Influence of dietary calcium stress on lethality of avian influenza viruses for laying chickens.

The effect of calcium stress was studied in an attempt to reproduce lethal infections in laying chickens with A/Chicken/Alabama/75 (H4N8) influenza virus and with two nonpathogenic H5N2 influenza viruses from the 1983-84 outbreak in the eastern United States. Hens were fed calcium-deficient or standard diets for 7 to 14 days; then the calcium-deficient feed was replaced with standard feed supplemented with ad libitum oyster shell, and both groups of hens were inoculated with virus. When hens were infected with the H4N8 virus, respective mortalities of those on the calcium-deficient and standard diets were 19% (27/141) and 5% (7/143). The H5N2 viruses did not kill hens fed either diet. In standard pathogenicity tests, Alabama H4N8 viruses reisolated from the hens that died generally were more lethal for 4-week-old chickens than the stock virus. These results argue for characterization of the Alabama H4N8 virus as pathogenic rather than nonpathogenic as originally determined.