Highly pathogenic virus recovered from chickens infected with mildly pathogenic 1986 isolates of H5N2 avian influenza virus.

A combination of in vitro and in vivo selection procedures was used to examine the possibility that certain mildly pathogenic field isolates of avian influenza (AI) virus may contain minority subpopulations of highly pathogenic virus. Two mildly pathogenic H5N2 isolates, A/chicken/New Jersey/12508/86 (NJ12508) and A/chicken/Florida/27716/86 (FL27716), recovered from chickens epidemiologically associated with urban live-bird markets, were cloned in trypsin-free chicken embryo fibroblast cultures. Selected clones were inoculated intranasally and intratracheally (IN/IT) into specific-pathogen-free laying hens, and virus reisolated from the hens that died was serially passed in hens by IN/IT inoculation. Several highly pathogenic reisolates were recovered from hens infected with the cloned NJ12508 or FL27716 virus. A highly pathogenic NJ12508 reisolate killed 19 of 24 IN/IT-inoculated hens, and a FL27716 reisolate killed all 24 inoculated hens; signs and lesions were typical of fowl plague. In contrast, uncloned NJ12508 stock virus killed 1 of 24 hens and FL27716 stock virus killed 4 of 24 hens, and neither produced the complete spectrum of lesions associated with fowl plague. Recovery of highly pathogenic viruses from these isolates demonstrates the coexistence of pathogenically distinct subpopulations of virus. Competition for dominance among such subpopulations could explain the variable pathogenicity of some AI viruses.