An outbreak in three-yellow chickens with clinical tumors of high mortality caused by the coinfection of reticuloendotheliosis virus and Marek’s disease virus: a speculated reticuloendotheliosis virus contamination plays an important role in the case

Both reticuloendotheliosis and Marek's disease are neoplastic diseases of chickens caused by reticuloendotheliosis virus (REV) and Marek's disease virus (MDV), respectively. The infection of REV or MDV may lead to clinical tumors and also result in immunosuppression and easily allow secondary infection by other pathogens. Here, we investigated a breeder flock of three-yellow chickens in southern China that had been vaccinated with CVI988/Rispens at hatching and had experienced depression, weakness, reduction in weight gain, and an increased death rate after 120 d of age. The morbidity and mortality were 20% and 10%, respectively, at 140 d of age when this infection was diagnosed. The necropsy of the birds revealed significant tumor-like lesions in the heart, liver, spleen, and ceca. Peripheral blood lymphocytes and tumor-like tissues were sampled for PCR detection and for histopathological observation, for virus isolation and the subsequent immunofluorescent assay on the cell cultures and for gene sequencing of the isolated viruses. A REV isolate GX18NNR1 and a MDV isolate GX18NNM5 were both recovered from the sampled bird. Further phylogenetic analysis based on the env gene of REV and the meq gene of MDV demonstrated that GX18NNR1 was closely related to the reference REV strain MD-2, which was isolated from a contaminated commercial turkey herpesvirus vaccine. In addition, the GX18NNM5 was found to belong to the Chinese very virulent MDV strains' cluster. The coinfection of REV and MDV may contribute to tumor outbreaks with high morbidity and mortality in three-yellow chicken flocks.