2009 H1N1 Influenza A Virus

Soon after the emergence of the H1N1 virus in April 2009, ARS scientists at the National Animal Disease Center in Ames, Iowa, began research using virus samples provided by the Centers for Disease Control and Prevention (CDC). Shortly after the reports of the new H1N1 virus in North America, H1N1 virus outbreaks were reported on turkey breeder farms in Virginia and California in the United States as well as in Chile, Canada, and France. ARS scientists at the Southeast Poultry Research Laboratory (SEPRL) quickly evaluated the pathogenesis of the new H1N1 virus in bird species to determine the potential risk to the poultry industry. The first step was to evaluate whether current U.S H1N1 swine influenza vaccines can protect pigs from infection with the 2009 H1N1 influenza virus circulating in people. This research study also evaluated whether pre-existing titers in pigs previously infected with endemic H1N1 swine influenza viruses circulating in the U.S could protect against the 2009 H1N1 influenza virus. Second was to address whether meat, blood and tissue from pigs infected with the new 2009 H1N1 Influenza A Virus would be free of infectious virus.

Classical swine influenza virus infections are enzootic among pigs in North America. Sporadic cases of human infection with swine influenza virus have been reported in the United States and elsewhere. Worldwide, more than 50 human cases of swine influenza virus infection, mostly due to classical swine influenza virus, have been documented in the past 35 years, with the greatest risk of infection among people with occupational exposure to live pigs.

Experts believe pigs can act as a "mixing vessel" for the reassortment of avian, swine and human influenza viruses, and might play an important role in the emergence of novel influenza viruses that could be capable of causing a human pandemic similar to the virus in the current outbreak.

Between the 1930s and the 1990s, the most commonly circulating swine influenza virus among pigs-classical swine influenza A, known as H1N1-underwent little change.

However, by the late 1990s, multiple strains and subtypes of triple reassortant swine influenza viruses-whose genomes include combinations of avian, human and swine influenza virus gene segments-had emerged and became predominant among North American pigs. The 2009 H1N1 influenza virus is also a triple reassortent, but its lineage is different than the H1N1 influenza viruses currently circulating in U.S. pigs.

Background

The genetic makeup of swine influenza viruses is identical to other influenza A viruses and consists of 8 segments of RNA that code for different proteins. Influenza viruses have the ability to exchange these segments, creating new genetically different viruses. Two major surface glycoproteins (proteins with a carbohydrate attached), called hemagglutinin (H) and neuraminidase (N), are how influenza A viruses are identified. These glycoproteins also determine the host range, antigenicity and the pathogenicity of the viruses. The hemagglutinin and neuraminidase proteins are important targets for diagnostics and used to designate the subtype of the virus.

Currently 16 different hemagglutinins and 9 neuraminidases have been identified. The majority of these viral subtypes are found in waterfowl, with only a few combinations being found in humans and swine.

Swine influenza virus (SIV) is one of the primary causes of respiratory disease in growing pigs and can lead to major economic losses. Currently, only H1N1, H1N2, and H3N2 subtypes are circulating in the U.S. swine population.

Pigs have long been considered a potential source for new and novel influenza viruses that infect humans, as they have receptors on their cells that bind both mammalian and avian influenza viruses, increasing the opportunity for the exchange of genetic segments of the virus.

Previously, the CDC have reported approximately one case of human infection with a swine influenza virus every one to two years.

Recent ARS Research Results: 2009 H1N1 Influenza Virus

Project 1: Serologic cross-reactivity of serum samples from U.S pigs against the new 2009 H1N1 influenza virus

This study addressed whether U.S commercial swine herds are susceptible to the 2009 A/H1N1 influenza viruses isolated from persons in California, New York, and Mexico. ARS researchers tested serum samples from pigs inoculated with swine influenza viruses that are known to circulate in the U.S or vaccinated with commercial vaccines to determine if U.S commercial swine herds are susceptible to the new H1N1 influenza virus. They found that there was limited cross reactivity against the new 2009 A/H1N1 influenza viruses. This suggests that pre-existing immunity induced by swine influenza viruses previously circulating in the U.S may not protect pigs against the new 2009 A/H1N1 influenza viruses presently circulating in people. Importantly, vaccines currently used to protect pigs on U.S swine farms operations against swine influenza viruses may not be effective against the new 2009 A/H1N1 influenza viruses.

Next step: ARS scientists will test the efficacy of a select subset of swine influenza virus vaccines tested in this first study, and evaluate their effectiveness in a pig vaccination challenge study to determine whether measurable antibody titers in pigs correlate with protection against the new A/H1N1 Influenza Virus.

More information about this study

Project 2: Four-pig pathogenesis study with the 2009 A/H1N1 influenza virus

This study addressed whether meat, blood, and tissue from pigs infected with the new 2009 A/H1N1 influenza virus are free of infectious virus. ARS researchers tested four 5-week-old cross-bred pigs from a herd free of swine influenza virus. The pigs were inoculated with an infective dose of the 2009 A/H1N1 influenza virus isolated from persons in California. Pigs were observed daily for clinical signs of disease and nasal swabs and fresh samples from lung, tonsil, inguinal lymph node, liver, spleen, kidney, skeletal muscle (ham), and colon contents were tested by the most sensitive virus detection assays. Live 2009 A/H1N1 influenza virus was only detected in the respiratory tract of infected pigs and the virus did not appear to spread and replicate in other tissues.

Next step: ARS scientists will conduct a larger study to evaluate tissues at additional time points (1, 3, 5, and 7 days dpi).

More information about this study

Project 3: Thirty-pig pathogenesis study with 2009 A/H1N1 influenza virus isolates from California and Mexico

This second H1N1 pathogenesis study was much larger than the first 4-pig study (Project 2) and tested viral isolates from California (2009 A/H1N1 influenza) and Mexico A/Mexico/4108/2009). A total of 30 five-week-old cross-bred pigs from a herd free of swine influenza virus were inoculated to determine the susceptibility of swine to the human virus and also address whether meat, blood, and tissue from pigs infected with the pandemic 2009 A/H1N1 influenza virus are free of infectious virus. Pigs were observed daily for clinical signs of disease and nasal swabs and fresh samples from lung, tonsil, inguinal lymph node, liver, spleen, kidney, skeletal muscle (ham), and colon contents were tested by the most sensitive virus detection assays at 3, 5, and 7 days post-infection. Live 2009 A/H1N1 influenza virus was only detected in the respiratory tract of infected pigs and the virus did not appear to spread and replicate in other tissues.

More information about this study

Project 4: Efficacy of Selected H1N1 Vaccines in Pigs

The pandemic H1N1 Influenza A (A/H1N1) virus emerged in 2009 in the human populations of North America (Mexico, United States, and Canada). Pandemic A/H1N1 virus,which has evolved into a new strain distinct from its Eurasian lineage, could become endemic in U.S. swine. This is due to a phenomenon known as genetic reassortment-the mixing of a virus' genetic material into new combinations-and ongoing genetic changes (antigenic drift). As a result, the virus causes the production of antibodies that are different from those of previous A/H1N1 viruses This presents a unique threat to previously unexposed human and swine populations. The development of a vaccine strain selection system is critical to controlling the virus and reducing the risk of future genetic reassortment and subsequent development of new A/H1N1 virus strains. Following up on previous research that demonstrated the limited application of existing assays to the 2009 pandemic A/H1N1 virus, scientists compared the disease protection and immunity conferred by three existing commercial, multivalent vaccines (with application across multiple strains) against an experimental inactivated homologous vaccine prepared from the 2009 pandemic A/H1N1 virus. As expected, the experimental vaccine prepared from the 2009 virus provided optimal protection against the 2009 pandemic A/H1N1 virus in all parameters evaluated: rectal temperatures, fever, viral shedding, and pneumonia. Moreover, this vaccine was the only one to cause the production of robust antibodies specific to the 2009 pandemic A/H1N1 virus. Although all three commercial vaccines tested provided partial protection, ranging from reduction of pneumonia lesions to reduced viral replication in the lung and nose, none were able to prevent all nasal shedding or clinical disease. It is important to note that none of the commercial vaccines caused aggravated pneumonia, a phenomenon occasionally seen with mismatched inactivated virus vaccines. Based on the findings here, development of monovalent homologous vaccines ( which would provide protection only to the current virus strain) is needed to vaccinate and protect all age groups of unexposed pigs against pandemic A/H1N1. This would protect the U.S. swine population and limit the potential transmission of pandemic A/H1N1 virus to pigs from humans, among pigs, or from pigs back to people.

More information about this study

Project 5: Susceptibility of poultry species to the 2009 novel H1N1 influenza A virus

The presence of avian and swine influenza virus genes in the 2009 novel H1N1 pandemic virus (pH1N1) raises questions about the potential for poultry becoming infected after being exposed to H1N1-infected humans or swine. This is especially important for turkeys because of their known susceptibility to type A influenza viruses and the history of infection with triple reassortant viruses. To study the possibility for transmission and infection of the pH1N1 virus in poultry, ARS researchers intranasally challenged turkeys, chickens, domestic ducks and Japanese quail with a pH1N1 virus. No clinical disease was produced in any of the species, and virus replication was infrequent and detected only in the oropharyngeal swabs of intranasally inoculated Japanese quail. There was no contact transmission of the virus to any of the species. This initial study suggests turkeys, chickens, and domestic ducks have low risk for field infection, but Japanese quail might become infected. Several other studies have showed that turkeys were resistant to infection when the virus was given intranasally, which is considered the natural route of infection. However, pH1N1 outbreaks have been reported on turkey breeder farms in Virginia and California in the United States as well as in Chile, Canada, and France, where they caused drops in egg production. In a second ARS study, laying turkey hens were inoculated with pH1N1 virus by intranasal, intracloacal and intrauterine routes. In this study, the turkeys became infected through the intracloacal and intrauterine routes, but not through the intranasal route. Virus replication in the reproductive tract of turkey hens after intrauterine inoculation caused decreased egg production but the birds present no clinical signs. The interuterineroute of exposure is a realistic concern since, in modern turkey production, turkey hens undergo intrauterine insemination in order to produce fertile eggs. This could explain virus outbreaks in such turkey flocks.

Next step: ARS scientists will conduct more studies on transmission of pH1N1 viruses in turkey hens and on vaccination against H1N1 virus.

ARS Swine Influenza Research Program

The ARS research program focuses on tracking the evolution of swine influenza viruses and their potential impact on the swine industry. The Agency also tracks the ability of current vaccines to protect swine from new virus isolates.

Because of the emergence of novel influenza viruses in U.S. pigs, USDA-ARS at NADC in collaboration with USDA's Animal and Plant Health Inspection Service and CDC created an interagency agreement in 2008 to address the need for monitoring the swine influenza virus. Having this agreement already in place has enabled the rapid response to evaluate the zoonotic potential of the 2009 H1N1 influenza virus.

ARS also supports a proposed USDA-APHIS swine influenza surveillance program by providing additional characterization of emerging swine influenza virus (SIV) isolates as they are identified through the surveillance program.

ARS Research Projects

Genetic and Antigenic Characterization of North American H1 Swine Influenza Viruses

Study of Influenza A Viruses

Characterization of Swine Influenza Virus Isolates

Influenza Pandemic Preparedness

ARS Publications

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

Detection of anti-influenza A nucleoprotein antibodies in pigs using a commercial influenza epitope-blocking enzyme-linked immunosorbent assay developed for avian species

Efficacy of inactivated swine influenza virus vaccines against the 2009 A/H1N1 influenza virus in pigs

One-step real-time RT-PCR for pandemic influenza A virus (H1N1) 2009 matrix gene detection in swine samples

Absence of 2009 Pandemic H1N1 Influenza A Virus in Fresh Pork

The Role of Swine in the Generation of Novel Influenza Viruses

Characterization of an Influenza A Virus Isolated from Pigs During an Outbreak of Respiratory Disease in Swine and People During a County Fair in the United States

Swine Influenza Viruses: A North American Perspective

Protection of Weaned Pigs by Vaccination with Human Adenovirus 5 Recombinant Viruses Expressing the Hemagglutinin and the Nucleoprotein of H3N2 Swine Influenza Virus

Experimental Infection of Pigs with the Human 1918 Pandemic Influenza Virus

Overcoming Maternal Antibody Interference by Vaccination with Human Adenovirus 5 Recombinant Viruses Expressing the Hemagglutinin and the Nucleoprotein of Swine Influenza Virus

Isolation of Reassortant H2N3 Avian/Swine Influenza Virus from Pigs in the United States

Limited Susceptibility and Lack of Systemic Infection by an H3N2 Swine Influenza Virus In Intranasally Inoculated Chickens

Failure of Protection And Enhanced Pneumonia with a US H1N2 Swine Influenza Virus in Pigs Vaccinated With an Inactivated Classical Swine H1N1 Vaccine

Related Graphics

Epidemiology of SIVs in North America. Click the image for larger version.

Triple reassortment. Click the image for larger version.

Epidemiology of SIVs in North America. Click the image for larger version.

Diagram of influenza A viruses. Click the image for larger version.

H1N1 Take Home Message Click the image for larger version.