Effects of chicken embryo age on time to death following infection by avian influenza viruses: implications for distinguishing highly pathogenic isolates

When white leghorn (WL) chick embryos ranging in age from 8 to 13 days were inoculated with a variety of avian influenza virus (AIV) isolates, strain-specific differences in embryo mean death times (MDT) were observed. Non-highly pathogenic (nHP) strains killed 8 or 9 day-old embryos much more rapidly than 12 or 13 day-old embryos. Highly pathogenic (HP) strains, however, were less sensitive to embryo age resulting in similar MDTs in both older and younger embryos. These observations were consistent over a broad range of virus doses for both HP and nHP strains. When a HP derivative of H5N2 AIV was compared to its nHP parent, the derivative killed older embryos more rapidly than the parent virus, while MDTs in younger embryos were the same for both parent and derivative. The two strains further exhibited clear differences in the structure of their respective hemagglutinin, a previously described pathogenicity determinant for this virus. Thus it may be possible to readily demonstrate the HP phenotype in AIV strains based on MDT measurements in WL embryos.