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 characteri zed by light yellow coloration of the dorsal cephalothorax area and ge nerally pale or bleached appearance of affected prawns. The yellow col or in the cephalothorax region results from the underlying yellow hepa topancreas showing through the translucent carapace in moribund shrimp . In histological preparations of moribund yellow-head specimens for t he light microscope, no consistent bacterial, fungal or parasitic agen ts 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 den sely basophilic, globose cytoplasmic inclusions located adjacent to so me of the hypertrophied nuclei. Similar basophilic inclusions were fou nd in interstitial hepatopancreatic tissue, in connective tissue under lying the mid gut, in cardiac tissue, in gill tissue and in hematopoet ic 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 wer e also present in intercellular spaces. The virions were similar to th ose of the insect granulosis viruses (Baculoviridae) in terms of cytop lasmic location, size, morphology and development. However, they were not occluded by granulin.