| Summary: | Immune-stimulants are chemical substances which activate white blood cells(leukocytes). Such substances may also, but not necessarily, render animals more resistant to infectiousdiseases and reduce the risk of disease outbreaks if administrated prior to situations known to result instress 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 microorganismsand parasites (e.g. spring and autumn blooms in the marine environment, high stockingdensity). In addition, aquaculture may benefit from the use of such immune-stimulants when they areused prior to, and during, developmental phases when the organisms are particularly susceptible toinfectious agents (e.g. the larvae phase of shrimp and marine fish, smoltification in salmon, sexualmaturation). Immune-stimulants may act in synergy with antibiotics and their effects may be enhancedby nutritional factors (e.g. vit. C, selenium), but there is no nutritional factor which on its own can bedefined as an immune-stimulant. Compounds with highly diverse chemical structures have been shownto stimulate white blood cells in vitro, but the majority of such compounds have no relevance forpractical use, due to high toxicity, obscure mode of action and unpredictable effects under farmingconditions. The most promising group of immune-stimulants are the b-1,3/1,6-glucans, because theyhave a well-defined chemical structure and mode of action on the immune system, described in a greatnumber 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 simplestinvertebrates to man. b-1,3/1,6-Glucans are active not only when injected, but also when administered inthe feed, or on mucosal surfaces. The paper will present data on such effects with reference mainly toexperience from fish and shellfish farming, and discuss how a high-molecular substance, which is nottaken up into the body fluids, can induce a systemic effect.
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