Immunostimulation in crustaceans: does it really protect against infection?

There is a growing need to control, prevent or minimise the devastating effects of disease in crustacean culture, without recourse to toxic chemicals or antibiotics. In keeping with approaches to disease control in fish and higher mammals, interest is developing in compounds that confer protection and/or enhance immune reactivity to likely pathogens in shellfish (sometimes, erroneously, referred to as ‘shellfish vaccines’). The agents currently under scrutiny for crustaceans include glucans, lipopolysaccharides and killed bacterial cells. They are thought to act as ‘immunostimulants’ because of their known effects on the crustacean immune system in vitro. A number papers are now appearing in the literature claiming to demonstrate their positive impact on immunity and disease resistance. This review article considers the problem of disease and its control in crustacean farming, describing the types of immunostimulatory compounds claimed to have positive effects and evaluating their merit in enhancing immune capability in cultured species. Analysis of the validity of the results of many of the published studies raises questions about the value of these compounds for cost-effective control of infection in aquaculture, especially for long lasting protection in both adults and juveniles. This review further discusses the potential risks to the wellbeing of the stock animals from repeated use of these agents and makes the case for rigorous testing of putative stimulants, at the gene, protein and functional levels, as well as for the need to consider alternative strategies and approaches to disease control.