Co-infection of Marek’s disease virus serotype 2 and reticuloendotheliosis virus induces tumorigenesis in broiler breeders (China)

Marek’s disease (MD) and reticuloendotheliosis (RE) are chicken tumors caused by MD virus (MDV) and RE virus (REV), respectively. MD tumor is caused by MDV-1, while MDV-2 is considered non-oncogenic. Although MDV-1 and REV co-infections are common, no reports on REV and MDV-2 co-infection exist. This study investigated broiler breeder flock in Heilongjiang Province, China, vaccinated against MD but suspected of developing tumor diseases. Postmortem examination and histopathological observations revealed substantial infiltration and proliferation of tumor-like lesions in the liver and kidneys. MDV plaques were observed 3 days after the peripheral blood lymphocytes of seven infected chickens were inoculated into chicken embryo fibroblast cells. The results of virus isolation revealed that the MDV-2 positive rate was 100% among seven different chickens, with isolation of two REV strains from MDV-2-infected samples (2/7). PP24 sequence analysis of the isolated strain MDV-2-HLJ-202302 showed 51.7%–51.9% nucleotide homology with MDV-1 reference strains and 98.2%–98.4% with MDV-2 reference strains. The results further indicated that the isolated strain MDV-2-HLJ-202302 belonged to MDV serotype 2. The isolated strain REV-HLJ-202305 had a relatively high homology (98.6%) with reference strains GQ415646.2, 0K631657.1, and KJ756349.1 from China, Brazil, and USA, respectively. This study is the first to report natural MDV-2 and REV co-infection in chickens with clinical tumors, offering new insights into MDV and REV prevention and diagnosis.