Shrimp Health Research in the Asia-Pacific:
Present Status and Future Directives

Celia R. Lavilla-Pitogo

Fish Health Section, Aquaculture Department,
Southeast Asian Fisheries Development Centre
Tigbauan, Iloilo, Philippines

Lavilla-Pitogo, C.R. 1996. Shrimp health research in the Asia-Pacific: present status and future directives. In Health Management in Asian Aquaculture. Proceedings of the Regional Expert Consultation on Aquaculture Health Management in Asia and the Pacific. R.P. Subasinghe, J.R.Arthur & M. Shariff (eds.), p. 41–50. FAO Fisheries Technical Paper No. 360. Rome, FAO. 142 p.

Abstract

Shrimp harvests from intensive aquaculture have recently declined in areas which have been productive for many years. Because the most convenient, although not necessarily factual, explanation for these crop failures has been the occurrence of infectious diseases, there is a need to consider shrimp health from a holistic point of view. The classical method used for the study of shrimp disease dealt mainly with identification of the causative organism and the search for methods of prevention and control through chemotherapy. The adverse affects resulting from the use of chemicals in aquaculture have led to a clamor for alternative approaches to disease management. For effective shrimp health maintenance and surveillance, the following components need consideration: development of rapid and sensitive methods for pathogen detection; establishment of shrimp tissue cultures for virology; immunological studies, toxicological studies and drug efficacy evaluation. The epidemiological approach to disease management should augment the classic approach to shrimp pathology, and this calls for multidisciplinary cooperation.

INTRODUCTION

In 1991 and 1992, over 700 000 metric tonnes (mt) of cultured shrimp were produced worldwide, with 80% of production coming from Asia. Production was expected to increase in succeeding years with the opening of new farms and adoption of more intensive culture methods. This expectation was not met, however. Global shrimp production in 1993 was only 639 000 mt, about 12% lower than the 1992 production. Crop failure in China in the summer of 1993 greatly affected the world's farmed shrimp production. The increase in production seen in 1994 was mainly due to record crops in Thailand, India and Vietnam (Rosenberry, 1994). This production, however, still did not surpass the harvests obtained in 1991 and 1992. Table 1 shows the production records of major shrimp-producing countries in Asia from 1991–1994.

Black tiger shrimp, Penaeus monodon, is the main species cultured in Asia except in China and Japan, where the main crops are P. chinensis and P. japonicus, respectively. Other penacids grown in farms are P. semisulcatus, P. penicillatus, P. merguiensis and P. indicus. The phenomenal growth of the shrimp farming industry in less than two decades has created various problems. Disease epizootics of economic significance are a major constraint to the industry, not only because they affect the quantity of harvest, but because disease also affects the quality and regularity of production. At present, the main goal of the shrimp industry is to meet the growing demand in a sustainable manner without damaging the environment. The role that shrimp disease research must play to attain such a goal is now being seriously considered by both the private and government sector, and by national and international organizations. It is clear that collaborative effort involving all sectors is needed to attain sustainability. However, a sustainable industry can only be achieved if considerable investment in research is provided by many sectors.

MAJOR HEALTH PROBLEMS IN SHRIMP CULTURE FACILITIES IN THE ASIA-PACIFIC

Epizootics of both infectious and non-infectious etiology have continuously plagued the various sectors of the industry. Although it is generally recognized that intensive culture systems often encounter serious disease problems, more recent experiences have shown that low-density culture systems can also be severely affected.

A recent review on shrimp diseases has been made by Lightner (1993). In the context of Asian shrimp culture, excellent papers on diseases were presented at the Workshop on Diseases of Cultured Penaeid Shrimp in Asia and the United States (Fulks and Main, 1992). Diseases of both infectious and non-infectious etiology have been described, but their effects on shrimp and impacts on culture activities remain poorly understood. Diseases caused by viruses, bacteria, fungi and protistans are considered very significant to shrimp culture. The following list is largely based on Lightner (1993) and Lightner et al. (1994).

Diseases Due to Infectious Organisms

• Viruses

  At least 15 viruses are known to infect cultured and wild marine penaeid shrimp. Reported types include parvoviruses, baculoviruses, reoviruses, togaviruses and rhabdoviruses. Three systemic baculoviruses have recently been described for penaeid shrimp: yellowhead virus (YBV) (Boonyaratpalin et al., 1993), hemolymph baculovirus (Owens, 1993) and systemic ectodermal and mesodermal baculovirus (SEMBV) (Wongteerasupaya et al., 1995). YBV and SEMBV infections have caused drastic mortalities resulting in severe economic losses in shrimp culture facilities in Thailand, Indonesia and India. Also described is a rod-shaped nuclear virus of Penaeus japonicus (RV-PJ) (Inouye et al., 1994) which has been implicated in mass mortalities of cultured P.japonicus in Japan in 1993.

• Bacteria

  Due to the economic losses following epizootics, bacteria are considered the most economically significant disease agents of shrimp. A review of the bacterial diseases affecting different aquaculture commodities in Asia is presented by Lavilla-Pitogo (in press). Infections due to Vibrio spp. affect shrimp in hatchery and growout facilities, causing chronic or mass mortalities and shell deformities which affect marketability. Although bacteria are easily isolated from diseased shrimp, the mechanism for pathogenicity remains unclear. The course of action taken by most farmers to arrest bacterial infections is to use antibacterials. These products have been used indiscriminately, creating resistance problems. Issues, problems and constraints regarding the use of chemicals in aquaculture facilities were raised during a workshop at the Symposium on Diseases in Asian Aquaculture held in Bali, Indonesia in 1990 (Shariff et al., 1992).

• Fungal, protistan and other parasitic diseases

  Fungi like Lagenidium, Haliphthoros and Sirolpidium caused epizootics in shrimp hatcheries (Baticados et al., 1990), resulting in heavy mortalities in larval stocks within two days. Infections due to the parasitic microsporidian Agmasoma penaei have been reported to cause high mortality in cultured shrimp (Flegel et al., 1992). Protistans, such as gregarines and ciliates, are also associated with shrimp, but their general impact is less because of their relatively slow development.

Non-Infectious Diseases and Syndromes

Several nutritional diseases and syndromes have been reported in shrimp, such as chronic soft-shell syndrome (Baticados et al., 1986) and red disease (Lightner and Redman, 1985). Various nutritional, toxic and environmental diseases are discussed in Sindermann and Lightner (1988), Baticados et al. (1990), Flegel et al. (1992) and Limsuwan (1993).

Reports of mortalities due to toxins and pesticides are emerging. Flegel et al. (1992) describe the toxic effects of methyl-parathion and cypermethrin pesticides used in rice culture on postlarvae of Penaeus monodon. The emphasis placed on studying these pesticides is due to the fact that they can find their way into shrimp farms through run-off.

Environmental Problems

Problems resulting from the effects of shrimp culture on the natural environment have been raised (Primavera, 1993). Moreover, concern is also directed toward the effects of shrimp culture practices on the pond environment and towards the conflict among users of the same water source for various purposes. More detailed discussions are given by Phillips (this volume).

THE IMPORTANCE OF RESEARCH

In the past, high production returns from shrimp farming were recorded in areas where new farms with good water source were opened. Opening of new areas is now limited, however, and future production will largely depend on already utilized resources. Successful production ventures will now largely depend on effective management utilizing technologies developed through meticulous research.

As with fish pathology, the classic approach to shrimp pathology is descriptive in nature. Most published studies on shrimp disease deal with the isolation and morphological or biochemical characterization of the pathogen. Recently, these activities have been carried out through molecular methods using gene probes. For effective disease control and surveillance programs, the following components should be considered:

Diagnostics

The use of new gene probe technologies that rely on demonstrating specific nucleic acid sequences offers an opportunity to detect viruses and other pathogens at much earlier stages of infection and to study how particular pathogens are transmitted to and within the shrimp host. Shrimp health status certification can also be performed with greater confidence in the outcome.

Epidemiology

The study of the distribution and determinants of health and disease in the natural setting is analogous to the pathogenesis of disease in individuals (Meek and Martin, 1991). It is clear that the classic approach to fish pathology should be augmented by epidemiology. Epidemiological surveys, which are essential for managing the spread of shrimp disease agents, need urgent attention. The availability of standardized diagnostic methods with predictable results will make surveillance and diagnostic work easier. The epidemiological approach calls for cooperation between farmers and pathologists in data-gathering to assess situations pre-empting a disease outbreak. These data will be the basis for rational decisions for the prevention and/or control of disease.

Tissue Culture Establishment

Although this work is expensive and difficult, cell lines are important because they allow in vitro study of host-pathogen interactions.

Immunologic Studies

For effective diagnosis and control of infectious shrimp diseases, sufficient knowledge of immunology is needed. The latest information and recommended research strategies for marine molluscan and crustacean immunology are discussed by Bachere et al. (1995). They emphasize the importance of immunology as a tool for studying host-pathogen interactions at the cellular and molecular levels, and as a key to selection of pathogen-resistant animals.

Toxicology and Drug Efficacy Evaluation

The need for approved chemotherapeutants in shrimp culture is obvious. However, the approval process required is complicated, time-consuming, and expensive. Bell (1992) thoroughly discusses the issues related to drugs for shrimp diseases. Equal to the need for approval is effective enforcement of laws regarding the use of drugs in shrimp culture.

Collaborative efforts with other disciplines will also lead to better shrimp health management. Husbandry techniques to produce fry and broodstock of good quality, as well as better nutrition and feeding strategies, will greatly support hatchery and farming activities. On the farm, environmentally friendly water and soil disinfection strategies are needed to get rid of pathogens in the water source from over-subscribed areas. Effluent clean-up schemes need to be devised to reduce nutrient discharge and prevent eutrophication in receiving waters. Mialhe et al. (1995) also emphasize that beyond the strategy of prophylaxis based on diagnostics for preventing and managing epidemics, another strategy must be developed for selecting pathogen-resistant strains.

The Importance of Collaboration in Shrimp Health Research

As a field of research, shrimp pathology has limited manpower compared to disciplines like breeding, seed production and husbandry. Solutions to production constraints due to infectious and non-infectious disease agents are not easily met, thus creating a feeling of uncertainty among investors.

It is clear that to solve enormous problems with limited manpower capability, there is a need for cooperation and collaboration among institutions. The concept of an “international team” to work on problems related to infectious diseases in marine invertebrate aquaculture was put forward by Mialhe et al. (1995) to avoid duplication of work.

There is also a need for increased interdisciplinary research. Pathologists can work with nutritionists in determining the nutritive values of new feed ingredients or the effects of anti-nutritional factors in these substances.

The role of scientific societies in carrying out plans for collaborative research is also discussed by M. Shariff (this volume). In Asia, the Fish Health Section of the Asian Fisheries Society serves as an effective mechanism for the exchange of ideas in research. However, private sector participation in this exchange is minimal. Research institutions, universities, government agencies and some private companies have contributed significantly to shrimp health research in Asia. Further strengthening of collaborative mechanisms to focus on shrimp diseases is still necessary to find fast track solutions to problems that beset the industry.

Effective collaborative projects have been initiated at the Aquatic Animal Health Research Institute (AAHRI) in Bangkok, Thailand with funding assistance from the Overseas Development Administration of the United Kingdom. The International Development Research Centre of Canada (IDRC) also funded research projects on shrimp disease at the Southeast Asian Fisheries Development Center (SEAFDEC) and the Philippine Bureau of Fisheries and Aquatic Resources (BFAR).

The role of universities in shrimp research is effectively carried out through the training of graduate students. This mechanism has boosted the manpower capability in shrimp pathology in Asia. Many universities and institutions in Japan, Taiwan, Malaysia, Australia and Thailand have contributed in training manpower in shrimp health research.

RECOMMENDATIONS FOR THE FUTURE

It is important for research to come up with information to secure the stability, sustainability and profitability of the shrimp industry. This could be done through:

• developing rapid and sensitive methods to detect pathogens within the animal and in its environment;

• increasing the availability and accessibility of disease diagnosis, prevention, and treatment services;

• increasing the availability of quality fry for stocking (fry which are disease resistant, fast growing, tolerant to sub-optimum environmental conditions etc.);

• developing a reliable method to assess fry quality;

• increasing the availability of environmentally sound and cost-effective culture methods (determination of pond carrying capacity, fine tuning of feeding strategies, waste recycling methods, polyculture etc.);

• undertaking studies on pond dynamics and its influence on shrimp health, and

• developing or verifying technologies for the use of probiotics, bioaugmentation and bioremediation products for shrimp aquaculture.

Sufficient resources to develop and transfer technologies to support an industry faced with huge problems should be made available. Government funds, as well as contributions from international funding agencies, have initially funded research. Closer interaction with industry representatives should be made in order to come up with schemes to help finance expensive research.

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¹ 1994 figures are from Rosenberry (1994); 1991–1993 figures are from C.P. ShrimpNews (March 1994).
² Other countries which also produce pond-grown shrimp include Japan, Korea, SriLanka, Australia and Malaysia.