2.2 Fish Health and Quarantine
Disease outbreaks are being increasingly recognized as a significant constraint to aquaculture production and trade and are affecting economic development of the sector in many countries of the world. Disease is now considered to be the most limiting factor in the shrimp culture sub-sector. Some figures are available on direct economic losses which indicate the significance of the problem, although social and other related impact, such as trade and employment issues, chemical and drug use, and environmental costs, has never been properly quantified. Estimates of economic losses suggest that developing countries in Asia lost at least US$1.4 thousand million due to diseases in 1990 alone. Since then, losses have increased. Reports from China suggest losses in 1993 of US$1 thousand million due to shrimp viral disease outbreaks (ADB/NACA, in press). A 1995 estimate suggests that aquatic animal disease and environment-related problems may cause annual losses to aquaculture production in Asian countries of more than US$3 thousand million per year (ADB/NACA, in press). According to a recent World Bank report, global losses due to shrimp disease are around US$3 thousand million and the Bank recommends investment of US$275 million in shrimp disease research over the next 15 years (Lundin, 1996).
The above figures provide an indication of the overall economic significance of aquatic animal diseases. However, there is need for a much more systematic review and analysis of information on their social and economic impact, and on the cost-benefits of alternative control strategies. There is also a need to develop a better framework or methodology for routine data collection and reporting on the incidence and socio-economic impact of aquatic animal diseases.
Major diseases and epizootics
Epizootic ulcerative syndrome (EUS) is one of the most severe fish diseases in Asia; it has a complex infectious aetiology and causes a seasonal epizootic condition in over 100 species of wild and farmed freshwater and brackishwater fish. The disease has caused severe economic losses in Asia and is now endemic to Southeast and South Asia, and has recently extended to West Asia. Control of EUS in natural waters is probably impossible, although various preventative measures can be used to reduce the risk of spread of EUS to some natural water bodies. The causative agent of EUS is the fungus Aphanomyces variably termed A. piscicida and A. invaderis (OIE, 1997). From the little epidemiological data available, it appears that the disease is spread by movement of water or, in certain cases, movement of fish without adequate quarantine and health certification (Arthur, 1996).
Monodon baculo virus (MBV) outbreak in Taiwan Province of China in 1988, followed by a series of shrimp viral disease outbreaks--Yellow-head virus (YHV) in 1992 in Thailand, Taura Syndrome virus (TSV) in 1992 in Ecuador, white spot virus (WSV) in 1993 in China and Thailand, and the same virus in a number of other Asian countries since 1993--have caused severe production losses to the global shrimp culture industry. At present over 20 viruses have been identified as important to shrimp, the most threatening being WSV in Asia and TSV in the Americas.
Many more disease outbreaks have been reported in other parts of the world, under culture and in the wild. Notable are furunculosis in salmonids, sea lice infections in European cage-cultured marine fish, and the recent Noda virus epizootic in seabass. Besides reported losses at epizootic levels, day-to-day loss of production due to infectious and non-infectious diseases, both at commercial and subsistent production levels and in the wild, is probably highly significant.
Need for a new approach to health management
Environmental factors and poor water quality resulting from increased effluent discharge, movement of aquatic animals, inadequate farm management, rapid proliferation of farms, etc., have been implicated in major disease outbreaks occurring in epizootic conditions. However, the underlying causes of such epizootics are highly complex and difficult to pinpoint. An understanding of the relationship between host, pathogen and environment is important in this regard. Since aquatic animal disease is the end result of a series of linked events, treatment of disease should go beyond consideration of the pathogen alone. Conventional approaches have so far had limited success in the prevention or cure of aquatic disease. Recent experience in trying to control disease outbreaks clearly demonstrates the importance of the linkage with other components of the production system, including the need for, inter alia, broader ecosystem management approaches to control farm-level environmental deterioration and to take preventative measures against the introduction of pathogens--the "systems management approach" (SMA) to aquatic animal health (Phillips, 1996).
The emphasis of SMA should be on better management for prevention, which is likely to be more cost effective than cure, involving both on-farm management and the management of the environment where farms are located. Government inputs are essential for regulation of resource use, particularly land and water, and for helping to provide legal and institutional arrangements that minimize resource-use conflicts and control environmental impact of and on aquaculture. Wider adoption of such approaches may lead to sustainable solutions that can be adopted by farmers, and to less reliance on the use of chemicals, which largely treat the symptom of the problem and not the cause. In addition, research, training programs, extension, and information exchange can be more effective and responsive to farmers' needs if based on SMA. The FAO’s Code of Conduct for Responsible Fisheries is an ideal platform to link SMA and national/international cooperation in harmonizing aquatic animal health management activities (Subasinghe et al., in press).
Recent developments in disease control
Over the years, significant achievements have been made through collaboration and cooperation between developed and developing countries in finding tools to combat or control diseases in aquaculture. However, there is still considerable scope for further improvement of such collaboration in developing new tools and perfecting existing ones. Major areas for further research include: a) quality control and more efficient and cost-effective use of inputs, such as water, seed and feed; b) the role of good nutrition in improving aquatic animal health; c) harnessing the host's specific and non-specific defence mechanisms in controlling aquatic animal diseases; d) development of affordable yet efficient vaccines for economically important tropical fish; e) use of immunostimulants and non-specific immune-enhancers to reduce susceptibility to disease; and f) use of probiotics and bioaugmentation for the improvement of aquatic environmental quality. The results of this research will undoubtedly help reduce chemical and drug use in aquaculture, and will not only contribute towards reducing negative environmental impact of chemical use, but will also make aquaculture products more acceptable to consumers.
Immunological assays, including fluorescent antibody techniques (FAT) and enzyme linked immunosorbent assays (ELISA), are presently used for sensitive and rapid detection of various fish pathogens. In addition, new assays from genetic engineering using nucleic acid probes are evolving from medical diagnostics. Reports have shown that polymerase chain reaction (PCR) techniques--which can greatly multiply minute quantities of DNA--combined with DNA probes, are effective in detecting the presence of nucleic acid sequences of fish and shrimp pathogens from infected tissues. The recent development of a PCR for the detection of WSV is a major breakthrough in combating the shrimp viral epizootic. There are two trends: (a) kits are becoming available for farmers to use on location to obtain information quickly about the presence of pathogens; and (b) central laboratories with trained staff are being organized to receive fresh and preserved samples from distant locations for processing and identification of unusual or difficult-to-detect antigens or nucleic acid sequences. Over the next decade, the potential for developing cost-effective and affordable, yet sensitive and effective, rapid diagnostic tools for use in developing-country situations will increase, as will joint commercial ventures between developed and developing countries for this purpose.
The application of genetic technologies in aquaculture is a recent practice for all but a few aquatic species. These technologies can be applied as part of SMA to fish health, to increase disease resistance, and to act as diagnostic tools to confirm the presence or absence of specific pathogens.
Quarantine and health certification
Quarantine and health certification programs form part of a first line of defense against possible adverse effects resulting from the introduction or transfer of exotic fish and shellfish. As such, they must be developed within the context of larger national, regional, and international plans addressing this problem. "Codes of practice" for the introduction and transfer of aquatic organisms that have been developed by international organizations, provide a starting point for designing national fish health legislation and for international agreements to prevent the spread of disease. To succeed, such efforts must be accompanied by the development of regionally agreed-upon lists of certifiable pathogens, the standardization of diagnostic techniques and the production of health certificates of unambiguous meaning. Establishment of intra- and inter-regional fish health information systems which could be linked with those of relevant regional and international agencies would be highly desirable for the success of such efforts. Strong commitment by national governments and the cooperation of importers/exporters are keys to the success of these programs. Successful disease prevention will also be directly related to the ability of countries to reduce their dependence on imported broodstock and fry for aquaculture.
FAO, in collaboration with the Network of Aquaculture Centres in Asia-Pacific, has developed a strategy for a cooperative framework, comprising key fisheries and veterinary authorities in countries of the region, and which may be used to implement the adopted uniform regional quarantine practices (Subasinghe et. al., in press). Agencies such as the Office International des Epizooties (OIE), UK Department of International Development, Australian Centre for International Agricultural Research, Fish Health Section of the Asian Fisheries Society, and Aquatic Animal Health Research Institute are supporting this initiative. The main objective of the above strategy is to develop practical and effective regional and national health certification and quarantine guidelines that will minimize the negative impact of aquatic animal diseases on production from aquaculture, inland and coastal capture fisheries, and indigenous aquatic biodiversity, as well as increase the income of aquaculturists in the region, the majority being small-scale farmers. There are activities underway to establish similar regional networks in Latin America with possible assistance from agencies such as the World Bank, European Union and FAO (Subasinghe and Arthur, 1997).
Use of drugs and other chemicalsMany chemicals are being used in aquaculture. Some are essential for successful and efficient farm and hatchery management, and some have been widely used in agriculture without adverse impacts. Most chemicals used do not appear to carry any significant potential for adverse effects on human health or environment, provided that they are applied in a technically appropriate manner (GESAMP, 1997). Although, in general, use of antimicrobial drugs in aquaculture is rapidly decreasing in major producing countries such as Norway ( Figure 2.2.1) (Anon., 1997), there are considerable constraints to the promotion of safe and effective use of drugs and chemicals in aquaculture in developing countries. They include: a) lack of trained manpower and related capacity-building schemes and support services to disseminate information on fish health management; b) the misapplication of some chemicals (e.g. the excessive prophylactic use of antibacterials); c) insufficient understanding of mode of action and efficacy of certain chemicals, especially under tropical aquaculture conditions; and d) uncertainties with regard to legal and institutional frameworks to govern chemical use in aquaculture (Barg and Lavilla-Pitogo, 1996). Implications for international trade
The use of drugs and other chemicals in aquaculture, quarantine and health certification of live aquatic animals during their transboundary movements, and screening of aquatic animals and animal products for human pathogens have become significant issues in international trade following the recent General Agreement on Tariffs and Trade and the "international agreement on application of sanitary and phytosanitary measures" (SPS Agreement) (GATT Secretariat, 1994). The SPS Agreement recognizes international agencies such as FAO, WHO, and OIE as reference points in solving trade disputes over such issues. Over the next decade, these agencies will play significant roles not only in solving trade disputes, but also in developing relevant technical guidelines and providing technical assistance to their member countries on issues relevant to the mandates of these organizations.
ADB/NACA. (In press). Final Report on the Regional Study and Workshop on Aquaculture Sustainability and the Environment (RETA 5534). Manila, Asian Development Bank, and Bangkok, Network of Aquaculture Centres in Asia-Pacific.
Anon. 1997. Aquaculture in Norway. Paper presented by the Government of Norway at the FAO, NACA, WHO, Study Group on Food Safety Issues Associated with Products from Aquaculture. 22-26 July 1997. Bangkok, Thailand.
Arthur, J.R. 1996. Fish and shellfish quarantine: the reality for Asia-Pacific, p.11-28. In R.P Subasinghe, J.R. Arthur, and M. Shariff (eds.). Health Management in Asian Aquaculture. FAO Fisheries Technical Paper No. 360. Rome, FAO. 142p.
Barg, U. and Lavilla-Pitogo, C.R. 1996. The use of chemicals in aquaculture: A summary brief of two international expert meetings. FAO Aquaculture Newsletter 14: 12-13.
GATT Secretariat. 1994. The results of the Uruguay Round of multilateral trade negotiations. The legal texts. Geneva, Secretariat of the General Agreement on Trade and Tariffs. 558p.
GESAMP (IMO/FAO/UNESCO/WMO/WHO/IAEA/UN/UNEP Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection). 1997. Towards safe and effective use of chemicals in coastal aquaculture. GESAMP Reports and Studies No. 65. Rome, FAO. 40 p.
Lundin, C.G. 1996. Global attempt to address shrimp disease (Draft). Marine/Environmental Paper No. 4. Land, Water and Natural Habitats Division, Environment Department, The World Bank. 45pp.
OIE. 1997. International aquatic animal health code and diagnostic manual for aquatic animal diseases. Second edition. Paris, Office International des Epizooties.
Phillips, M.J. 1996. Better health management in the Asia-Pacific through systems management, p.1-10. In R.P. Subasinghe, Arthur, J.R., and Shariff, M. (eds.). Health Management in Asian Aquaculture. FAO Fisheries Technical Paper No. 360. Rome, FAO. 142p.
Subasinghe, R.P. and J.R. Arthur. 1997. Introducing AAPQIS: the FAO’s Aquatic Animal Pathogen and Quarantine Information System. FAO Aquaculture Newsletter 16: 3-6.
Subasinghe, R.P., U. Barg, M.J. Phillips, D. Bartley and A. Tacon. (In press). Aquatic animal health management: investment opportunities within developing countries. Journal of Applied Ichthyology.