The emergence of the infection of subgroup J avian leucosis virus escalated the tumour incidence in commercial Yellow chickens in Southern China in recent years.

A total of 81 clinical cases of suspected tumours were submitted to our laboratory from Yellow chicken farms in southern China during the years 2010 through 2017. The tumour-like tissue samples were closely examined for common oncogenic avian viruses in cell culture and further analysed using polymerase chain reaction (PCR). During 2010-2012, Marek's disease virus (MDV) mono-infection was found to be the dominant cause of the tumour incidences (52.4%, 11/21) followed by co-infection of MDV+ALVs (19.1%, 4/21). Starting from the year 2013 the mono-infection of avian leucosis virus subgroup J (ALV-J) became the dominant agent of the tumour cases (83.3%, 5/6). During the most recent four years (2014-2017), co-infections involving ALV-J and MDV or between ALV subgroups have increased (23.4% and 18.5%, respectively), but each of the co-infections was still slightly lower than the ALV-J mono-infection incidence (33.3%). In contrast to the dominant MDV mono-infection cases before 2013, more recently, the emerging ALV-J mono-infection and ALV-J co-infections were largely responsible for the occurrence of avian virus-induced tumour incidences in the commercial local Yellow breeds of chickens in southern China. These results indicate that eradication measures against ALV on all chicken farms, especially on farms with the Yellow chickens, ought to be enhanced to reverse this trend.