Synergistic pathogenic effects of co-infection of subgroup J avian leukosis virus and reticuloendotheliosis virus in broiler chickens

To study interactions between avian leukosis virus subgroup J (ALV-J) and reticuloendotheliosis virus (REV) and the effects of co-infection on pathogenicity of these viruses, 1-day-old broiler chicks were infected with ALV-J, REV or both ALV-J and REV. The results indicated that co-infection of ALV-J and REV induced more growth retardation and higher mortality rate than ALV-J or REV single infection (P < 0.05). Chickens co-infected with ALV-J and REV also showed more severe immunosuppression than those with a single infection. This was manifested by significantly lower bursa of Fabricius and thymus to body weight ratios and lower antibody responses to Newcastle disease virus and H9-avian influenza virus (P < 0.05). Perihepatitis and pericarditis related to severe infection with Escherichia coli were found in many of the dead birds. E. coli was isolated from each case of perihepatitis and pericarditis. The mortality associated with E. coli infection in the co-infection groups was significantly higher than in the other groups (P < 0.05). Among 516 tested E. coli isolates from 58 dead birds, 12 serotypes of the O-antigen were identified in two experiments. Different serotypes of E. coli strains were even isolated from the same organ of the same bird. Diversification of O-serotypes suggested that perihepatitis and pericarditis associated with E. coli infection was the most frequent secondary infection following the immunosuppression induced by ALV-J and REV co-infection. These results suggested that the co-infection of ALV-J and REV caused more serious synergistic pathogenic effects, growth retardation, immunosuppression, and secondary E. coli infection in broiler chickens.