Genetic and antigenic evolution of swine influenza viruses in Europe and evaluation of their zoonotic potential.

Swine influenza is a highly contagious respiratory viral infection of pigs that has become enzootic in areas densely populated with pigs. Like other influenza A viruses, swine influenza virus (SIV) is genetically unstable and able to accumulate antigenic drifts and/or antigenic shifts. The pig is susceptible to both avian and human influenza viruses and can serve as an intermediate host in influenza virus ecology. Zoonotic agents may emerge in pigs following the modification of an established swine strain, adaptation of a strain of avian origin to the mammalian host, or reassortment between human and avian influenza viruses. Three different subtypes, H1N1, H3N2 and H1N2, are at present circulating in Europe. They differ from those found in North America and Asia and various lineages can be distinguished within each subtype. To date, European SIVs have not produced a global outbreak of influenza in humans but sporadic cases of SIV infection have been reported. This review presents a historical record of the genetic and antigenic evolution of SIVs in Europe. Our present understanding of the transmission of European SIVs from pigs to other animal species and to humans, together with the factors that limit inter-species transmission, is described.