INVESTIGATIONS OF PENAEUS-STYLIROSTRIS DISEASE (SYNDROME-93) IN NEW-CALEDONIA, EXPLORING A VIRAL HYPOTHESIS

Semi-intensive farming of Penaeus stylirostris (Mexican strain) is a d eveloping industry in New Caledonia. Since 1993 crops have experienced mortality episodes (named 'Syndrome 93') which are strongly affecting the industry. A search for bacteria in the hemolymph of moribund shri mps revealed bacterial septicemia. The Vibrio spp. initially isolated were Variable and usually weakly pathogenic. Therefore, another resear ch was investigated. Histological sections of moribund shrimps reveale d not only classical vibriosis lesions, but also numerous pycnotic cel ls and intracytoplasmic basophilic bodies, which resemble those descri bed in the case of 'yellow head disease' and 'Taura syndrome'. These o bservations suggested a possible participation of viral agents in 'Syn drome 93'. To explore this viral hypothesis, five cephalothorax from m oribund shrimps caught in 1994 were ground, centrifuged and filtered o n a 0.22 mu m membrane. Filtered aliquots were kept at - 76 degrees C, then injected into groups of presumably healthy P. stylirostris (1 g) , from New Caledonia and specific pathogen-free P. stylirostris and P. vannamei from Tucson, AZ. While 100% mortality was observed in groups injected with the filtrate 6 h to 5 days after injection, no signific ant mortality occurred in the control groups. Basophilic bodies simila r to those described above were observed in experimentally infected mo ribund shrimps. This demonstrates the existence of ultrafiltrable enti ties in diseased shrimps which can experimentally reproduce the lesion s, symptoms and mortalities characteristic of 'Symdrome 93'. Specific gene probe were used (dot blot) to detect Infectious Hypodermal and He matopoietic Necrosis Virus (IHHNV) and showed that 94 out of 95 moribu nd shrimps were positive but the detected levels were very low. Purifi cation trials using centrifugation on sucrose and cesium chloride grad ients failed to detect IHHNV which is probably present in small quanti ties. However, some fractions revealed an abundance of calibrated part icles of about 100 mm of variable shape following examination of negat ive staining under transmission electron microscopy. Nucleic acid extr action trials and electrophoresis of these fractions on agarose gel re vealed small quantities of a monocatenary DNA molecule of about 10 Kb, probably of viral origin. Results suggest the presence of the IHHNV, and probably of a second virus in shrimp crops in New Caledonia. The e tiological role of each of those entities in 'Syndrome 93' is currentl y being studied. (C) 1998 Elsevier Science B.V. All rights reserved.