Review 

Avian leukosis is a major neoplastic disease caused by avian leukosis viruses (ALVs), which are classified into 11 subgroups (ALV-A to ALV-K). Among them, ALV subgroup J (ALV-J) has undergone significant epidemiological changes since its introduction into China in 1999. It initially transmitted among broilers and then rapidly spread to commercial layer chickens and local chicken breeds. ALV-J infection typically induces myeloid leukosis in chickens and, in some layers, can also lead to hemangiomas. As a retrovirus, ALV-J exhibits high genetic variability. Compared with the prototype strain HPRS-103, the prevalent ALV-J strains in China show notable mutations in the gp85 gene, U3 region, and untranslated region (UTR). The variations in gp85 have led to the emergence of distinct evolutionary clusters of strains derived from layers and local chicken breeds, significantly enhancing viral replication and transmission. Additionally, a 205-nucleotide deletion in UTR and key mutations in the U3 region contribute to increased viral pathogenicity. For disease control, China has adopted an integrated strategy focusing on surveillance and eradication, supported by advanced ALV detection and eradication technologies. This review systematically summarizes the epidemiological evolution, molecular variations, and control measures of ALV-J in China over the past two decades, providing critical insights into its biological characteristics and guiding the development of more effective control strategies.

Keywords: avian leukosis; avian leukosis virus subgroup J; epidemiology; molecular variation; prevention and control strategies.