Endogenous viral genes influence infection with avian leukosis virus.

The influence of endogenous viral (ev) genes on avian leukosis virus (ALV) infection was studied in ALV-free white leghorn chickens exposed to chicks from ALV shedding dams. The study included four lines, each segregating for one ev gene, one line free of ev genes, and four commercial stocks segregating for a number of ev genes. Genes ev12 and ev21 that produce the complete endogenous virus were associated with significant reductions in antibody response to ALV. In commercial stocks with ev21 in all birds, both antibody response and ALV in oviducts were significantly influenced by the genomic background (stock). In one commercial stock, 37.6% of 133 birds with ev21 and only 5.8% of 120 birds without the gene were test-positive for virus in their oviducts at 180 days of age. No such differences were associated with ev1, ev3 or ev6 that do not produce complete endogenous virus. Thus, virus-producing ev genes were a predisposing factor for shedding ALV.