The pig as an intermediate host for inf luenza A viruses between birds and humans

Swine influenza (SI) was first observed at the time of the pandemic in humans in 1918, and since that time, subtypes H1N1 and H3N2 have been widely reported in pigs, frequently associated with respiratory disease. These include classical swine H1N1, avian-like H1N1 and humanand avianlike H3N2 viruses. Swine husbandry practices influence directly the evolution of SI viruses through reduced immune pressure and constant availability of susceptible hosts. The pig has been the leading contender for the role of intermediate host for reassortment of influenza A viruses, since they are the only mammalian species which are susceptible to, and allow productive replication of avian and human influenza viruses due to the presence of receptors for both virus types (a2,3 and a2,6, respectively), and this can result in modification of the receptor binding specificities of avian influenza viruses from a2,3 to a2,6 linkage, thereby providing a potential link from birds to humans. Several independent introductions of avian viruses to pigs have occurred, primarily involving H1N1 that has led to the establishment of stable lineages, but more recently with H4N6 and H9N2 viruses. Although H3N2 viruses related closely to early human strains continue to circulate long after their disappearance from the human population, there have also been frequent transmissions of the prevailing human strains and these may also be able to persist. In addition, reassortant viruses of H3N2 and H1N2 subtype derived from mixed lineages and usually containing genes encoding surface glycoproteins from human viruses have been reported widely in pigs. Numerous transmissions of virus from pigs to humans have occurred but without apparent secondary spread. Pigs may be important in the generation of ‘new’ strains, some of which may have the potential to transmit to other species including humans. Crown Copyright D 2001 Published by Elsevier Science B.V. All rights reserved.