Determining the role of beta-glucan in Nile tilapia (Oreochromis niloticus) immunological response against streptococcal disease

Streptococcus iniae, a Gram-positive aerobic cocci, has been identified as one of the principal etiological agents of streptococcal disease in cultured fish. The bacterium can be isolated from brain, eye, or kidney of fish naturally infected. A vaccine for the ARS-98-60 isolate has been used for the prevention of streptococcal infection in Nile tilapia, Oreochromis niloticus. In addition, beta-glucan has been known to increase protection against bacterial infection when used as an immunostimulant in fish; therefore, a combination of the S. iniae vaccine and beta-glucan should be more effective in increasing disease resistance of Nile tilapia than administration of one of the two as individual treatments. Three experiments were performed to determine the effect intraperitoneal (IP) administration and dietary supplementation of beta-glucan on Nile tilapia. Specific immune responses, serum antibody as measured by indirect ELISA and agglutination, were not affected by dietary beta-glucan and the interaction between beta-glucan and immunization, but the effects of immunization with the S. iniae ARS-98-60 vaccine on the serum antibody level were positive. Antibody levels at 21 days post-booster immunization (PBI) and 14 PC were significantly enhanced by immunization with S. iniae vaccine, leading to increased protection against S. iniae infection. The results of the 10-week feeding suggest that oral administration of beta-glucan had no effect on stimulating the S. iniae-specific antibody levels and resistance of Nile tilapia against S. iniae infection, and showed that beta-glucan supplemented feed did not improve specific immune defenses provided by vaccination. During the 4-week feeding, beta-glucan effect on macrophage activity and mortalities due to various concentrations of bacteria was also observed. There was no effect on mortalities due to beta-glucan supplementation in the feed; in addition, dilutions (1 x 109 to 1 x 10 6 CFU/ml) of S. iniae ARS-98-60 isolate did not produce significantly different mortality levels. Serum lysozyme activity was not effected by administration of beta-glucan in the feed; however, antibody levels were slightly but significantly lower in fish fed the 100 mg beta-glucan/kg diet by 28 days post-feeding and 14 days post-challenge with the S. iniae ARS-98-60 isolate. Peritoneal macrophages had a decrease in their ability to kill S. iniae when fish were fed 100 mg beta-glucan/kg diet. The results suggest that oral administration of beta-glucan may negatively effect resistance of Nile tilapia against S. iniae infection by partial inhibition of macrophage activity and specific antibody levels following infection. (Abstract shortened by UMI.)...