The ecology of influenza viruses: a WHO memorandum.

Influenza A viruses continue to be isolated from man, pigs, horses, and a wide range of avian species, especially ducks. The recent isolation of an influenza A virus from seals has added an additional mammal to the list of natural hosts for these viruses. In contrast, influenza B viruses have been isolated only from man.The haemagglutinin of a virus isolated from gulls in the United States of America could not be identified with reference antisera and may constitute a new haemagglutinin subtype.Studies in swine suggest that H1N1 viruses related to A/New Jersey/8/76 continue to circulate in some countries of the world, and that the H3N2 and H1N1 viruses can be transmitted from man to pigs. The isolation from pigs of an H1N1 virus that is antigenically similar to avian H1N1 isolates and the isolation of "classical" swine influenza virus from a piglet in Czechoslovakia are of particular interest and raise the question of the source of the viruses.Preliminary studies on equine influenza viruses (H3N8) (Heq2Neq2) from horses in Europe suggest that significant antigenic drift has occurred in viruses of this subtype since the prototype was identified in 1963.The role of influenza viruses from lower animals and birds in the genesis of new human strains is unknown, and the genes responsible for host range and virulence have not yet been identified. The availability of techniques for analysis of all the RNAs and the gene products will permit further studies on these important questions.