Search

The movement of live aquatic animals is one of the principal reasons for the rising number of disease emergencies in Asia and the Pacific. Contingency planning is crucial in the effective management of such emergencies. Within Asia, the Asia Regional Technical Guidelines on Health Management and the Responsible Movement of Live Aquatic Animals provide the basic framework for national and regional efforts to reduce the risk of introducing disease across borders. However, in following these guidelines, the progress made by countries on contingency planning has, to date, been rather limited. In this paper, the authors broadly examine the implementation of health management programmes, highlight some regional initiatives that support contingency planning and describe the present status of contingency planning in the Asia-Pacific region.

Asia contributes more than 90% to the world's aquaculture production. Like other farming systems, aquaculture is plagued with disease problems resulting from its intensification and commercialization. This paper describes the various factors, providing specific examples, which have contributed to the current disease problems faced by what is now the fastest growing food-producing sector globally. These include increased globalization of trade and markets; the intensification of fish-farming practices through the movement of broodstock, postlarvae, fry and fingerlings; the introduction of new species for aquaculture development; the expansion of the ornamental fish trade; the enhancement of marine and coastal areas through the stocking of aquatic animals raised in hatcheries; the unanticipated interactions between cultured and wild populations of aquatic animals; poor or lack of effective biosecurity measures; slow awareness on emerging diseases; the misunderstanding and misuse of specific pathogen free (SPF) stocks; climate change; other human-mediated movements of aquaculture commodities. Data on the socio-economic impacts of aquatic animal diseases are also presented, including estimates of losses in production, direct and indirect income and employment, market access or share of investment, and consumer confidence; food availability; industry failures. Examples of costs of investment in aquatic animal health-related activities, including national strategies, research, surveillance, control and other health management programmes are also provided. Finally, the strategies currently being implemented in the Asian region to deal with transboundary diseases affecting the aquaculture sector are highlighted. These include compliance with international codes, and development and implementation of regional guidelines and national aquatic animal health strategies; new diagnostic and therapeutic techniques and new information technology; new biosecurity measures including risk analysis, epidemiology, surveillance, reporting and planning for emergency response to epizootics; targeted research; institutional strengthening and manpower development (education, training and extension research and diagnostic services).

Since production from capture fisheries cannot meet the demands of exports, aquaculture has subsequently played a major role in securing the raw materials for the world's food industries. Aquaculture has rapidly developed from extensive systems to semi-intensive, intensive and super-intensive systems. This has introduced the use of chemicals and drugs into the systems, which cause residual problems in the products. In the developed world, food safety has become a major issue of concern. The world market now demands healthy aquaculture products from farm to table. To achieve these requirements and to keep their markets, countries involved in aquaculture have implemented control measures such as farm licensing, code of conduct for sustainable aquaculture, hazard analysis and critical control point (HACCP) and good aquaculture practice. However, infectious diseases in aquaculture are of major concern to the industry and are typically controlled by eradication of the pathogen, treatment with antibiotic or chemotherapeutics, and/or by preventative measures such as the use of probiotics or vaccines. To limit the use of chemicals and antibiotics, good farm management is highly recommended. In terms of treatment, chemicals and antibiotics should be evaluated to establish recommended doses and withdrawal periods, otherwise alternative treatments should be developed. Environmentally-friendly probiotics have been introduced to aquaculture practice in the last decade to replace pathogenic bacteria with beneficial bacteria transient in the gut. Micro-organisms have also been prepared for the purpose of biocontrol and bioremediation. The application of probiotic, biocontrol and bioremediation seem promising; however considerable efforts of further research in terms of food and environmental safety are needed. Vaccination has proved highly effective in controlling diseases in the salmon industry mainly in Europe, America and Japan. In other Asian countries, this practice seem to be slower to develop due to differences in the aquaculture systems used and the economic value of the fish species farmed. Because shrimp farming is a large industry in this region, much effort has been put into developing a vaccine for viral diseases of shrimp. The efficacy of those vaccines remains inconclusive. Immunostimulants are another option developed for use in the shrimp industry in the region; however their efficacy also remains unclear.

Aphanomyces invadans (Saprolegniaceae) is a peronosporomycete fungus associated with the serious fish disease, epizootic ulcerative syndrome (EUS), also known as mycotic granulomatosis. In this study, interspecific relationships were examined between A. invadans isolates and other aquatic animal pathogenic Saprolegniaceae, and saprophytic Saprolegniaceae from EUS-affected areas. Restriction fragment length polymorphisms and sequences of ribosomal DNA confirmed that A. invadans is distinct from all other species studied. A sequence from the internal transcribed spacer region ITS1, unique to A. invadans, was used to design primers for a PCR-based diagnostic test. Intraspecific relationships were also examined by random amplification of polymorphic DNA using 20 isolates of A. invadans from six countries. The isolates showed a high degree of genetic homogeneity using 14 random ten-mer primers. This provides evidence that the fungus has spread across Asia in one relatively rapid episode, which is consistent with reports of outbreaks of EUS. Physiological distinctions between A. invadans and other Aphanomyces species based on a data set of 16 growth parameters showed remarkable taxonomic congruence with the molecular phylogeny.

Epizootic ulcerative syndrome (EUS) is one of the most destructive diseases of fresh and brackish water farmed and wild fish in the Asia-Pacific region. The in vitro germination and growth of the propagules of the EUS pathogen, Aphanomyces invadans (= A. piscicida), were assessed in the presence of the head-kidney macrophages, serum, and serum heated to inactivate complement proteins, of three EUS-susceptible and one resistant fish species. The susceptible species were: striped snakehead (Channa striata), giant gourami (Osphronemus gouramy) and silver barb [Barbodes (= Puntius) gonionotus], and the resistant species was Nile tilapia (Oreochromis niloticus). Fish of all species were acclimatised to either low temperature (20 degrees C +/- 1.6) at which EUS is known to occur, or to high temperature (32 degrees C +/- 5.0) at which EUS does not occur, except for giant gouramis which were only studied at low temperature. The respiratory burst of the macrophages was assessed in the presence of A. invadans or the stimulant phorbol myristate acetate (PMA), and compared to that of controls. Anti-A. invadans antibody concentrations were assessed in all species except silver barbs. All assays were carried out at the same temperature, regardless of the temperature that the fish were kept at. Macrophages of all species other than snakeheads inhibited fungal germination at both temperatures, though only silver barb and gourami macrophages could inhibit germling growth. PMA increased the respiratory burst in nearly all cases. The respiratory burst in the presence of A. invadans was consistently lower than that of controls, though the difference was only significant in the case of snakeheads. The respiratory burst of all macrophage treatments was higher at a low temperature. Except in the case of PMA-stimulated macrophages, regressions between respiratory burst and inhibitory action were only found in susceptible species, suggesting that the respiratory burst is important in those species, but is unable to prevent the proliferation of A. invadans. Serum inhibited fungal germination in all cases other than low temperature tilapia, indicating that the EUS resistance of tilapia is not due to the serum. Inhibition of germling growth by serum only occurred in silver barbs and gourami. Heated serum did not inhibit germination in any case except that of high temperature snakehead, and in fact stimulated germination in the case of tilapia. Heating serum did not affect the growth inhibiting activity of silver barbs and gouramis, but it stimulated growth in some groups. At high temperatures snakeheads had high anti-A. invadans antibody concentrations, which may explain the inhibitory activity of their heated serum. A role for complement and antibodies in defence against A. invadans in susceptible species is suggested.

A mass mortality of up to 60% per tank occurred in 28-day-old Macrobrachium rosenbergii postlarvae cultured under intensive conditions in a hatchery in Thailand. Grossly affected prawns displayed a diffuse 'milky' white body opacity; histologically and ultrastructurally, segmental myofibre (myofibrillar and sarcoplasmic) necrosis unassociated with any infectious agents was observed, and was diagnosed as idiopathic muscle necrosis. Avoidance of overstocking and increased dissolved oxygen in tank water have proved effective in preventing subsequent recurrences of the condition. [ABSTRACT FROM AUTHOR]