The search for immunostimulants in fish feeds have been one of the most popular thesis topics of both undergraduate and graduate students in aquaculture in the Philippines and abroad. Its popularity goes hand in hand with the search for the best anti-cancer, anti-aging, and anti-almost-anything for humans. As a result, the industry for neutraceuticals have emerged as a very lucrative business for the simple reason that it does not need any approval of a regulatory body such as the Bureau of Food and Drug Administration. One just has to add in the label of whatever form of neutraceutical he/she is selling that the there has been No Proven Pharmaceutical Value.
I guess for aquaculture, feed manufacturers are not so easily tempted to incorporate certain unknown ingredients unless they have been shown to promote immune enhancement. It is unlike the pharmaceuticals for humans which only need a little convincing from friends to say they are really effective simply because they work for them. But what is the acceptable proof of immunostimulants for fish or shrimp in aquaculture?
The interest in immunostimulants could have started when the shrimp industry in Asia and elsewhere suffered losses due to bacterial and viral attacks. But then again, I have not come across a reading about this and it should stay as a suspicion on my part. Shrimps are very good subject for such kind of tests because of the absence of what we call adaptive immune response or the ability to produce an antibody very specific to a certain kind of antigen. Thus, crustaceans lack the cell memory to recognize a certain pathogen and thus unable to combat it once it reappears in their system. The absence of the adaptive immune system makes it a lot easier for the researcher to measure quite conveniently only the indices for the other kind of defense, the first line of defense, the innate or non-adaptive immune system. The investigators are able to avoid complicated conclusions arising from both systems and thus are able to isolate the response of one system and are able to make more conclusive findings.
The innate defense system is a generalized kind of defense and does not act upon specific antigen and for this reason they are alternatively called the nonspecific immune system. Indices for the innate immune system includes the ability to produce the superoxide that oxidizes microbes to death, rate of phagocytosis (engulfing of microbe organisms by specialized body cells), the level of activity of enzymes responsible for producing the killer substance e.g. phenoloxidase or its dormant form prophenoloxidase, or whether the presence of the ingredient in question can inhibit the growth of the pathogenic bacteria on petridishes (i.e. in vitro), among other indices. So if all these indices increased their levels when the ingredient is administered to a group of shrimps, and the increase in level is considerably more than that of a control group of shrimps, the experimenter then concludes that the ingredient in question is an immunostimulant.
Is the finding conclusive that the compound in question is now considered immunostimulant? When this field of research was at its early stage, acceptance of publications in scientific journals of this type of research has relied on conclusions based on the above-mentioned immune responses. But now, I have the impression that more stringent journals consider only the findings to be conclusive if there is a parallel study. It is a challenge test in which the crustaceans, following administration, are deliberately challenged with the pathogenic organism in question under laboratory conditions. If in addition to increased levels of immune responses (not necessarily all of them), the crustaceans are able to demonstrate acceptable rate of survival, only then can the reviewers consider the compound immunostimulant.
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