Interspecies transmission of influenza viruses

In this report we examine the hypothesis that aquatic birds are the primordial source of all influenza viruses in other species. Two partly overlapping reservoirs of influenza A viruses exist in migrating water-fowl and shorebirds throughout the world. These species harbor influenza viruses of all the known hemagglutinin and neuraminidase subtypes. In contrast to the rapid, progressive changes in both the nucleotide and amino acid sequences of mammalian virus gene lineages, avian virus genes show far less variation and, in most cases, appear to be in evolutionary stasis. There are periodic exchanges of influenza virus genes or whole viruses between species, giving rise to pandemics of disease in humans, lower animals, and birds. The periodic exchange of influenza viruses between species has been illustrated by the appearance of new pandemic influenza viruses in humans, including the Spanish influenza of 1918, the Asian influenza of 1957, and the Hong Kong influenza of 1968. Transmission of avian influenza viruses to swine in Europe in 1979 has resulted in the appearance of human-avian reassortant influenza viruses in pigs in Italy and in children in the Netherlands. These studies provide evidence supporting the possibility that pigs serve as a mixing vessel for reassortment between influenza viruses in mammalian and avian hosts and raise the question of whether the avian influenza viruses now circulating in European swine are the precursors of the next human pandemic virus.