Generation of a highly pathogenic avian influenza A virus from an avirulent field isolate by passaging in chickens

Highly virulent avian influenza viruses can arise from avirulent strains ma intained in poultry, but evidence to support their generation from viruses in wild birds is lacking. The most likely mechanism for the acquisition of virulence by benign avian viruses is the introduction of mutations by error -prone RNA polymerase, followed by the selection of virulent viruses, To in vestigate whether this mechanism could apply to wild waterfowl, we studied an avirulent wild-swan virus that replicates poorly in chickens. After 24 c onsecutive passages by air sac inoculation, followed by five passages in ch icken brain, the avirulent virus became highly pathogenic in chickens, prod ucing a 100% mortality rate. Sequence analysis at the hemmaglutinin cleavag e site of the original isolate revealed a typical avirulence type of sequen ce, R-E-T-R, which progressed incrementally to a typical virulence type of sequence, R-R-K-K-R, during repeated passages in chickens. These results de monstrate that avirulent viruses maintained in wild waterfowl in nature and bearing the consensus avirulence type sequence R-E-T-R have the potential to become highly pathogenic while circulating in chickens.