HETEROGENEITY IN THE HEMAGGLUTININ GENE AND EMERGENCE OF THE HIGHLY PATHOGENIC PHENOTYPE AMONG RECENT H5N2 AVIAN INFLUENZA-VIRUSES FROM MEXICO

Molecular changes in the haemagglutinin (HA)coding regions and proteol ytic cleavage sites from multiple H5N2 subtype viruses isolated during a recent outbreak of avian influenza (Al) in central Mexico have been characterized, Eighteen isolates, collected during a 15 month period (October 1993 to January 1995) from six central states, were sequenced , None of the 18 predicted HA(1) amino acid sequences were identical a nd changes were not restricted to a specific region of the sequence, P hylogenetic analyses of the HA, sequences demonstrated two virus linea ges, designated Puebla and Jalisco, with sequence variation as high as 10.5% for amino acid and 6.2% for nucleotide sequences, During the la tter months of the surveillance period, highly pathogenic (HP) strains of Al emerged causing lethal disease in commercial poultry Rocks, In each of the HP strains isolated, the HA protein was cleaved in chicken embryo fibroblast cells in the absence of trypsin, and two alteration s not found in earlier non-HP isolates were detected, In the HA protei n, HP strains all had a glutamic acid --> lysine substitution at amino acid position 324 and an insertion of arginine and lysine as new resi dues 325 and 326, The insertion appears to be due to a duplication of the nucleotide sequence AAAGAA at nucleotide positions 965-970 of the HA(1)-coding region, Computer-assisted secondary structure analyses pl ace the target for the insertion in a predicted RNA stem-loop structur e, A mechanism is suggested by which the polymerase duplicates the seq uence.