HISTOLOGY AND ULTRASTRUCTURE REVEAL A NEW GRANULOSIS-LIKE VIRUS IN PENAEUS-MONODON AFFECTED BY YELLOW-HEAD DISEASE

A recently reported disease syndrome of Penaeus monodon in Thailand is called 'yellow-head' or 'hua leung' in Thai. It is usually characterized by light yellow coloration of the dorsal cephalothorax area and generally pale or bleached appearance of affected prawns. The yellow color in the cephalothorax region results from the underlying yellow hepatopancreas showing through the translucent carapace in moribund shrimp. In histological preparations of moribund yellow-head specimens for the light microscope, no consistent bacterial, fungal or parasitic agents could be found. The lymphoid organs of yellow-head specimens showed extensive abnormalities. These included obviously necrotic cells and vacuolated cells with hypertrophied nuclei. Also evident were very densely basophilic, globose cytoplasmic inclusions located adjacent to some of the hypertrophied nuclei. Similar basophilic inclusions were found in interstitial hepatopancreatic tissue, in connective tissue underlying the mid gut, in cardiac tissue, in gill tissue and in hematopoetic tissue. Transmission electron micrographs revealed the presence of previously undescribed rod-shaped, enveloped virions in the cytoplasm adjacent to the nuclei of cells from various tissues. Free virions were also present in intercellular spaces. The virions were similar to those of the insect granulosis viruses (Baculoviridae) in terms of cytoplasmic location, size, morphology and development. However, they were not occluded by granulin.