Pathogenicity of marine birnavirus (MABV) on fingerlings of rock bream (Oplegnathus fasciatus)

To investigate the pathogenicity of marine birnavirus (MABV) on rock bream (Oplegnathus fasciatus), two different sized rock bream fingerlings were artificially infected with MABV. The experimental MABV strain previously isolated from asymptomatic olive flounder (Paralichthys olivaceus), was not pathogenic for olive flounder. Experimental infection of rock bream fingerlings with the virus elicited typical signs of birnaviral disease and yielded high cumulative mortality. The presence of MABV in infected rock bream fingerlings was confirmed by electron microscopy. These results indicate that rock bream is susceptible to MABV isolated from asymptomatic olive flounder. There were no differences in gene sequence encoding VP5-pVP2 region between the viruses isolated from diseased rock bream and the virus cultured in fish cell line. These results indicate that the virulence of the present MABV strain is variable according to host fish species, and the gene sequence encoding VP5-pVP2 region is not related at least by itself to the susceptibility of rock bream to the disease caused by the present MABV strain.