Transmission of influenza A viruses between pigs and people, Iowa, 2002-2004

Modeling transmission of avian influenza viruses at the human-animal-environment interface in Cuba

Introduction

The increasing geographical spread of highly pathogenic avian influenza viruses (HPAIVs) is of global concern due to the underlying zoonotic and pandemic potential of the virus and its economic impact. An integrated One Health model was developed to estimate the likelihood of Avian Influenza (AI) introduction and transmission in Cuba, which will help inform and strengthen risk-based surveillance activities.

Materials and methods

The spatial resolution used for the model was the smallest administrative district ("Consejo Popular"). The model was parameterised for transmission from wild birds to poultry and pigs (commercial and backyard) and then to humans. The model includes parameters such as risk factors for the introduction and transmission of AI into Cuba, animal and human population densities; contact intensity and a transmission parameter (β).

Results

Areas with a higher risk of AI transmission were identified for each species and type of production system. Some variability was observed in the distribution of areas estimated to have a higher probability of AI introduction and transmission. In particular, the south-western and eastern regions of Cuba were highlighted as areas with the highest risk of transmission.

Discussion

These results are potentially useful for refining existing criteria for the selection of farms for active surveillance, which could improve the ability to detect positive cases. The model results could contribute to the design of an integrated One Health risk-based surveillance system for AI in Cuba. In addition, the model identified geographical regions of particular importance where resources could be targeted to strengthen biosecurity and early warning surveillance.

Characteristics of two zoonotic swine influenza A(H1N1) viruses isolated in Germany from diseased patients

Interspecies transmission of influenza A viruses (IAV) from pigs to humans is a concerning event as porcine IAV represent a reservoir of potentially pandemic IAV. We conducted a comprehensive analysis of two porcine A(H1N1)v viruses isolated from human cases by evaluating their genetic, antigenic and virological characteristics. The HA genes of those human isolates belonged to clades 1C.2.1 and 1C.2.2, respectively, of the A(H1N1) Eurasian avian-like swine influenza lineage. Antigenic profiling revealed substantial cross-reactivity between the two zoonotic H1N1 viruses and human A(H1N1)pdm09 virus and some swine viruses, but did not reveal cross-reactivity to H1N2 and earlier human seasonal A(H1N1) viruses. The solid-phase direct receptor binding assay analysis of both A(H1N1)v showed a predominant binding to α2-6-sialylated glycans similar to human-adapted IAV. Investigation of the replicative potential revealed that both A(H1N1)v viruses grow in human bronchial epithelial cells to similar high titers as the human A(H1N1)pdm09 virus. Cytokine induction was studied in human alveolar epithelial cells A549 and showed that both swine viruses isolated from human cases induced higher amounts of type I and type III IFN, as well as IL6 compared to a seasonal A(H1N1) or a A(H1N1)pdm09 virus. In summary, we demonstrate a remarkable adaptation of both zoonotic viruses to propagate in human cells. Our data emphasize the needs for continuous monitoring of people and regions at increased risk of such trans-species transmissions, as well as systematic studies to quantify the frequency of these events and to identify viral molecular determinants enhancing the zoonotic potential of porcine IAV.

Seroprevalence of Swine Influenza A Virus (swIAV) Infections in Commercial Farrow-to-Finish Pig Farms in Greece

Swine influenza is a highly contagious respiratory disease caused by influenza A virus infection. Pigs play an important role in the overall epidemiology of influenza because of their ability to transmit influenza viruses of avian and human origin, which plays a potential role in the emergence of zoonotic strains with pandemic potential. The aim of our study was to assess the seroprevalence of Swine Influenza Viruses (swIAVs) in commercial pig farms in Greece. A total of 1416 blood samples were collected from breeding animals (gilts and sows) and pigs aged 3 weeks to market age from 40 different swIAV vaccinated and unvaccinated commercial farrow-to-finish pig farms. For the detection of anti-SIV antibodies, sera were analyzed using an indirect ELISA kit CIVTEST SUIS INFLUENZA®, Hipra (Amer, Spain). Of the total 1416 animals tested, 498 were seropositive, indicating that the virus circulates in both vaccinated (54% seroprevalence) and unvaccinated Greek pig farms (23% seroprevalence). In addition, maternally derived antibody (MDA) levels were lower in pigs at 4 and 7 weeks of age in unvaccinated farms than in vaccinated farms. In conclusion, our results underscore the importance of vaccination as an effective tool for the prevention of swIAV infections in commercial farrow-to-finish pig farms.

Evidence of influenza A infection and risk of transmission between pigs and farmworkers

Interspecies transmission of influenza A virus (IAV) between pigs and people represents a threat to both animal and public health. To better understand the risks of influenza transmission at the human–animal interface, we evaluated 1) the rate of IAV detection in swine farmworkers before and after work during two human influenza seasons, 2) assessed risk factors associated with IAV detection in farmworkers and 3) characterized the genetic sequences of IAV detected in both workers and pigs. Of 58 workers providing nasal passage samples during 8‐week periods during the 2017/18 and 2018/19 influenza seasons, 33 (57%) tested positive by rRT‐PCR at least once. Sixteen (27%) workers tested positive before work and 24 (41%) after work. At the sample level, 58 of 1,785 nasal swabs (3.2%) tested rRT‐PCR positive, of which 20 of 898 (2.2%) were collected prior to work and 38 of 887 (4.3%) after work. Although farmworkers were more likely to test positive at the end of the working day (OR = 1.98, 95% CI 1.14–3.41), there were no influenza‐like illness (ILI) symptoms, or other risk indicators, associated with IAV detection before or after reporting to work. Direct whole‐genome sequencing from samples obtained from worker nasal passages indicated evidence of infection of a worker with pandemic 2009 H1N1 of human‐origin IAV (H1‐pdm 1A 3.3.2) when reporting to work, and exposure of several workers to a swine‐origin IAV (H1‐alpha 1A 1.1) circulating in the pigs on the farm where they were employed. Our study provides evidence of 1) risk of IAV transmission between pigs and people, 2) pandemic H1N1 IAV infected workers reporting to work and 3) workers exposed to swine harbouring swine‐origin IAV in their nasal passages temporarily. Overall, our results emphasize the need to implement surveillance and transmission preventive protocols at the pig/human interface.

Detection of swine influenza virus in nasal specimens by reverse transcription-loop-mediated isothermal amplification (RT-LAMP)

Detection of swine influenza virus (SIV) in commercial swine herds is important for understanding the infection status of the herd and for controlling disease. Current molecular diagnostics require that specimens be submitted to a laboratory which provides results to the growers after some time which is generally too late to intercede in disease control. Moreover, current diagnostic assays are time-consuming, typically costly, and require skilled technical expertise. We have instituted a reverse transcription loop-mediated isothermal amplification (RT-LAMP) diagnostic assay based on conserved regions of the SIV matrix (M) gene and H1N1 hemagglutinin (HA) sequences. The RT-LAMP assay was optimized to use both colorimetric and fluorescent endpoints and was validated. The M and HA RT-LAMP assays have a limit-of-detection (LOD) sensitive to 11 and 8-log-fold dilutions of viral RNA, respectively, and are capable of discriminating between H1 and H3 strains of SIV. Additionally, the RT-LAMP assay was optimized for direct amplification of SIV from field samples without the need for viral RNA isolation. The direct RT-LAMP detected >86 % of qRT-PCR validated SIV samples, and >66 % of negative samples when spiked with viral RNA or SIV. The diagnostic RT-LAMP assay is a rapid, sensitive, specific, and cost-effective method for the detection of SIV in herds substantially aiding diagnosis and surveillance.

Characterization of Swine Influenza A(H1N2) Variant, Alberta, Canada, 2020

Influenza strains circulating among swine populations can cause outbreaks in humans. In October 2020, we detected a variant influenza A subtype H1N2 of swine origin in a person in Alberta, Canada. We initiated a public health, veterinary, and laboratory investigation to identify the source of the infection and determine whether it had spread. We identified the probable source as a local pig farm where a household contact of the index patient worked. Phylogenetic analysis revealed that the isolate closely resembled strains found at that farm in 2017. Retrospective and prospective surveillance using molecular testing did not identify any secondary cases among 1,532 persons tested in the surrounding area. Quick collaboration between human and veterinary public health practitioners in this case enabled a rapid response to a potential outbreak.

Animal models for the risk assessment of viral pandemic potential

Pandemics affect human lives severely and globally. Experience predicts that there will be a pandemic for sure although the time is unknown. When a viral epidemic breaks out, assessing its pandemic risk is an important part of the process that characterizes genomic property, viral pathogenicity, transmission in animal model, and so forth. In this review, we intend to figure out how a pandemic may occur by looking into the past influenza pandemic events. We discuss interpretations of the experimental evidences resulted from animal model studies and extend implications of viral pandemic potentials and ingredients to emerging viral epidemics. Focusing on the pandemic potential of viral infectious diseases, we suggest what should be assessed to prevent global catastrophes from influenza virus, Middle East respiratory syndrome coronavirus, dengue and Zika viruses.

Experimental H1N1pdm09 infection in pigs mimics human seasonal influenza infections

Pigs are anatomically, genetically and physiologically comparable to humans and represent a natural host for influenza A virus (IAV) infections. Thus, pigs may represent a relevant biomedical model for human IAV infections. We set out to investigate the systemic as well as the local immune response in pigs upon two subsequent intranasal infections with IAV H1N1pdm09. We detected decreasing numbers of peripheral blood lymphocytes after the first infection. The simultaneous increase in the frequencies of proliferating cells correlated with an increase in infiltrating leukocytes in the lung. Enhanced perforin expression in αβ and γδ T cells in the respiratory tract indicated a cytotoxic T cell response restricted to the route of virus entry such as the nose, the lung and the bronchoalveolar lavage. Simultaneously, increasing frequencies of CD8αα expressing αβ T cells were observed rapidly after the first infection, which may have inhibited uncontrolled inflammation in the respiratory tract. Taking together, the results of this study demonstrate that experimental IAV infection in pigs mimics major characteristics of human seasonal IAV infections.

Conserved HA-peptide NG34 formulated in pCMV-CTLA4-Ig reduces viral shedding in pigs after a heterosubtypic influenza virus SwH3N2 challenge

Swine influenza viruses (SIVs), the causal agents of swine influenza, are not only important to control due to the economic losses in the swine industry, but also can be pandemic pathogens. Vaccination is one of the most relevant strategies to control and prevent influenza infection. Current human vaccines against influenza induce strain-specific immunity and annual update is required due to the virus antigenic shift phenomena. Previously, our group has reported the use of conserved hemagglutinin peptides (HA-peptides) derived from H1-influenza virus as a potential multivalent vaccine candidate. Immunization of swine with these HA-peptides elicited antibodies that recognized and neutralized heterologous influenza viruses in vitro and demonstrated strong hemagglutination-inhibiting activity. In the present work, we cloned one HA-peptide (named NG34) into a plasmid fused with cytotoxic T lymphocyte-associated antigen (CTLA4) which is a molecule that modifies T cell activation and with an adjuvant activity interfering with the adaptive immune response. The resulting plasmid, named pCMV-CTLA4-Ig-NG34, was administered twice to animals employing a needle-free delivery approach. Two studies were carried out to test the efficacy of pCMV-CTLA4-Ig-NG34 as a potential swine influenza vaccine, one in seronegative and another in seropositive pigs against SIV. The second one was aimed to evaluate whether pCMV-CTLA4-Ig-NG34 vaccination would overcome maternally derived antibodies (MDA). After immunization, all animals were intranasally challenged with an H3N2 influenza strain. A complete elimination or significant reduction in the viral shedding was observed within the first week after the challenge in the vaccinated animals from both studies. In addition, no challenged heterologous virus load was detected in the airways of vaccinated pigs. Overall, it is suggested that the pCMV-CTLA4-Ig-NG34 vaccine formulation could potentially be used as a multivalent vaccine against influenza viruses.

Importance of 1918 virus reconstruction to current assessments of pandemic risk

Reconstruction of the 1918 influenza virus has facilitated considerable advancements in our understanding of this extraordinary pandemic virus. However, the benefits of virus reconstruction are not limited to this one strain. Here, we provide an overview of laboratory studies which have evaluated the reconstructed 1918 virus, and highlight key discoveries about determinants of virulence and transmissibility associated with this virus in mammals. We further discuss recent and current pandemic threats from avian and swine reservoirs, and provide specific examples of how reconstruction of the 1918 pandemic virus has improved our ability to contextualize research employing novel and emerging strains. As influenza viruses continue to evolve and pose a threat to human health, studying past pandemic viruses is key to future preparedness efforts.

Serological study of influenza viruses in veterinarians working with swine in Mexico

Humans and swine are both affected by influenza viruses, and swine are considered a potential source of new influenza viruses. Transmission of influenza viruses across species is well documented. The aim of this study was to evaluate the seroprevalence of different influenza virus subtypes in veterinarians working for the Mexican swine industry, using a hemagglutination inhibition test. All sera tested were collected in July 2011. The data were analysed using a generalized linear model and a linear model to study the possible association of seroprevalence with the age of the veterinarian, vaccination status, and biosecurity level of the farm where they work. The observed seroprevalence was 12.3%, 76.5%, 46.9%, and 11.1% for the human subtypes of pandemic influenza virus (pH1N1), seasonal human influenza virus (hH1N1), the swine subtypes of classical swine influenza virus (swH1N1), and triple-reassortant swine influenza virus (swH3N2), respectively. Statistical analysis indicated that age was associated with hH1N1 seroprevalence (P < 0.05). Similarly, age and vaccination were associated with pH1N1 seroprevalence (P < 0.05). On the other hand, none of the studied factors were associated with swH1N1 and swH3N2 seroprevalence. All of the pH1N1-positive sera were from vaccinated veterinarians, whereas all of those not vaccinated tested negative for this subtype. Our findings suggest that, between the onset of the 2009 pandemic and July 2011, the Mexican veterinarians working in the swine industry did not have immunity to the pH1N1 virus; hence, they would have been at risk for infection with this virus if this subtype had been circulating in swine in Mexico prior to 2011.

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.

Severe acute respiratory infection caused by swine influenza virus in a child necessitating extracorporeal membrane oxygenation (ECMO), the Netherlands, October 2016

In October 2016, a severe infection with swine influenza A(H1N1) virus of the Eurasian avian lineage occurred in a child with a previous history of eczema in the Netherlands, following contact to pigs. The patient's condition deteriorated rapidly and required life support through extracorporeal membrane oxygenation. After start of oseltamivir treatment and removal of mucus plugs, the patient fully recovered. Monitoring of more than 80 close unprotected contacts revealed no secondary cases.

Maternally-derived antibodies do not prevent transmission of swine influenza A virus between pigs

A transmission experiment involving 5-week-old specific-pathogen-free (SPF) piglets, with (MDA(+)) or without maternally-derived antibodies (MDA(-)), was carried out to evaluate the impact of passive immunity on the transmission of a swine influenza A virus (swIAV). In each group (MDA(+)/MDA(-)), 2 seeders were placed with 4 piglets in direct contact and 5 in indirect contact (3 replicates per group). Serological kinetics (ELISA) and individual viral shedding (RT-PCR) were monitored for 28 days after infection. MDA waning was estimated using a nonlinear mixed-effects model and survival analysis. Differential transmission rates were estimated depending on the piglets' initial serological status and contact structure (direct contact with pen-mates or indirect airborne contact). The time to MDA waning was 71.3 [52.8-92.1] days on average. The airborne transmission rate was 1.41 [0.64-2.63] per day. The compared shedding pattern between groups showed that MDA(+) piglets had mainly a reduced susceptibility to infection compared to MDA(-) piglets. The resulting reproduction number estimated in MDA(+) piglets (5.8 [1.4-18.9]), although 3 times lower than in MDA(-) piglets (14.8 [6.4-27.1]), was significantly higher than 1. Such an efficient and extended spread of swIAV at the population scale in the presence of MDAs could contribute to swIAV persistence on farms, given the fact that the period when transmission is expected to be impacted by the presence of MDAs can last up to 10 weeks.

Modeling risk of occupational zoonotic influenza infection in swine workers

Zoonotic transmission of influenza A virus (IAV) between swine and workers in swine production facilities may play a role in the emergence of novel influenza strains with pandemic potential. Guidelines to prevent transmission of influenza to swine workers have been developed but there is a need for evidence-based decision-making about protective measures such as respiratory protection. A mathematical model was applied to estimate the risk of occupational IAV exposure to swine workers by contact and airborne transmission, and to evaluate the use of respirators to reduce transmission.  The Markov model was used to simulate the transport and exposure of workers to IAV in a swine facility. A dose-response function was used to estimate the risk of infection. This approach is similar to methods previously used to estimate the risk of infection in human health care settings. This study uses concentration of virus in air from field measurements collected during outbreaks of influenza in commercial swine facilities, and analyzed by polymerase chain reaction.  It was found that spending 25 min working in a barn during an influenza outbreak in a swine herd could be sufficient to cause zoonotic infection in a worker. However, this risk estimate was sensitive to estimates of viral infectivity to humans. Wearing an excellent fitting N95 respirator reduced this risk, but with high aerosol levels the predicted risk of infection remained high under certain assumptions.  The results of this analysis indicate that under the conditions studied, swine workers are at risk of zoonotic influenza infection. The use of an N95 respirator could reduce such risk. These findings have implications for risk assessment and preventive programs targeting swine workers. The exact level of risk remains uncertain, since our model may have overestimated the viability or infectivity of IAV. Additionally, the potential for partial immunity in swine workers associated with repeated low-dose exposures or from previous infection with other influenza strains was not considered. Further studies should explore these uncertainties.

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.

The global antigenic diversity of swine influenza A viruses

Swine influenza presents a substantial disease burden for pig populations worldwide and poses a potential pandemic threat to humans. There is considerable diversity in both H1 and H3 influenza viruses circulating in swine due to the frequent introductions of viruses from humans and birds coupled with geographic segregation of global swine populations. Much of this diversity is characterized genetically but the antigenic diversity of these viruses is poorly understood. Critically, the antigenic diversity shapes the risk profile of swine influenza viruses in terms of their epizootic and pandemic potential. Here, using the most comprehensive set of swine influenza virus antigenic data compiled to date, we quantify the antigenic diversity of swine influenza viruses on a multi-continental scale. The substantial antigenic diversity of recently circulating viruses in different parts of the world adds complexity to the risk profiles for the movement of swine and the potential for swine-derived infections in humans.

DOI:http://dx.doi.org/10.7554/eLife.12217.001

Influenza viruses, commonly called flu, infect millions of people and animals every year and occasionally causes pandemics in humans. The immune system can neutralise flu viruses by recognising the proteins on the virus surface, generically referred to as antigens. These antigens change as flu viruses evolve to escape detection by the immune system. These changes tend to be relatively small such that exposure to one flu virus generates immunity that is still effective against other related flu viruses. However, over time, the accumulation of these small changes can result in larger differences such that prior infections no longer provide protection against the new virus.

Influenza A viruses infect a wide variety of birds and mammals. Viruses can also transmit from one species to another, which may result in the introduction of viruses with antigens that are new to the recipient species and which have the potential to cause substantial outbreaks. Pig flu viruses have long been considered to be a potential risk for human pandemic viruses and were the source of the 2009 pandemic H1N1 virus. Importantly, humans often transmit flu viruses to pigs. Understanding the dynamics and consequences of this two-way transmission is important for designing effective strategies to detect and respond to new strains of flu.

Influenza A viruses of the H1 and H3 subtypes circulate widely in pigs. However, it was poorly understood how closely related swine and human viruses circulating in different regions were to one another and how much the antigens varied between the different viruses.

Lewis, Russell et al. have now analysed the antigenic variation of hundreds of H1 and H3 viruses from pigs on multiple continents. The antigenic diversity of recent swine flu viruses resembles the diversity of H1 and H3 viruses observed in humans over the last 40 years. A key factor driving the diversity of the H1 and H3 viruses in pigs is the frequent introduction of human viruses to pigs. In contrast, only one flu virus from a bird had contributed to the observed antigenic diversity in pigs in a substantial way.

Once in pigs, human-derived flu viruses continue to evolve their antigens. This results in a tremendous diversity of flu viruses that can be transmitted to other pigs and also to humans. These flu viruses could pose a serious risk to public health because they are no longer similar to the current human flu strains. These findings have important implications not only for developing flu vaccines for pigs but also for informing the development of more-effective surveillance and disease-control strategies to prevent the spread of new flu variants.

DOI:http://dx.doi.org/10.7554/eLife.12217.002

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.

Investigation of an Outbreak of Variant Influenza A(H3N2) Virus Infection Associated With an Agricultural Fair-Ohio, August 2012

BACKGROUND

In 2012, one third of cases in a multistate outbreak of variant influenza A(H3N2) virus ([H3N2]v) infection occurred in Ohio. We conducted an investigation of (H3N2)v cases associated with agricultural Fair A in Ohio.

METHODS

We surveyed Fair A swine exhibitors and their household members. Confirmed cases had influenza-like illness (ILI) and a positive laboratory test for (H3N2)v, and probable cases had ILI. We calculated attack rates. We determined risk factors for infection, using multivariable log-binomial regression.

RESULTS

We identified 20 confirmed and 94 probable cases associated with Fair A. Among 114 cases, the median age was 10 years, there were no hospitalizations or deaths, and 82% had swine exposure. In the exhibitor household cohort of 359 persons (83 households), we identified 6 confirmed cases (2%) and 40 probable cases (11%). An age of <10 years was a significant risk factor (P < .01) for illness. One instance of likely human-to-human transmission was identified.

CONCLUSIONS

In this (H3N2)v outbreak, no evidence of sustained human-to-human (H3N2)v transmission was found. Our risk factor analysis contributed to the development of the recommendation that people at increased risk of influenza-associated complications, including children aged <5 years, avoid swine barns at fairs during the 2012 fair season.

Swine worker awareness and behavior regarding prevention of zoonotic influenza transmission

ABSTRACT A convenience survey of swine workers on large and small commercial farms in the Northeast and Midwest United States regarding zoonotic influenza awareness and precautions was conducted. Workers reported low levels of concern regarding the risk of contracting influenza from swine, and were generally not aware of national guidelines for influenza prevention. Use of personal protective equipment (PPE) varied by task, N95 respirator use was rare, and no workers were enrolled in respirator programs. Reported influenza vaccination coverage was greater than the national average in 2009-2010, but declined in 2010-2011. Workers on large farms were more likely to use PPE in some tasks and to report using more precautions when pigs appeared ill. Although reporting low levels of concern regarding zoonotic influenza and low adherence to national influenza guidelines, swine workers reported making task-based and risk-based decisions about use of PPE, suggesting opportunities for enhanced prevention of zoonotic disease transmission.

Population dynamics of cocirculating swine influenza A viruses in the United States from 2009 to 2012

BACKGROUND

Understanding the ecology and evolution of influenza A viruses (IAV) in mammalian hosts is critical to reduce disease burden in production animals and lower zoonotic infection risk in humans. Recent advances in influenza surveillance in US swine populations allow for timely epidemiological, phylogenetic, and virological analyses that monitor emergence of novel viruses and assess changes in viral population dynamics.

METHODS

To better understand IAV in the North American swine population, we undertook a phylogenetic analysis of 1075 HA, 1049 NA, and 1040 M sequences of IAV isolated from US swine during 2009-2012 through voluntary and anonymous submissions to the US Department of Agriculture IAV swine surveillance system.

RESULTS

Analyses revealed changes in population dynamics among multiple clades of A/H1N1, A/H3N2, and A/H1N2 cocirculating in US swine populations during 2009-2012. Viral isolates were categorized into one of seven genetically and antigenically distinct hemagglutinin lineages: H1α, H1β, H1γ, H1δ1, H1δ2, H1pdm09, and H3 cluster IV. There was an increase in occurrence of H1δ1 in samples submitted, with a concurrent decrease in H1pdm09. H3 cluster IV exhibited increasing diversification, warranting a re-evaluation of phylogenetic nomenclature criteria. Although H3N2 represented 25% of identified viruses, this subtype was reported in increasing proportion of sequenced isolates since late 2011.

CONCLUSIONS

Surveillance and reporting of IAV in US swine have increased since 2009, and we demonstrate a period of expanded viral diversity. These data may be used to inform intervention strategies of vaccine and diagnostic updates and changes in swine health management.

Evaluation of Screening Assays for the Detection of Influenza A Virus Serum Antibodies in Swine

Increased surveillance of influenza A virus (IAV) infections in human and swine populations is mandated by public health and animal health concerns. Antibody assays have proven useful in previous surveillance programmes because antibodies provide a record of prior exposure and the technology is inexpensive. The objective of this research was to compare the performance of influenza serum antibody assays using samples collected from pigs (vaccinated or unvaccinated) inoculated with either A/Swine/OH/511445/2007 γ H1N1 virus or A/Swine/Illinois/02907/2009 Cluster IV H3N2 virus and followed for 42 days. Weekly serum samples were tested for anti-IAV antibodies using homologous and heterologous haemagglutination-inhibition (HI) assays, commercial swine influenza H1N1 and H3N2 indirect ELISAs, and a commercial influenza nucleoprotein (NP)-blocking ELISA. The homologous HIs showed 100% diagnostic sensitivity, but largely failed to detect infection with the heterologous virus. With diagnostic sensitivities of 1.4% and 4.9%, respectively, the H1N1 and H3N2 indirect ELISAs were ineffective at detecting IAV antibodies in swine infected with the contemporary influenza viruses used in the study. At a cut-off of S/N ≤ 0.60, the sensitivity and specificity of the NP-blocking ELISA were estimated at 95.5% and 99.6%, respectively. Statistically significant factors which affected S/N results include vaccination status, inoculum (virus subtype), day post-inoculation and the interactions between those factors (P < 0.0001). Serum antibodies against NP provide an ideal universal diagnostic screening target and could provide a cost-effective approach for the detection and surveillance of IAV infections in swine populations.

PhyloFlu, a DNA microarray for determining the phylogenetic origin of influenza A virus gene segments and the genomic fingerprint of viral strains

Recent evidence suggests that most influenza A virus gene segments can contribute to the pathogenicity of the virus. In this regard, the hemagglutinin (HA) subtype of the circulating strains has been closely surveyed, but the reassortment of internal gene segments is usually not monitored as a potential source of an increased pathogenicity. In this work, an oligonucleotide DNA microarray (PhyloFlu) designed to determine the phylogenetic origins of the eight segments of the influenza virus genome was constructed and validated. Clades were defined for each segment and also for the 16 HA and 9 neuraminidase (NA) subtypes. Viral genetic material was amplified by reverse transcription-PCR (RT-PCR) with primers specific to the conserved 5' and 3' ends of the influenza A virus genes, followed by PCR amplification with random primers and Cy3 labeling. The microarray unambiguously determined the clades for all eight influenza virus genes in 74% (28/38) of the samples. The microarray was validated with reference strains from different animal origins, as well as from human, swine, and avian viruses from field or clinical samples. In most cases, the phylogenetic clade of each segment defined its animal host of origin. The genomic fingerprint deduced by the combined information of the individual clades allowed for the determination of the time and place that strains with the same genomic pattern were previously reported. PhyloFlu is useful for characterizing and surveying the genetic diversity and variation of animal viruses circulating in different environmental niches and for obtaining a more detailed surveillance and follow up of reassortant events that can potentially modify virus pathogenicity.

Outbreak of influenza A (H3N2) variant virus infection among attendees of an agricultural fair, Pennsylvania, USA, 2011

Avoiding or limiting contact with swine at agricultural events may help prevent A(H3N2)v virus infections in such settings.

During August 2011, influenza A (H3N2) variant [A(H3N2)v] virus infection developed in a child who attended an agricultural fair in Pennsylvania, USA; the virus resulted from reassortment of a swine influenza virus with influenza A(H1N1)pdm09. We interviewed fair attendees and conducted a retrospective cohort study among members of an agricultural club who attended the fair. Probable and confirmed cases of A(H3N2)v virus infection were defined by serology and genomic sequencing results, respectively. We identified 82 suspected, 4 probable, and 3 confirmed case-patients who attended the fair. Among 127 cohort study members, the risk for suspected case status increased as swine exposure increased from none (4%; referent) to visiting swine exhibits (8%; relative risk 2.1; 95% CI 0.2–53.4) to touching swine (16%; relative risk 4.4; 95% CI 0.8–116.3). Fairs may be venues for zoonotic transmission of viruses with epidemic potential; thus, health officials should investigate respiratory illness outbreaks associated with agricultural events.

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.

Influenza A virus infection dynamics in swine farms in Belgium, France, Italy and Spain, 2006-2008

Avian-like H1N1 and reassortant H3N2 and H1N2 influenza A viruses with a human-like haemagglutinin have been co-circulating in swine in Europe for more than a decade. We aimed to examine the infection dynamics of the three swine influenza virus (SIV) lineages at the farm level, and to identify possible regional and seasonal variations in their circulation. Sera were collected from six successive generations of fattening pigs (2006-2008) in a total 80 farrow-to-finish herds in Belgium, Italy, France and Spain and examined for antibodies against the three SIVs in haemagglutination inhibition tests. Overall, in all regions and periods, 9.7% of all farms were negative for SIV, 49% were infected with one subtype, 38% with two subtypes and 3.9% with all three SIVs. We found serological evidence for the circulation of all three subtypes in Belgium, Italy and Spain, while only infections with H1N1 and H1N2 SIVs were detected in France. Despite temporary changes in the circulation of H1N2 in Belgium and in Spain, there was no true seasonal variation. The exact combination of subtypes on the same farm differed in each of the sampling periods. On the other hand, 21 farms were found to be consistently infected with the same SIV subtype throughout the study. This can either be explained by the persistence of the virus in a farm, or by the periodical re-introduction of SIVs of the same subtype.

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.

Study of influenza A virus in wild boars living in a major duck wintering site

Wild birds, which are reservoirs of influenza viruses, are believed to be the original source of new influenza viruses-including highly pathogenic ones-that can be transmitted to domestic animals as well as humans and represent a potential epizootic and/or pandemic threat. Despite increasing knowledge on influenza A virus dynamics in wild birds, the viral circulation in wild boars remains largely unknown. This is of particular interest since pigs can be infected with both human and avian viruses; upon co-infection, they can act as a mixing vessel through reassortment, a mechanism that resulted in the emergence of the pandemic H1N1 virus in 2009. The Camargue (Southern France) appears as an ideal study area to investigate inter-species transmission of influenza A viruses from wild birds and possibly humans to wild boars. Indeed, the important local wild boar population shares wetland use with humans and the largest concentration of wintering ducks in France, that are both susceptible to infection by influenza A viruses. Additionally, wild boars occasionally prey on ducks. We conducted a virological and serological survey on wild boars in the Camargue (Southern France) between September 2009 and November 2010. No influenza A virus was detected in the collected nasal swabs (n=315) and no influenza specific antibodies were observed in the serological samples (n=20). As the study was mainly focused on viral excretion, which is limited in time, we cannot exclude that low or occasional influenza A virus circulation took place during the study period. Although, wild boars did not seem to be a key element in the dynamics of influenza A virus circulation in the Camargue, wild boar influenza A virus infections should be more widely studied to determine if the pattern observed here represents the normal situation or an exceptional one.

The M segment of the 2009 new pandemic H1N1 influenza virus is critical for its high transmission efficiency in the guinea pig model

A remarkable feature of the 2009 pandemic H1N1 influenza virus is its efficient transmissibility in humans compared to that of precursor strains from the triple-reassortant swine influenza virus lineage, which cause only sporadic infections in humans. The viral components essential for this phenotype have not been fully elucidated. In this study, we aimed to determine the viral factors critical for aerosol transmission of the 2009 pandemic virus. Single or multiple segment reassortments were made between the pandemic A/California/04/09 (H1N1) (Cal/09) virus and another H1N1 strain, A/Puerto Rico/8/34 (H1N1) (PR8). These viruses were then tested in the guinea pig model to understand which segment of Cal/09 virus conferred transmissibility to the poorly transmissible PR8 virus. We confirmed our findings by generating recombinant A/swine/Texas/1998 (H3N2) (sw/Tx/98) virus, a representative triple-reassortant swine virus, containing segments of the Cal/09 virus. The data showed that the M segment of the Cal/09 virus promoted aerosol transmissibility to recombinant viruses with PR8 and sw/Tx/98 virus backgrounds, suggesting that the M segment is a critical factor supporting the transmission of the 2009 pandemic virus.