide while emphasizing an understanding of the routes of transmission and factors that contribute to virus spread and dissemination within and between pig populations. In addition, we describe transmission events between pigs and other species including people. Understanding transmission is crucial for designing effective control strategies and for making well-informed recommendations for surveillance.