Submitted to: Journal of General Virology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/1996
Publication Date: N/A
Interpretive Summary: A recent outbreak of avian influenza in Mexico has caused great concern in the US poultry industry. In order to better understand the nature of such a natural outbreak, we have analyzed 18 isolates taken during a 15 month period from 6 central Mexican states. Our findings indicate that these viruses mutate quite rapidly in the field, with the primary surface protein of the viruses, the hemagglutinin (HA), undergoing substantial change. The changes occur at sites all along the HA protein, many of them affecting areas involved in immune response. As such, these viruses are much like the human influenza strains in that constant mutation results in production of strains which can elude the bird's immune system. This is the first such direct demonstration of this phenomenon in avian species. In this study we also identified a mutational event which resulted in the insertion of new genetic information into the hemagglutinin gene. Following this insertion event, the viruses shifted from the non- pathogenic to the highly pathogenic type, causing severe illness and death in commercial chickens. It is believed that this mutational insertion represents a new mechanism by which avian influenza viruses become virulent and may have significant consequences in future avian influenza outbreaks. Computer assisted analysis of the virus HA gene further allowed molecular predictions as to how the insertion could occur.
Technical Abstract: Molecular changes in the hemagglutinin (HA) coding regions and proteolytic cleavage sites from multiple H5N2 subtype viruses isolated during a recent outbreak of avian influenza (AI) 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 HA1 amino acid sequences were identical and changes were not restricted to a specific region. Phylogenetic analyses of the HA1 sequences demonstrated two viral lineages, designated Puebla and Jalisco, with sequence variations as high as 10.5 percent for amino acid and 6.2 percent for nucleotide sequences. During the latter months of the surveillance period, highly pathogenic (HP) strains of AI emerged causing lethal disease in commercial poultry flocks. In each of the HP strains isolated, the HA protein was cleaved in chicken embryo fibroblast cells in n the absence of trypsin, and two alterations not found in earlier non- highly pathogenic (nHP) isolates were detected. In the HA protein, highly pathogenic strains all had a glutamic acid to lysine substitution at amino acid position 324 and an insertion of an arginine-lysine as new residues 325 and 326. The insertion appears to be due to a duplication of the nucleotide sequence AAAGAA at nucleotide positions 965-970 of the HA1 coding region.