Phylogenetic Analysis and Pathogenicity Assessment of the Emerging Recombinant Subgroup K of Avian Leukosis Virus in South China

In recent years, cases of avian leukosis virus (ALV) infection have become more frequent in China. We isolated 6 ALV strains from yellow feather broiler breeders in south China from 2014 to 2016. Their full genomes were sequenced, compared, and analyzed with other reference strains of ALV. The complete genomic nucleotide sequences of GD150509, GD160403, GD160607, GDFX0601, and GDFX0602 were 7482 bp in length, whereas GDFX0603 was 7480 bp. They shared 99.7% to 99.8% identity with each other. Homology analysis showed that the gag, pol, long terminal repeats (LTRs), and the transmembrane region (gp37) of the env genes of the 6 viruses were well conserved to endogenous counterpart sequences (>97.8%). However, the gp85 genes displayed high variability with any known chicken ALV strains. Growth kinetics of DF-1 cells infected with the isolated ALV showed viral titers that were lower than those infected with the GD13 (ALV-A), CD08 (ALV-B), and CHN06 (ALV-J) on day 7 post-infection. The infected Specific-pathogen-free (SPF) chickens could produce continuous viremia, atrophy of immune organs, growth retardation and no tumors were observed. These subgroup ALVs are unique and may be common in south China. The results suggested that updating the control and eradication program of exogenous ALV for yellow feather broiler breeders in south China needs to be considered because of the emergence of the new subgroup viruses.