The effect of dietary immunomodulation upon Edwardsiella tarda vaccination in healthy and immunocompromised Indian major carp (Labeo rohita)

In order to determine the impact on disease resistance of four dietary immunomodulators viz., β-1,3 glucan, levamisole, vitamins C and E, in an important farmed Indian major carp species, rohu (Labeo rohita Ham.), fish were fed diets containing various levels of these substances during a 60 day trial. Aflatoxin B1(AFB1) at 1·25 mg kg−1body weight was injected intraperitoneally (i.p.) into fish to induce an immunosuppressive state on the first day of the experiment in some individuals. The fish were vaccinated against formalin-killedEdwardsiella tarda vaccine on day 30 of the experiment. Specific immunity, as measured by bacterial agglutination titre and disease resistance against E. tarda, was determined at the end of the trial. The results demonstrate that all the four immunomodulators were capable of significantly (P< 0·05) increasing specific immunity and reducing mortality in immunocompromised fish but failed to enhance specific immunity and protection in healthy fish. The increased bacterial agglutination titre by β-1,3 glucan, and reduced mortality losses by both β-1,3 glucan and levamisole were marked in healthy vaccinated fish compared with their controls. Similarly, all four substances significantly reduced the mortality rates in immunocompromised and healthy unvaccinated fish. Out of these four substances, glucan was recorded to be the most effective immunomodulator in rohu. The present results suggest that the introduction of these substances into the diet of fish grown in farms under immunosuppressive/stressful conditions could increase their resistance to infection by reducing mortality rates and offer economic benefits.