Changes in the haemagglutinin and the neuraminidase genes prior to the emergence of highly pathogenic H7N1 avian influenza viruses in Italy

Outbreaks of avian influenza due to an H7N1 virus of low pathogenicity occu rred in domestic poultry in northern Italy from March 1999 until December 1 999 when a highly pathogenic avian influenza (HPAI) virus emerged. Nucleoti de sequences were determined for the HA1 and the stalk region of the neuram inidase (NA) for viruses from the outbreaks. The HPAI viruses have an unusu al multibasic haemagglutinin (HA) cleavage site motif, PEIPKG (SRVR) under bar RGLF. Phylogenetic analysis showed that the HPAI viruses arose from low pathogenicity viruses and that they are most closely related to a wild bir d isolate, A/teal/Taiwan/98. Additional glycosylation sites were present, a t amino acid position 149 of the HA for two separate lineages, and at posit ion 123 for all HPAI and some low pathogenicity viruses. Other viruses had no additional glycosylation sites. All viruses examined from the Italian ou tbreaks had a 22 amino acid deletion in the NA stalk that is not present in the N1 genes of the wild bird viruses examined. We conclude that the Itali an HPAI viruses arose from low pathogenicity strains, and that a deletion i n the NA stalk followed by the acquisition of additional glycosylation near the receptor binding site of HA1 may be an adaptation of H7 viruses to a n ew host species i.e. domestic poultry.