Abstract
In conjunction with good health management, vaccination is a powerful tool for disease control in modern day fish farming. Vaccination of fish has become a standard operating procedure in most countries in Europe and North America.
In Asia, with the exception of Japan, vaccines are not commonly used for fish disease control. This is inconsistent with the large quantities of fish that are being produced in this region. There are several reasons for the lack of vaccine products in Asia. Firstly, more resources are needed to understand the basic epidemiology of diseases and the immune system of many species. Secondly, most of the farms are operated on a small scale with little technical support. Farmers focus more on treatment than prevention as antibiotics are largely available. In addition, since development and commercialization of vaccines requires a great deal of time and resources, only few companies are committed and specialized in this field.
The major advantages of vaccination over therapeutic treatments are that vaccines provide long-lasting protection and leave no adverse residues in the product or the environment. A critical milestone in vaccine development is the understanding of the disease etiology and epidemiology. At present, more and more information is being generated by governmental institutes, universities and the private sector. In the foreseeable future, this knowledge will lead to successful development of vaccines specifically for the Asian aquaculture industry.
Introduction
While the intensification of aquaculture has led to remarkable improvements in productivity, it is also associated with disease epidemics, involving bacterial, fungal, viral and parasitic pathogens. Disease is undoubtedly one of the biggest constraints on production, development and expansion of the aquaculture industry. Diseases can be controlled in a number of ways, for example, introduction of specific-pathogen-free (SPF) broodstock, optimization of feed, improvement of husbandry techniques and good sanitation. In conjunction with good health management, prophylactic immunization (vaccination) is an indispensable tool for disease control in aquaculture (Evelyn, 1997, 2002; Gudding et. al., 1999).
Vaccination has become an increasingly important aspect of aquaculture. Several bacterial and viral vaccines, either mono- or multivalent, have been successfully developed and commercialized (Bostock, 2002; Evelyn, 2002). They have proved to be cost effective. In salmonid farming, the use of vaccines is now so widespread that basically all fish stocked in sea cages have been vaccinated. Taking Norwegian salmon farming as an example, the use of antibiotics has dropped to virtually zero and production has increased tremendously (Bostock, 2002, Markestad and Grave, 1996). While the success of the Norwegian salmon industry is directly associated with advances in culture methods, feeding strategies, processing technology, marketing, and legislation of disease prevention, vaccination has certainly played a significant role.
Norwegian salmon farming is often taken as an example of how things should or could progress in aquaculture. However, the production of fish in tropical and subtropical areas is quite different. Differences involve not only in the species cultured, but also (and mainly) the scientific knowledge that is available on reproduction, husbandry, feed requirements, diseases and immunology specific to the farmed species. Taking these differences into account, the knowledge that has been gathered in salmon vaccinology can be used to advance the science more efficiently in other farmed species. In this paper, an overview is given on the current situation of fish vaccination with an emphasis on fish cultured in tropical areas.
Comparison of Salmon Production with Asian Aquaculture
Salmon is an anadromous fish species, i.e., it spends most of its life in the marine environment but reproduces in freshwater. The larvae and fry are produced in freshwater and subsequently migrate to the seawater environment. The most economically significant diseases (e.g., furunculosis, classical vibriosis, infectious pancreas necrosis and coldwater vibriosis) occur in the marine environment. This compartmentalized development of salmon provides a convenient vaccination window to assist disease prevention. The fry are vaccinated during the freshwater phase well before their transfer to seawater so that they have time to develop protective immunity against the disease agents that they will encounter during the grow-out phase in seawater. With the available adjuvanted multivalent vaccines, a single intraperitoneal injection in juveniles can confer long-term protection in seawater stage (Evelyn, 2002). Most fish species in Asia are either cultured solely in seawater or freshwater and therefore the specific vaccination opportunity that exists for salmon is not available for these species.
Asian aquaculture is characterized by an enormous diversity of species, with over one hundred species being farmed. In other regions, the number of species cultured is far less, i.e., in Northern Europe, the only family cultured until recently was salmonids. Consequently, all resources available in Western countries were spent on the optimization of the culture for salmonids including disease control. In Asia, given the large number of species cultured, resources are spread thinly across species, resulting in sporadic and fragmented knowledge on each individual species.
The intensification of salmon production has led to separation of fry production and ongrowing sites, optimized feed and feeding strategies, good quality fingerlings that are virtually disease free and good farm management. In Asia, most farms produce different species of fish at the same site. No segregation in year classes is made, something that is obligatory for salmon in Europe, trash fish is widely used as feed, fry are often caught wild or derived from wild-caught broodstock and the culture techniques per species are not yet established. Furthermore, legislation and implementation regarding farming license and zoning policy are not in place in most Asian countries. With the gold rush mentality, this often results in too many fish and too many farms in a concentrated area that promotes the spread of diseases. The combination of all these factors together with the diversity of organisms in tropical waters leads to a truly challenging disease situation with a variety of entry points for pathogens. While the use of vaccines will make a contribution, all other aspects of farming operations must be improved for Asian aquaculture to remain sustainable.
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March 2007
