Efficient Transmission of Swine-Adapted but Not Wholly Avian Influenza Viruses among Pigs and from Pigs to Ferrets

Background. Pigs are considered to be intermediate hosts for the transmission of avian influenza viruses (AIVs) between birds and humans, but the transmissibility of AIVs among pigs and from pigs to other mammals remains largely unexplored. Methods. In an initial study, we confirmed the replication potential of various low-pathogenic AIVs in pigs and ferrets, which we used as a model for humans. We then examined the transmissibility of 6 selected low-pathogenic AIVs among pigs (by direct contact) and from pigs to ferrets (by air contact). Results. Although H1N1 and H3N2 swine influenza viruses spread readily from inoculated pigs to contact pigs and ferrets, evidence for AIV transmission was extremely rare. The fact that infected pigs shed lower amounts of AIVs than of swine influenza viruses may contribute to the reduced transmissibility of AIVs. Conclusions. Our data indicate that swine-adapted influenza viruses spread readily among pigs and from pigs to other susceptible mammalians and support the notion that AIVs undergo genetic adaptation to efficiently cross the species barrier. Our transmission models hold potential to study the factors that lead to the generation of pandemic influenza viruses.