Since it first appeared in 1992, white spot syndrome virus (WSSV) has become the most threatening infectious agent in shrimp aquaculture. Within a decade, this pathogen has spread to all the main shrimp farming areas and has caused enormous economic losses amounting to more than seven billion US dollars. At present, biosecurity methods used to exclude pathogens in shrimp farms include disinfecting ponds and water, preventing the entrance of animals that may carry infectious agents and stocking ponds with specific pathogen-free post-larvae. The combination of these practices increases biosecurity in shrimp farming facilities and may contribute to reduce the risk of a WSSV outbreak. Although several control methods have shown some efficacy against WSSV under experimental conditions, no therapeutic products or strategies are available to effectively control WSSV in the field. Furthermore, differences in virulence and clinical outcome of WSSV infections have been reported. The sequencing and characterization of different strains of WSSV has begun to determine aspects of its biology, virulence and pathogenesis. Knowledge on these aspects is critical for developing effective control methods. The aim of this review is to present an update of the knowledge generated so far on different aspects of WSSV organization, morphogenesis, pathology and pathogenesis.

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Vibrio species' infections are a common sequelae to environmental stress or other disease processes in shrimp, but the mechanism by which the shrimp eliminate the bacteria is poorly understood. In this study, the penetration, fate and the clearing of V. vulnificus were investigated in Penaeus monodon. A bacterial disease isolate from a shrimp farm was identified as V. vulnificus biotype I. Polyclonal antiserum was raised in rabbits against the bacterium and the specificity was verified by ELISA and immunoblot against a range of Vibrio spp. and other Gram-negative bacteria. The bacteria were then administered to P. monodon juveniles by injection, immersion and oral intubation. An indirect immunoperoxidase technique was employed in a time course study to follow the bacteria and bacterial antigens in the tissue of the shrimp. Bacteria were cleared by a common route, regardless of the method of administration. Observations in immersion challenge were similar to a combination of those for oral and injection challenges. With immersion, bacteria entered the shrimp through damaged cuticle or via insertion points of cuticular setae. Shortly after entry, whole bacterial cells were observed in the haemolymph and connective tissue. They were either phagocytosed by haemocytes, or broken down outside host cells. Haemocytes containing bacterial cells or antigens (HCB) were observed in the connective tissue and haemolymph. HCB accumulated around the hepatopancreas, midgut, midgut-caecum, gills, heart and lymphoid organ. Free bacterial antigens also accumulated in the heart and lymphoid organ. Bacteria entering through the mouth by oral intubation or immersion were broken down so that only soluble or very fine particles entered the hepatopancreas. Bacterial antigens passed through the hepatopancreas into the haemolymph. Antigens were initially observed in the haemolymph sinuses and subsequently accumulated in the heart and lymphoid organ. Bacterial antigens were released from the shrimp, initially through the gills and subsequently through hepatopancreatic B-cells, branchial podocytes and sub-cuticular podocytes. KEY WORDS: Shrimp · Vibrio · Penaeus monodonResale or republication not permitted without written consent of the publisher

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Summary White‐spot disease is a relatively recent but widespread epizootic of cultured shrimp in Asia. This epizootic probably began in China in 1993 and subsequently spread from there to Japan, Taiwan, and the rest of Asia as far as India. Pond‐side losses in 1996 in Thailand alone reached approximately 70000 metric tonnes, valued at over half a billion US dollars. For all of Asia, the loss must amount to several billion dollars. The Asian science community reacted quickly to this disease outbreak. Within one year, the causative agent was identified as a bacilli‐form virus and within 3 years diagnostic DNA probes had been developed by several groups working in Japan, Taiwan, and Thailand. The virus has been included in the nonclassified virus group and is now generally referred to as white‐spot syndrome virus (WSSV). Previous experience with another serious epizootic caused by yellow‐head virus (YHV) helped to prime the science community for the advent of WSSV. Using the DNA diagnostic probes developed, many sources of WSSV were identified. The list now includes over 40 reservoir crustaceans and perhaps even aquatic insects. All of the commonly cultivated shrimp in Asia can be infected with the virus and for many, the infection can be deadly. In addition to noncultivated carriers, the postlarvae used to stock rearing ponds have also been found to be potential sources of rearing pond contaniination. Knowledge of the virus and its host species has brought about revolutionary changes in hatchery and farming practice, in order to implement preventive measures. These include the use of polymerase chain reaction (PCR) technology to screen broodstock and larvae, and the widespread adoption of closed or semiclosed cultivation. It is believed that without widespread application of these and other preventive measures, the losses in Thailand in 1996 would have been much higher. The WSSV epizootic has increased the awareness of the shrimp farming industry to the urgent need for further research and development work, if the shrimp industry is to become sustainable. Prime needs include domestication of broodstock leading to breeding programs; a better understanding of shrimp defences against disease; a better understanding of shrimp nutrition; and a better understanding of the shrimp pond environment so that growth can be optimized and negative impacts on the environment eliminated.