Isolation of influenza A(H3N2) virus with "O"-->"D" phase variation

We report the isolation of two influenza A(H3N2) virus strains which were unable, in the first passages in MDCK cell culture, to agglutinate chicken erythrocytes, though reacting with guinea pig and turkey red blood cells. This observation demonstrates that the occurrence of this phenomenon is not exclusive to influenza A(H1N1) viruses, as previously reported. In order to investigate the molecular basis of this phenomenon, we analysed the nucleotide sequence of the HA-1 region, presumed to be involved in the switch of haemagglutination properties, from the virus present in the original samples and in the corresponding strains isolated and cultivated in MDCK cells and in embryonated eggs. The substitution of amino acid 138 (Ala-->Ser) in MDCK cells could be related to the change in haemagglutination characteristics.