Mycobacteriosis, caused by Mycobacterium spp, is a common disease in snakehead (Channa striata) and Siamese fighting fish (Betta splendens). In Thailand, snakehead is an economically important food species, while Siamese fighting fish is one of the main ornamental species cultured for export. Mycobacteriosis is not confined to fish and can also be transmitted to humans. M. marinum has been reported to be the main species involved in mycobacteriosis in both fish and man. The results from this study showed that M. fortuitum as well as M. marinum, is involved in Mycobacterium infections observed on Siamese fighting fish and snakehead fish farms. Both M. fortuitum and M. marinum were detected in samples collected from the fish s environment, including water and fish food, although M. fortuitum appeared to be the more prevalent of the two species. However, a large number of the Mycobacterium spp. detected in the samples collected from the farms remains unspeciated. Workers on the Siamese fighting fish farms were affected by skin lesions, while those on snakehead fish farms were not unaffected. When biopsies of the lesions were analysed by polymerase chain reaction and reverse cross blot hybridisation, M. fortuitum together with unspeciated Mycobacterium sp. appeared to be the main aetiological agents associated with the lesions.

Over the last 10 years, viruses have been isolated from fish imported into Singapore and also from indigenous fish in Singapore. In 1993, the marble goby Oxyeleotris marmorata** underwent viral diagnostic investigation as there were frequent reports of high losses in that species after importation into Singapore. Viruses, identified as birnaviruses based on their size and a double-stranded RNA genome, were isolated in BF-2 cells from marble gobies which were imported from four other Asian countries. The viruses underwent serotyping using antisera against all nine serotypes in birnavirus Serogroup A and the single serotype in Serogroup B. The viruses did not cross react at all with any of the antisera. However, they were neutralised with an antiserum against a birnavirus isolated in 1991 from a single giant snakehead, Channa (=Ophicephalus) micropeltes, held in a Singapore farm. That birnavirus was also not neutralised by any birnavirus Serogroup A or B antisera, and Serogroup A or B viruses were not neutralised by the antiserum against the virus isolated from C. micropeltes. On that basis the marble goby and snakehead birnaviruses should be classified as members of a new birnavirus Serogroup.

Floating net-cage culture of fish was established in the early 1970s in Southeast Asia and has become very popular since. Although several species of marine and diadromous fishes have been successfully cultured, only a few species, particularly the seabass (Lates calcarifer) and grouper (Epinephelus spp.), are widely cultured. The expansion of commercial culture of marine and estuarine finfishes has been accompanied by the occurrence of diseases of infectious and noninfectious etiologies. Among the recognized causative agents of infectious diseases are protozoa, fungi, bacteria and viruses. Bacterial diseases, caused by Vibrio species (particularly V. parahemlyticus, V. anguillarum, and V. vunificus), and myxobacterial infection due to Flexibacter sp. and Flavobacterium sp. have continued to accompany the growth of marine fish cage-culture industry. Viral diseases include Lymphocystis, viral encephalitis and iridovirus infection. This paper reviews the major bacterial and viral pathogens of economically important finfishes, with emphasize on seabass and grouper.

There have been relatively few studies on the diseases of molluscs in the Asian region, and those that have been carried out are often difficult to access. Therefore, our current knowledge of mollusc diseases in Asia is, at best, fragmentary. Despite this, serious viral diseases have been reported from hard clams (Meretrix lusoria) in Taiwan, flat oysters (Ostrea angasi, Tiostrea chilensis) and rock oysters (Saccostrea glomerata) in Australia and New Zealand, pearl oysters (Pinctada fucata martensii) in Japan, abalone (Nordotis discus discus) in Japan, pearl mussels (Hyriopsis cumingi) in China, and mussels (Perna canaliculus) and scallops (Pecten novaezelandiae) in New Zealand. Prokaryote infections (Rickettsiales-like organisms, Vibrio spp., Nocardia crassostreae), and fungal infections (Steinhausia mytilovum) have also been reported. Several serious and potentially serious protist infections (Haplosporidium spp., Bonamia sp., Mikrocytos roughleyi, an unidentified haplosporidian, Marteilia spp., Marteilioides spp., Perkinsus olseni) also occur in the region. It is concluded that as more research is carried out on the parasites and diseases of Asian molluscs, other undescribed serious pathogens will be encountered.

In Thailand, farming the giant freshwater prawn, Macrobrachium rosenbergii, is a viable aquaculture industry. In 1995-96, giant freshwater prawns collected from grow out ponds located in Supanburi Province, Thailand showed severe branchiostegite melanization associated with poor growth and high cumulative mortality in affected ponds. The objective this study was to report the histopathological features of branchiostegite melanization and associated lesions in affected giant freshwater prawns. The gross lesions displayed by the collected prawns were bilateral branchiostegite melanization in the medial surfaces with normal underlying gill lamellae. Histopathological examination revealed a massive number of sporonts in the gill tissue causing a hyperinfection. Enlargement of the sporonts occurred in the gills. High magnification using a light microscope revealed massive round microsporidia-like spores in the sporonts measuring 1-2 m in diameter. The muscles of collected prawns were normal showing no signs of opaque cottony or milky white coloration. The hepatopancreas, however, were yellow in color. Positive Giemsa and PAS stained spores were found in hemocytes and cytoplasm of hepatopancreas cells. Because these results do not include electron-micrographic data, more observation is needed before the spores can be identified as microsporidia.

The number of diseases affecting cultivated penaeid shrimp has increased steadily with expansion and intensification of large-scale commercial cultivation. In the 1990s the most serious losses have occurred from the emergence of new viral pathogens, like white spot virus (WSV) in Asia and Taura syndrome virus (TSV) in the Americas. These initially endemic diseases have quickly become global, probably via infected carriers, including aquaculture stocks. The most alarming of the newly emerging viruses are those like spawner mortality virus (SMV) that produce no pathognomonic lesions and require sophisticated molecular diagnostic techniques for detection. This requirement severely limits the prospects for successful control and it provides a high incentive for the development of more field friendly detection methods. In the coming years, more viral pathogens will undoubtedly be found, especially as cultivation expands geographically and non-native species are used for rearing. To lower risks of disastrous viral epizootics, we should be more cautious with the international movement of living shrimp for aquaculture. We should also lower risks by realistically assessing and limiting other possible modes of pathogen transfer, without unreasonably interfering with international trade. In comparison to viruses, bacteria constitute a much less serious threat to shrimp farmers because most can be controlled by appropriate pond management techniques and because proper chemotherapy is possible in the event of management failure. In spite of this, new pathogens are still being described and too little is known about them. Particularly threatening are new intracellular forms (e.g., molecutes) that may often be overlooked because they cannot be detected in the absence of sophisticated molecular techniques. Even for more traditional pathogens like Vibrio species, we still do not understand why some strains are lethal and others are not. However, some progress is beginning to be made regarding the toxins from Vibrio harveyi and V. penaeicida, the genes controlling toxin production and the possible transfer of these genes amongst strains by bacteriophages. Much remains to be done. To date, fungi and parasites come far behind the viruses and bacteria in the ranking of pathogens that threaten farmed shrimp. However, the example of crayfish plague suggests that this may not always be the case. Control of all these diseases requires a level of understanding that we still lack, not only for the pathogens, but also for the shrimp host. Admirable progress is being made with respect to crayfish cellular defence mechanisms based on pattern recognition proteins targeted against bacterial and fungal cell wall components. Cultivated shrimp species seem to have similar defence systems, but work on them is uneven and much still remains to be confirmed or characterised. In contrast to bacteria, our knowledge of the shrimp response to viral pathogens is almost totally lacking and this is a serious shortcoming given the staggering losses the viruses have caused. We must accelerate investigations at the molecular level for both the host shrimp and its pathogens in order to rationally assess and improve the benefits of proposed therapies and preventative measures for disease. This work must go hand in hand with a concerted effort to develop certified domesticated shrimp stocks and secure, but affordable, cultivation systems.

Penaeus monodon postlarvae (stage 10) were reared with Artemia nauplii enriched with 50,100 and 150 mg/l (ppm) of L-ascorbyl-dipotassium-2- sulfate dihydrate for 5 days. Postlarvae in the control group were given non-enriched Artemia nauplii. Responses of postlarvae to toxic (ammonia and methyl parathion exposure) and stressful (salinity and dissolved oxygen stress) conditions, as well as their resistance to challenge with Vibrio harveyi, were compared. Acute toxicity tests using ammonia and methyl parathion revealed better tolerance of postlarvae given enriched Artemia at 100 and 150 ppm L-ascorbyl-dipotassium-2- sulfate dihydrate for 5 days. Transfer of postlarvae from 30 to 0 ppt water caused 100% mortality in all groups after 2 hr of transfer, but postlarvae that were transferred from 30 to 5 ppt showed significantly different mortality rates after 48 hr. Postlarvae that were given enriched Artemia had lower mortality compared with the control. In the oxygen depletion experiment, mortality rates were significantly different between enriched and non-enriched postlarvae. However, on the fourth day, all enriched and control shrimp died. Challenge with pathogenic Vibrio harveyi by bath challenge for 7 days showed significantly different mortality rates among postlarval groups. Postlarvae given 100 and 150 ppm L-ascorbyl-dipotassium-2- sulfate dihydrate for 5 days had lower mortality. Results from this study indicate the benefits of L-ascorbyl-dipotassium-2-sulfate dihydrate enrichment through Artemia in improving the survival of P. monodon postlarvae against some stressful conditions and bacterial challenge.

Due to increasing stringency of environmental regulations, agricultural production systems have switched from persistent pesticides to products which degrade more rapidly. In spite of that, new pesticide products can affect non-target organisms by continuous exposure to non-lethal concentrations, producing alterations such as reduced growth rate, impairment of the defense mechanisms and increased susceptibility to viral and bacterial infections. Experiments were carried out using Litopenaeus vannamei, juveniles (approximately 1g), methyl parathion and Vibrio parahaemolyticus isolated from a diseased shrimp. Initially, oral LC₅₀ and LD₅₀ of injected bacteria were determined in the shrimp to provide the guidelines to the experimental design. For the final experiment, treatments applied were: 1) shrimp fed with pellets containing 14.043 µg of methylparathion per gram of feed for 10 days, which were injected intramuscularly with sterile saline solution on the fifth day; 2) shrimp fed with pellets containing 14.043 µg of methylparathion per gram of feed for 10 days, which were injected intramuscularly with V. parahaemolyticus on the fifth day; 3) shrimp fed with pellets containing acetonitrile (the solvent for methyl parathion) for 10 days, which were injected intramuscularly with V. parahaemolyticus on the fifth day; and 4)

Quorum sensing is a gene regulatory mechanism by which certain pathogenic and symbiotic bacteria respond to population density and host association. By mediating the ability of bacteria such as Vibrio harveyi to infect larval shrimp, quorum sensing could have application to the problem of vibriosis in shrimp mariculture. Knowledge of quorum sensing developed from studies of luminescence in the marine bacteria Vibrio fischeri and V. harveyi. In both species, luminescence is controlled by a quorum-sensing mechanism involving two signal molecules and several genetically defined and physiological factors. In V. fischeri, luminescence is regulated by LuxR protein and the Luxl-dependent 3-oxo-hexanoyl-homoserine lactone (3- oxo-C6-HSL), which together activate transcription of luxICDABEG (the lux operon, genes for LuxI and the luminescence enzymes). A second acyl-HSL, octanoyl-HSL, product of AinS, blocks premature lux operon induction by interfering with 3-oxo-C6-HSL binding to LuxR. In V. harveyi, quorumsensing control of luminescence involves the luxLM-dependent 3-hydroxy- butyryl-HSL and an unidentified luxS-dependent signal molecule. These signals, through independent branches of a phosphorylation cascade, control the phosphorylation state of LuxO. In the absence of the signals, LuxO, in phosphorylated form, represses lux operon (luxCDABEGH) expression. The presence of the signals operates to dephosphorylate LuxO, relieving the repression. A LuxR protein then activates lux operon transcription. Targets for blocking the quorum-sensing mechanism in these bacteria and potentially blocking vibriosis include the activity, production and persistence of the signals, the production and activity of the response proteins, and the activity of specific downstream quorum-sensing controlled proteins involved in host infection.

Dot-blot nitrocellulose enzyme immunoassays (DB-NC-EIA) were developed for the detection of two shrimp viral pathogens, white-spot virus (WSV), and yellow-head virus (YHV). The assays utilized HRP-conjugated virus-specific polyclonal antibodies to detect virus antigen present in gill homogenates of infected shrimp spotted onto nitrocellulose membrane. The DB-NC-EIA was highly specific and sensitive enough to detect the presence of viral proteins before the appearance of overt symptoms. The assays are one of the simplest and most rapid detection methods available for WSV and YHV.