Observations on the geographic distribution, pathogenesis and morphology of the baculovirus from Penaeus monodon Fabricius

A baculovirus disease of Penaeus monodon Fabricius named monodon baculovirus (MBV) was found in populations of postlarval P. monodon that originated from three separate sources in the Southwestern Pacific region. The pathogenesis, lesion development and morphology of the virus was the same regardless of the origin of the P. monodon population. The apparent target organs and tissues for MBV are the hepatopancreatic tubule and duct epithelium of postlarvae, juveniles and adults, and the anterior midgut epithelium of very young postlarvae. It was not determined if MBV produces patent disease in the larval stages of P. monodon, nor how it is transferred from brood stock to their offspring. MBV disease may cause serious losses in compromised populations (especially in postlarvae and young juveniles), but under conditions favorable to the host, fewer mortalities occurred and only low-grade infections of the hepatopancreas were detected. Three stages in the pathogenesis of MBV disease were noted in the hepatopancreas. Stage 1 hepatopancreatocytes had slightly hypertrophied nuclei, but contained no occlusion bodies (a highly specialized type of inclusion body) and few completed virions; stage 2 cell nuclei also had hypertrophied nuclei, but contained developing occlusion bodies and completed virions; stage 3 cell nuclei contained mature occulusion bodies and abundant numbers of completed free and occluded virions. Cell necrosis and cytolysis or sloughing (releasing virus and occlusion bodies into the gut lumen) followed stage 3 development in living hosts. The morphology of MBV is consistent with members of the genus Baculovirus subgroup A. The MBV particles are rod-shaped, singly enveloped, and replicate in the nucleus, occur free or within proteinaceous polyhedral occlusion bodies, and are assumed to contain DNA as the nucleic acid type. The nucleocapsid of MBV measured 42 ± 3 nm by 246 ± 15 nm, while the enveloped virions measured 75 ± 4 nm by 324 ± 33 nm.