Replication-competent endogenous avian leukosis virus in commercial lines of meat chickens.

Group-specific (gs)-antigen-positive egg albumen in seven commercial lines of meat chickens was found to result from the presence of endogenous avian leukosis virus (ALV); these lines had resisted selection attempts to reduce the shedding rate. In two meat lines, exogenous as well as endogenous ALV contributed to the gs-antigen shedding. All hens that produced gs-antigen-positive albumen transmitted endogenous ALV to a high proportion of their embryos (20 to 100%). Hens shedding gs-antigen to albumen were negative for endogenous ALV in vaginal swabs and had no detectable antibody to subgroup E virus. Chickens hatched from these dams were negative for endogenous ALV in meconia but were viremic at 2 weeks of age. Replication-competent endogenous ALV was almost uniformly expressed in embryos of hens from nine meat lines that were negative for gs-antigen in albumen. Shedding of gs-antigen to albumen was not related to the level of endogenous ALV expression. Embryos from five meat lines tested were resistant to infection with ALV of subgroup E. The level of endogenous gs-antigen in albumen was consistently lower than the level of exogenous gs-antigen.