The use of immune-stimulants in fish and shellfish feeds

Immune-stimulants are chemical substances which activate white blood cells (leukocytes). Such substances may also, but not necessarily, render animals more resistant to infectious diseases and reduce the risk of disease outbreaks if administrated prior to situations known to result in stress and impaired general performance (e.g. handling, change of temperature and environment, weaning of larvae to artificial feeds) or prior to expected increase in exposure to pathogenic microorganisms and parasites (e.g. spring and autumn blooms in the marine environment, high stocking density). In addition, aquaculture may benefit from the use of such immune-stimulants when they are used prior to, and during, developmental phases when the organisms are particularly susceptible to infectious agents (e.g. the larvae phase of shrimp and marine fish, smoltification in salmon, sexual maturation). Immune-stimulants may act in synergy with antibiotics and their effects may be enhanced by nutritional factors (e.g. vit. C, selenium), but there is no nutritional factor which on its own can be defined as an immune-stimulant. Compounds with highly diverse chemical structures have been shown to stimulate white blood cells in vitro, but the majority of such compounds have no relevance for practical use, due to high toxicity, obscure mode of action and unpredictable effects under farming conditions. The most promising group of immune-stimulants are the b-1,3/1,6-glucans, because they have a well-defined chemical structure and mode of action on the immune system, described in a great number of scientific papers. In addition, b-1,3/1,6-glucans are non-toxic universal “alarm signals” which activate the immune system by the same basic mechanism in all animal groups, from the simplest invertebrates to man. b-1,3/1,6-Glucans are active not only when injected, but also when administered in the feed, or on mucosal surfaces. The paper will present data on such effects with reference mainly to experience from fish and shellfish farming, and discuss how a high-molecular substance, which is not taken up into the body fluids, can induce a systemic effect.