The use of immunostimulatory substances in fish and shellfish farming

Abstract

A number of different compounds with highly diverse chemical structures have been shown to elevate the overall resistance of animals to a number of infectious agents simultaneously. Such compounds, usually called immunostimulants, include bacterial cell wall fragments, ß‐1,3‐glucans of yeast and mycelial fungi, peptides, and a number of synthetic products. The present review paper describes the chemical structure of various immunostimulants and their mode of action on the immune system and disease resistance of fish and shrimp and discusses the current experience and future prospects of using immunostimulants to reduce disease threats in aquaculture. Immunostimulants are already in use as helper substances (adjuvants) in fish vaccines and as additives in aquafeeds, and there is a growing interest in the aquaculture industry to make use of immunostimulants as prophylactic agents prior to situations known to represent an elevated risk of disease outbreak. Such situations include transport and handling, sudden change of temperature and environment, weaning of young individuals to artificial diets, increased exposure to pathogenic microbes, and parasites and environmental stress that causes impaired general performance and reduced growth. Immunostimulants may become an important tool to reduce disease problems of shrimp aquaculture, because the concept of vaccination of crustaceans is not realistic. Moreover, the disease resistance of young fish, prior to the development stage when they can produce specific antibodies, may be boosted by immunostimulants. The science of immunostimulation in fish and shrimp is rapidly expanding and with a few specific products the mode of action and efficacy have been described in great detail. However, in many cases still there are only anecdotal information and poor documentation on efficacy. This may be one reason why there has been reluctance and skepticism to the use of immunostimulants under practical farming conditions. Another reason may be that the efficacy of immunostimulants very much depends on dose and mode of administration and that absence of efficacy due to unskilled use may have had a negative impact on the reputation of immunostimulants in practice. Moreover, the use of immunostimulants is a kind of risk insurance that will not provide a notable benefit when the environmental conditions are so good that no disease occurs. However, certain immunostimulants seem to be performance enhancers under such conditions, resulting in increased growth.