Abstract

Semi-intensive farming of Penaeus stylirostris (Mexican strain) is a developing 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 shrimps revealed bacterial septicemia. The Vibrio spp. initially isolated were variable and usually weakly pathogenic. Therefore, another research was investigated. Histological sections of moribund shrimps revealed not only classical vibriosis lesions, but also numerous pycnotic cells and intracytoplasmic basophilic bodies, which resemble those described in the case of `yellow head disease' and `Taura syndrome'. These observations suggested a possible participation of viral agents in `Syndrome 93'. To explore this viral hypothesis, five cephalothorax from moribund shrimps caught in 1994 were ground, centrifuged and filtered on a 0.22 mu m membrane. Filtered aliquots were kept at -76 degree 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 significant mortality occurred in the control groups. Basophilic bodies similar to those described above were observed in experimentally infected moribund shrimps. This demonstrates the existence of ultrafiltrable entities in diseased shrimps which can experimentally reproduce the lesions, symptoms and mortalities characteristic of `Symdrome 93'. Specific gene probe were used (dot blot) to detect Infectious Hypodermal and Hematopoietic Necrosis Virus (IHHNV) and showed that 94 out of 95 moribund shrimps were positive but the detected levels were very low. Purification trials using centrifugation on sucrose and cesium chloride gradients failed to detect IHHNV which is probably present in small quantities. However, some fractions revealed an abundance of calibrated particles of about 100 mm of variable shape following examination of negative staining under transmission electron microscopy. Nucleic acid extraction trials and electrophoresis of these fractions on agarose gel revealed 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 etiological role of each of those entities in `Syndrome 93' is currently being studied.