Pigs and Pandemic Influenza: Myths versus Facts

September 30, 2009 — 

Pigs and Pandemic Influenza: Myths versus Facts was the title of Dr. Kristien Van Reeth's Pijoan Lecture during the 2009 Al Leman Swine Conference. Dr. Van Reeth, from the faculty of Veterinary Medicine at Ghent University in Belgium, gave a very concise and informative presentation on the role of pigs in influenza pandemics. Her presentation casts doubt on the theory of the pig as a mixing vessel for influenza viruses and questions the significance of the pig's role in the emergence or future transmission of the novel 2009 H1N1 pandemic influenza virus.

Dr. Van Reeth offers a thorough explanation of the mechanisms and implications of antigenic shift and drift common to influenza viruses and how this genetic reassortment can lead to the emergence of novel subtypes with the potential for epidemic or pandemic consequences. She emphasizes that the consensus is that all novel influenza viruses arise from wild aquatic birds but that interspecies transmission is actually fairly uncommon.

Interestingly, although they have evolved along different pathways, the classical swine H1N1 (cH1N1) and the early human H1N1 viruses both derived from the 1918 pandemic influenza outbreak when clinical signs were observed simultaneously in swine and humans. This was the first report of influenza-like disease in swine although symptoms of human influenza had been recognized for centuries. It is unclear whether the 1918 virus arose in humans and was transmitted to pigs or vice versa.

The evolution of the human and swine H1N1 viruses, however, has resulted in antigenic differences between the recent swine and recent human seasonal H1N1 subtypes. The cH1N1 has further combined with avian and human viruses to produce an internal triple reassortant gene cassette.

Notably, the European H1N1 swine virus, derived from wild birds, is much different from the cH1N1 virus common in the U.S. The reassortment of the European avian-like H1N1 and the North American swine virus has resulted in the novel 2009 pandemic H1N1 influenza currently circulating in the global human population.

While swine influenza is recognized as a zoonotic disease, there have been only a handful of reported human cases. The true incidence of zoonotic SIV infections is unknown. According to Dr. Van Reeth, until recently, all SIVs lacked the capacity to spread from human to human.

Currently, there are 3 influenza viruses circulating in the human population: seasonal H1N1, H3N2, and the novel pandemic H1N1. It has been hypothesized that the reassortment of swine viruses occurs in the pig but this has never been proven. The emergence of the highly pathogenic avian influenza virus (H5N1) in 1998 challenged the theory of the pig as a mixing vessel. It became evident that humans were becoming infected directly from 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 efficiently transmit those viruses.

While it has been shown that the novel H1N1 pandemic virus can infect pigs and transmits readily between pigs, research shows there may be some level of partial immunity to the novel virus within at least the European swine population. Dr. Van Reeth's research shows serologic cross-reaction with 2 European H1N1 SIV vaccines and cross-protection with a North American H1N1 indicating that the virus may not become widespread in swine and that swine will likely not play a significant role in the transmission of this novel virus.

The organizers of the Leman Conference have made this presentation, as well as those of the other keynote speakers, available online for review on the SwineCast website. I would encourage you to take the time to view this timely and informative presentation at http://www.swinecast.com/dr-kristien-van-reeth-swine-influenza-recent-developments.