Emergence of highly pathogenic virus during selective chicken passage of the prototype mildly pathogenic chicken/Pennsylvania/83 (H5N2) influenza virus.

The prototype mildly pathogenic A/chicken/Pennsylvania/21525/83 (H5N2) avian influenza virus, which was isolated more than 5 months before the emergence of highly pathogenic virus in the major 1983 Pennsylvania outbreak, was examined for the presence of minority subpopulations of highly pathogenic virus. Selective serial passage of the parental mildly pathogenic virus in leghorn hens did not lead to recovery of highly pathogenic virus. However, several highly pathogenic reisolates were recovered from hens inoculated with either of two mildly pathogenic virus clones selected for their ability to efficiently produce plaques in trypsin-free chicken embryo fibroblasts. Unlike the parental virus, these reisolates caused high mortality in chickens and produced postmortem lesions typical of highly pathogenic avian influenza. Electrophoretic mobilities of the hemagglutinin glycoproteins of the highly pathogenic derivatives resembled those of the prototype highly pathogenic A/chicken/Pennsylvania/1370/83 (H5N2) virus isolated in October 1983. These results suggest that unrecognized subpopulations of highly pathogenic virus may have infected Pennsylvania chickens for several months before emerging as the clinically manifest component of the virus population.