Avian influenza in swine: a threat for the human population?

Until recently, it was thought that pigs were required as intermediate hosts for the transmission of avian influenza viruses to humans and this hypothesis is based on three suppositions: (1) Pigs are more susceptible to avian influenza viruses than humans. (2) Pigs are the single animal species with receptors preferred by both avian (alpha 2-3 linked sialic acid to galactose) and human (alpha 2-6 linked sialic acid) influenza viruses, which supports their role as "mixing vessels" for reassortment between human and avian viruses. In addition, influenza viruses from aquatic birds can adapt to "human" receptors in the pig. (3) Genetic reassortment between avian and human influenza viruses, which is an important mechanism for the emergence of new pandemic human strains, frequently occurs in pigs in nature. The first part of this paper presents some critical (counter) arguments for these suppositions. The second part focuses on the role of swine in recent cases of highly pathogenic avian influenza (HPAI) in humans in The Netherlands and in Southeast Asia. The respective HPAI viruses have spread directly from infected poultry to both humans and pigs, and pigs did not serve as an intermediate host between birds and humans. Fortunately, it is unlikely that these viruses would spread widely in the human population, unless mutations or genetic reassortment would occur. In theory such genetic changes might occur in the pig. However, it is currently impossible to analyse the risk of the pig in the introduction of new avian influenza strains in the human population, because the basic questions about the replication and pathogenesis of such viruses in swine are still unanswered.