In this review we examine the hypothesis that aquatic birds are the primordial source
of all influenza viruses in other species and study the ecological features that permit
the perpetuation of influenza viruses in aquatic avian species. Phylogenetic analysis
of the nucleotide sequence of influenza A virus RNA segments coding for the spike
proteins (HA, NA, and M2) and the internal proteins (PB2, PB1, PA, NP, M, and NS)
from a wide range of hosts, geographical regions, and influenza A virus subtypes support
the following conclusions. (i) Two partly overlapping reservoirs of influenza A viruses
exist in migrating waterfowl and shorebirds throughout the world. These species harbor
influenza viruses of all the known HA and NA subtypes. (ii) Influenza viruses have
evolved into a number of host-specific lineages that are exemplified by the NP gene
and include equine Prague/56, recent equine strains, classical swine and human strains,
H13 gull strains, and all other avian strains. Other genes show similar patterns,
but with extensive evidence of genetic reassortment. Geographical as well as host-specific
lineages are evident. (iii) All of the influenza A viruses of mammalian sources originated
from the avian gene pool, and it is possible that influenza B viruses also arose from
the same source. (iv) The different virus lineages are predominantly host specific,
but 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.
(v) The influenza viruses currently circulating in humans and pigs in North America
originated by transmission of all genes from the avian reservoir prior to the 1918
Spanish influenza pandemic; some of the genes have subsequently been replaced by others
from the influenza gene pool in birds. (vi) The influenza virus gene pool in aquatic
birds of the world is probably perpetuated by low-level transmission within that species
throughout the year. (vii) There is evidence that most new human pandemic strains
and variants have originated in southern China. (viii) There is speculation that pigs
may serve as the intermediate host in genetic exchange between influenza viruses in
avian and humans, but experimental evidence is lacking. (ix) Once the ecological properties
of influenza viruses are understood, it may be possible to interdict the introduction
of new influenza viruses into humans.