A ProMED-mail post (May 28, 2009)
The following is  a statement of the research program of the United States Department of Agriculture (USDA) Agricultural Research Service (ARS), and  the outcome of a study of the cross-reactivity of serum samples from US pigs against the new swine origin 2009 H1N1 influenza virus (S/O H1N1). The results of this analysis indicate that pre-existing immunity induced by swine influenza viruses circulating in the US may not protect pigs against the new S/O H1N1 influenza virus presently circulating in people.
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). The 1st step was to evaluate whether current US 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 US could protect against the 2009 H1N1 influenza 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 US pigs.
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 are 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 US 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, CDC has reported about one case of human infection with a swine influenza virus every one to 2 years.
Recent ARS research results: 2009 H1N1 influenza virus: ARS researchers tested serum samples from pigs previously infected with US swine influenza viruses or vaccinated with commercial vaccines to determine whether US commercial swine herds are susceptible to the new swine origin (S/O) H1N1 influenza virus. They found that there was limited cross reactivity against the new S/O H1N1 influenza virus. This suggests that pre-existing immunity induced by swine influenza viruses previously circulating in the US may not protect pigs against the new S/O H1N1 influenza virus presently circulating in people. Importantly, vaccines currently used to protect pigs on US swine farm operations against swine influenza viruses may not be effective against the new S/O H1N1 influenza virus.