Big liver and spleen disease of broiler breeders

Five adult broiler breeders were inoculated with faeces and tissue extracts from birds with big liver and spleen (BLS) disease. Five birds were placed in contact with the inoculated birds. After 8 weeks all inoculated and four of the five in-contact birds were shown to be infected as evidenced by the detection of an immune response and/or the demonstration of BLS antigen in spleen and tissue smears, by an immunofluorescence test and by agar gel immunodiffusion. Using appropriate antisera BLS antigen was found mainly in phagocytic liver cells and in splenic macrophages and dendritic cells. Fewer cells containing antigen were found in the kidney. Double staining of liver and spleen tissue for BLS antigen and chicken immunoglobulin indicated that many cells were positive for both. It is possible that BLS antigen in most cells may represent phagocytosed material rather than a replicating agent. If so, while this could explain the failure to detect virus-like particles, it implies that the primary site of replication of the BLS agent and its nature remain to be established.