by Jan Frericks, Leiber GmbH, Germany
The effect of stress caused by environmental pollution and farming conditions on the health and yield of fish in intensive aquacultures is becoming increasingly important.
Factors such as stocking density, contamination, toxins, pollutants and outbreaks of disease have a negative effect on the immune status of the fish. The consequence of this is an increased susceptibility to infection through bacterial, viral, fungal or parasitic pathogens. Increased loss rates and reduced growth performance result in lower profitability for the fish production industry.
As a consequence, the monetary and quantitative overheads for the vaccination and medicinal treatment of the fish increases. In many cases, antibiotics are given not only therapeutically, but also prophylactically as a standard additive in fish feed.
The intensive farming methods used for fish cultivation in aquatic environments with a direct connection to the groundwater are especially liable to facilitate the very rapid and direct spread of problematic production residues to humans. Resistant pathogens and germs do not just limit the effectiveness of therapeutic antibiotics for fish. The transfer of genes for resistance between different species of bacteria is accelerated, leading to an exacerbation of the problem of resistance in the treatment of human diseases worldwide.
Future-oriented production methods in the fish farming industry should therefore be targeted towards minimising the use of antibiotics and medicinal drugs. It is of great importance to analyse the negative effects caused by environmental pollution and farming methods.
More crucial still will be to influence the animal’s metabolism so that external toxins have a lesser impact, even under intensive conditions. A healthy gut and a functioning non-specific immune response are fundamental prerequisites for this.
Excellent for the gut
Brewers’ yeast cells are like miniature power houses, and are responsible for the alcoholic fermentation that takes place during the brewing stage of beer production. In the course of the fermentation of malt extract, high concentrations of minerals and trace elements, amino acids and nucleotides, B vitamins and enzymes, as well as many micro-nutrients accumulate within the cells of the yeast species Saccharomyces cerevisiae. Being organically bound ensures high availability of these active substances. Dried brewers’ yeast is used very often in fish nutrition due to the high bioavailability of the constituent compounds.
In addition to this, brewers’ yeast has cell walls that are composed of mannan-oligosaccharides (MOS). This complex network of mannans and ßglucans serves as a substrate for the beneficial gut flora. The fish’s limited digestive tract benefits in particular from the prebiotic properties of the yeast cell walls, which stabilise the gut and ensure a healthy balance of microflora (eubiosis). In addition to this, the mannan-oligosaccharides in brewers’ yeast are able to bind harmful toxins in the food, and thus inhibit their absorption and resultant metabolic harm. Last but not least, the formation of a biofilm on the intestinal mucosa enhances this protective barrier against pathogens.
Glucan and the immune system
The cell wall of brewers’ yeast comprises approximately 20-25 percent mannans and 25-30 percent ß-glucans. ß1,3/1,6(D)glucan molecules can be isolated from it using special hydrolytic processes. The molecules consist of characteristic (1,3)-beta-glycosidic linked D-glucose subunits connected with with irregular beta-(1,6)-linked side chains of various length. Only this free ß-glucan structure from Saccharomyces cerevisiae has an immunomodulatory effect on the metabolism.
In contrast with intact yeast cells or mannan-oligosaccharides, free ß1,3/1,6(D)glucan molecules are able to pass through the protective epithelial barrier in the gut with the help of specialised M cells. In gut-associated lymph tissue (GALT), ß-glucans act like antigens, stimulating specific macrophage receptors with their characteristic surface structures (epitopes) (Engstad and Robertsen 1993). A cascade of immune responses is triggered, and non-specific immune system cells such as monocytes, natural killer cells, B-cells, T-cells or lysozymes are released or activated. They put the animal on a high state of alert and preparedness to defend against all types of foreign attack. What differentiates this from an actual infection is that ß-glucan does not possess any pathogenic properties, and acts without causing any adverse health effects.
A quality criterion for the effectiveness of ß-glucan products is not only the source and characteristic molecular structure, but also the purity of the product. A minimum content of 70 percent pure ß1,3/1,6(D)glucan should be aimed for. The standard grade Leiber® BetaS has a ß-glucan content of 80 percent. The metabolic activity of this product has already been tested on many animal species and verified.
Tests were recently performed on several species of fish that confirm a stimulatory effect on the non-specific immune system. Rainbow trout and carp received 0.02 percent ßglucan (Leiber® BetaS) administered in their feed ration. The potential killing activity of phagocytes, the proliferative response of T and Blymphocytes and the concentration of immunoglobulins and lysozymes in the blood serum was measured four weeks and eight weeks after beginning administration. In two repetitions, a significant stimulation of these parameters was demonstrated in both rainbow trout and carp.
In subsequent infection studies using two bacterial (Aeromonas salmonicida; Yersinia ruckeri) and one viral pathogen (IPN virus), the survival rate of rainbow trout and carp with 0.02 percent Leiber® BetaS in their feed showed an absolute increase of 30-40 percent (Siwicki, et al. 2008; Siwicki, et al. 2009).
Oral administration tests
Purified ß1,3/1,6(D)glucans enhance the animals’ non-specific immune response. This is of particular value in the fish farming industry, as there are multiple stress and environmental factors that impact and stress the fish. If they become infected, the specific immune system is only able to respond slowly and inadequately. In such situations, a heightened non-specific capability can support or accelerate the specific immune response and the production of specific antibodies.
Fish are subject to similar, added stresses during vaccinations too. A study by Siwicki et al. (2011) investigated the effect of orally administered ß1,3/1,6(D)glucan (100mg or 200mg Leiber® BetaS per kg of feed) on the antibody secreting cells (ASC) and specific antibody titres after immunisation of rainbow trout fingerlings (Oncorhynchus mykiss) by immersion with anti-enteric redmouth disease vaccine (AquaVac ERM). Inoculation was performed one week after the start of administration with Leiber® BetaS.
These two parameters were measured on day seven, 14, 21, 28 and 40 in the blood serum and adrenal glands. Both dosage levels stimulated the number of specific ASCs and specific antibody levels, whereby 0.02 percent Leiber® BetaS in the fish feed was more effective. In each case the improvements were significant from day 21 onwards.
The beneficial effect of ß1,3/1,6(D)glucan is well known in the fish nutrition industry. The administration of 0.02 percent Leiber® BetaS in fish feed activates the non-specific immune status of the fish on the one hand, and on the other, acts as an adjuvant during vaccinations, thus enhancing the immunocompetence of the fish. Its safe and simple method of use, as well as the fact that ß-glucan from Saccharomyces cerevisiae is harmless to fish and the environment, will become yet more important in the future. ■