Fish Diseases and Management

5.4.5.3 BMN (Baculoviral midgut gland necrosis virus) and other type C baculoviruses

This group does not produce occlusion body in the nuclei of infected cells. Infections by these viruses can be diagnosed by necrotic  tubular, epithelial cells that have hypertrophied nuclei with marginated chromatin, diminished nuclear chromatin, nucleolar dissociation and no occlusion bodies. Type C baculoviruses have been manually recognised in P. japonicus and P. monodon. BMN has been extensively studied. Enveloped viruses averaging 72-310  may occur in dense aggregates especially near to nuclear membrane and surrounding virogenic areas.

BMN causes serious epizootics in hatchery reared P. japonicus in Southern Japan. The disease in characterised by sudden onset and high mortality rate. The disease appears from protozoea stage 2 and mysis larvae but is more severe up to PL 9 – PL 10. The disease subsides by PL 20. The infected larvae can be distinguished by its inactive floating on the surface and by a white turbid midgut line through the abdomen. Histological conformation of the infection could be demonstrated by the presence of necrotic hepatopancreatic tubules epithelial cells possessing hypertrophied nuclei with marginated and diminished chromatin, nucleolar dissociation and the absence of occlusion bodies.