Multiple genetic reassortment of avian and human influenza A viruses in European pigs, resulting in the emergence of an H1N2 virus of novel genotype

Novel H1N2 influenza A viruses which were first detected in pigs in Great B ritain in 1994 were examined antigenically and genetically to determine the ir origins and establish the potential mechanisms for genetic reassortment, The haemagglutinin (HA) of all swine H1N2 viruses examined was most closel y related to, but clearly distinguishable both antigenically and geneticall y from, the HA of human H1N1 viruses which circulated in the human populati on during the early 1980s, Phylogenetic analysis of the HA gene revealed th at the swine H1N2 viruses formed a distinct branch on the human lineage and were probably introduced to pigs shortly after 1980, Following apparent tr ansfer to pigs the HA gene underwent genetic variation resulting in the est ablishment and cocirculation of genetically and antigenically heterogeneous virus populations. Genetic analyses of the other RNA segments of all swine H1N2 viruses indicated that the neuraminidase gene was most closely relate d to those of early 'human-like' swine H3N2 viruses, whilst the RNA segment s encoding PB2, PB1, PA, NP, M and NS were related most closely to those of avian viruses, which have been circulating recently in pigs in Northern Eu rope. The potential mechanisms and probable progenitor strains for genetic reassortment are discussed, but we propose that the swine H1N2 viruses exam ined originated following multiple genetic reassortment, initially involvin g human H1N1 and 'human-like' swine H3N2 viruses, followed by reassortment with 'avian-like' swine H1N1 virus. These findings suggest multiple reassor tment and replication of influenza viruses may occur in pigs many years bef ore their detection as clinical entities.