Identification of the viral haemorrhagic disease virus of rabbits as a calicivirus

Liver tissue from rabbits which had died of viral haemorrhagic disease (VHD) was used to identify the causative agent of the disease. After extraction of liver homogenates and density gradient ultracentrifugation, viral particles with buoyant densities between 1.32 to 1.38 g/ml and estimated sedimentation coefficients between 100 S and 175 S were obtained. The isolated virions were determined to have a diameter of 40 nm. The particles morphologically resembled those of the Caliciviridae family, had positive reactions both in ELISA and haemagglutination assays, and were infective for rabbits. By immunoblotting, a major structural protein with a molecular weight of 60 kDa was identified. RNA of high purity and of approximately 8 kb was isolated from virions. Labelled cDNA of virion RNA detected two RNAs of 8 kb and 2 kb in Northern blots of liver RNA from animals infected with the VHD virus (VHDV). Finally, isolated virion RNA injected directly into the liver or rabbits produced a disease with clinical symptoms and pathological findings typical of VHD. A calicivirus originally designated "rabbit haemorrhagic disease virus (RHDV)" was thus identified as the causative agent of VHD. The agent causing the European brown hare syndrome (EBHS) is also a calicivirus (EBHSV). VHDV and EBHSV are different members of the Caliciviridae family; their relationship can be clearly demonstrated by using different approaches of investigation.