Experimental infection of snow crab Chionoecetes opilio with the Chionoecetes opilio bacilliform virus (CoBV)

Milky hemolymph syndrome (MHS) has been observed in both reared and wild populations of the snow crab Chionoecetes opilio from the Sea of Japan. The virus presumably associated with MHS and detected with electron microscopy was designated ‘Chionoecetes opilio bacilliform virus’ (CoBV), but little is known about its infectivity or pathogenicity in the snow crab. In this study, we performed an infectivity experiment in which snow crabs were injected with a viral inoculum prepared from MHS-affected crabs and then maintained at 2 °C or 2–8 °C. Cumulative mortality increased more rapidly at 2 °C than at 2–8 °C, reaching approximately 80% in both groups at the termination of the experiment. This result demonstrates the temperature-dependent pathogenicity of CoBV in the snow crab. CoBV-infected crabs displayed the clinical and histopathological signs of MHS. Mortality tended to increase as the viral load in the hemolymph increased at both 2 °C and 2–8 °C. The viral load in the hemolymph was significantly higher in dead crabs than in surviving crabs and was also elevated in various other organs. We conclude that CoBV is the causative agent of MHS and that severely affected crabs develop viremia that leads to death.