Persistence of genes of epidemical influenza viruses in natural populations in the USSR.

Antigenic analogues of all known epidemic human viruses (H1, H2 and H3) have been isolated from natural sources in the USSR. A 5 to 25-year persistence of genes coding haemagglutinins of these viruses in the genome structures of natural populations of viruses have been established. The viruses are more commonly isolated from water birds (ducks, gulls, terns, coots, shelducks, sandpipers), and the circulation of influenza viruses in nature among pigeons, crows, chickens and other birds have been identified. The viruses have been also isolated from bats, whales, squirrels, deer, fishes and lake water. The exchange between the genofond of natural populations of influenza A viruses and epidemic influenza viruses is discussed.