There is a rapidly increasing literature pointing to the success of probiotics,
immunostimulants, plant products and oral
vaccines in
immunomodulation, namely stimulation of the innate, cellular and/or humoral immune response, and the control of bacterial
fish diseases. Probiotics are regarded as live micro-organisms administered orally and leading to health benefits. However, in contrast with the use in terrestrial animals, a diverse range of micro-organisms have been evaluated in aquaculture with the mode of action often reflecting
immunomodulation. Moreover, the need for living cells has been questioned. Also, key subcellular components, including
lipopolysaccharides, have been attributed to the beneficial effect in fish. Here, there is a link with
immunostimulants, which may also be administered orally. Furthermore, numerous plant products have been reported to have health benefits, namely protection against disease for which stimulation of some immune parameters has been reported. Oral
vaccines confer protection against some diseases, although the mode of action is usually linked to humoral rather than the innate and cellular immune responses. This review explores the relationship between probiotics,
immunostimulants, plant products and oral
vaccines.