Avian leukosis virus infection and congenital transmission in lines of chickens resisting selection for reduced shedding.

Two lines (D and E) of three breeder lines of chickens that had resisted selection for reduced avian leukosis virus (ALV) congenital transmission on the breeder's premises did not resist the same selection procedures (tests for gs-antigen in albumen) under laboratory conditions. The incidence of ALV congenital transmission in the remaining third line (F) was spontaneously reduced from 13% to 0.9%. Environmental ALV exposure of uninfected chicks after hatching induced 7-10% of the progeny from lines E and F to become congenital transmitters but had negligible effects on line D. Neither errors in identifying dams nor horizontal transmission leading to congenital transmission were great enough to explain the lack of improvements in the three lines on the breeder's premises. Conditions of environmental exposure on the breeder's farm seem most likely to account for the resistance to reduced shedding. These findings suggest that the effectiveness of testing and selection procedures used to reduce ALV may be greatly influenced by the environment.