European swine researcher Kristien Van Reeth says there is reason to doubt the theory of the pig as a mixing vessel for influenza viruses. She also questions the significance of the pig's role in the emergence or future transmission of the novel 2009 H1N1 pandemic influenza virus.

Antigenic shift and drift common to influenza viruses can lead to the emergence of novel subtypes of flu with the potential for epidemic or pandemic consequences, says Van Reeth, from the faculty of Veterinary Medicine at Ghent University in Belgium. The consensus is that all novel influenza viruses stem from wild aquatic birds, but that interspecies transmission is actually fairly uncommon.

Van Reeth points out that interestingly, the classical swine H1N1 and the early human H1N1 influenza viruses both originated from the 1918 pandemic influenza outbreak, when signs of disease were observed simultaneously in swine and humans. This was the first report of flu-like disease in swine, although flu symptoms in humans had been observed for hundreds of years. But it's unknown whether the 1918 virus occurred in humans or swine first.

What is known, however, is that the evolution of the human and swine H1N1 viruses has produced an antigenic (protein or carbohydrate substance capable of producing an immune response) difference between the recent swine and recent human seasonal H1N1 subtypes. The classical H1N1 influenza virus has further combined with avian and human viruses to produce an “internal triple reassortant gene cassette.”

Of note is the fact that the European H1N1 swine virus, derived from wild birds, is much different from the classical H1N1 virus common for so many years in U.S. swine. The reassortant (having genetic material from two or more similar viruses) of the European avian-like H1N1 influenza virus and the North American virus has resulted in the creation of the novel 2009 pandemic H1N1 influenza virus currently circulating in humans worldwide.

While swine influenza virus (SIV) is known as a zoonotic disease, only a few cases have been confirmed in humans. The true incidence of zoonotic SIV infections is unknown. Van Reeth says until recently, all SIVs lacked the ability to spread from human to human.

There are currently three influenza viruses circulating in the human population: seasonal H1N1, H3N2 and the novel pandemic H1N1. The theory is that the reassortant of swine viruses occurs in the pig, but this has never been proven, she points out.

And the emergence of the highly pathogenic avian influenza virus (H5N1) challenged the theory of the pig as a mixing vessel, Van Reeth notes. It was evident that humans were becoming infected from direct contact with infected poultry. This observation led to the discovery in 2006 that the human respiratory tract contains receptors for both human and avian viruses (confined to the lungs). Recent research has shown that, in contrast to previous theory, pigs are not easily infected with avian viruses and do not transmit those viruses efficiently.

Van Reeth's research indicates the novel H1N1 influenza virus may not become widespread in swine and that swine will likely not play a major role in the transmission of this novel virus.

Van Reeth's talk was given at the Leman Swine Conference in mid-September in St. Paul, MN.