Diversity of influenza viruses in swine and the emergence of a novel human pandemic influenza A (H1N1)

Abstract

The novel H1N1 influenza virus that emerged in humans in Mexico in early 2009 and transmitted efficiently in the human population with global spread has been declared a pandemic strain. Here we review influenza infections in swine since 1918 and the introduction of different avian and human influenza virus genes into swine influenza viruses of North America and Eurasia. These introductions often result in viruses of increased fitness for pigs that occasionally transmit to humans. The novel virus affecting humans is derived from a North American swine influenza virus that has acquired two gene segments [Neuraminidase (NA) and Matrix (M)] from the European swine lineages. This reassortant appears to have increased fitness in humans. The potential for increased virulence in humans and of further reassortment between the novel H1N1 influenza virus and oseltamivir resistant seasonal H1N1 or with highly pathogenic H5N1 influenza stresses the need for urgent pandemic planning.

Figure 1

Evolution of swine influenza in North America from 1930‐present. Swine influenza remained relatively stable in the North American swine population from 1930 to 1998. It occasionally crossed over into the human population, but did not spread. After the emergence of the triple reassortant H3N2 viruses, these viruses transmitted to turkeys and continue to circulate in the turkey population. Further reassortment events resulted in two distinct H1N1 reassortants and two distinct H1N2 reassortants. The H1N1 with classical swine H1 and N1 and the H1N2 with human H1 and N2 have been isolated from humans but did not spread. The North American/Eurasian reassortant H1N1 virus is the first swine virus to establish sustained human to human transmission.