COMPARISONS OF HIGHLY VIRULENT H5N1 INFLUENZA-A VIRUSES ISOLATED FROMHUMANS AND CHICKENS FROM HONG-KONG

Genes of an influenza A (H5N1) virus from a human in Hong Kong isolate d in May 1997 were sequenced and found to be all avian-like (K. Subbar ao et al., Science 279:393-395, 1998). Gene sequences of this human is olate were compared to those of a highly pathogenic chicken H5N1 influ enza virus isolated from Hong Kong in April 1997. Sequence comparisons of all eight RNA segments from the two viruses show greater than 99% sequence identity between them. However, neither isolate's gene sequen ce was closely (>95% sequence identity) related to any other gene sequ ences found in the GenBank database. Phylogenetic analysis demonstrate d that the nucleotide sequences of at least four of the eight RNA segm ents clustered with Eurasian origin avian influenza viruses. The hemag glutinin gene phylogenetic analysis also included the sequences from a n additional three human and two chicken H5N1 virus isolates from Hong Kong, and the isolates separated into two closely related groups. How ever, no single amino acid change separated the chicken origin and hum an origin isolates, but they all contained multiple basic amino acids at the hemagglutinin cleavage site, which is associated with a highly pathogenic phenotype in poultry. In experimental intravenous inoculati on studies with chickens, all seven viruses were highly pathogenic, ki lling most birds within 24 h.All infected chickens had virtually ident ical pathologic lesions, including moderate to severe diffuse edema an d interstitial pneumonitis. Viral nucleoprotein was most frequently de monstrated in vascular endothelium, macrophages, heterophils, and card iac myocytes. Asphyxiation from pulmonary edema and generalized cardio vascular collapse were the most likely pathogenic mechanisms responsib le for illness and death. In summary, a small number of changes in hem agglutinin gene sequences defined two closely related subgroups, with both subgroups having human and chicken members, among the seven virus es examined from Hong Kong, and all seven viruses were highly pathogen ic in chickens and caused similar lesions in experimental inoculations .