Avian leukosis virus (ALV) is a retrovirus that induces tumours in infected birds; ALV is divided into different subgroups according to the env gene and cellular tropism. In general, ALV subgroup J (ALV-J) is considered to be the most pathogenic and prevalent subgroup while subgroup K (ALV-K), a newly identified subgroup, only causes mild symptoms. To illuminate the roles of the env viral gene and LTR sequence in pathogenic differences between ALV-J and ALV-K, rescued ALV-J strain rSDAU1005, rescued ALV-K strain rJS11C1, and recombinant strains rENV(J)-LTR(K) and rENV(K)-LTR(J) were characterized and investigated in this study. Among rescued viruses, rSDAU1005 had the highest replication efficiency while rJS11C1 replicated the slowest (replication efficiency rankings were rSDAU1005 >rENV(K)-LTR(J)>rENV(J)-LTR(K)>rJS11 C1). The luciferase reporter gene assay results showed that the promoter activity of ALV-K LTR was lower than that of the ALV-J LTR promoter, which may have accounted for the slower replication efficiency of ALV-K. Pathogenicity of the four rescued viruses was determined via inoculating the yolk sacs of specific-pathogen-free chickens. The results demonstrated that all four viruses were pathogenic; rSDAU1005 caused the most severe growth retardation and immunosuppression. rENV(J)-LTR(K) was more pathogenic when compared to rENV(K)-LTR(J), indicating that env and the LTR sequence play important roles in pathogenicity between ALV-K and ALV-J. Additionally, env seemed to especially play a role in ALV-K pathogenesis. This study provided scientific data and insight to improve detection methods and judgement criteria in ALV clearance and surveillance.

Psittacine beak and feather disease virus (PBFDV) has been reported in many countries, such as Australia, Poland, the United States, South Africa, etc. In this study, the complete genome of a PBFDV isolate was determined and characterized from budgerigars in China.