ABSTRACT: Eight groups of Oncorhynchus mykiss were fed diets either lacking or supplemented with ascorbic acid (AA, 500 mg kg^-1 diet) for 10 wk prior to vaccination against cryptobiosis. The concentrations of AA in livers and kidneys of the fish correlated with those of the exogenous dietary AA. Fish were vaccinated intraperitoneally with 100000 attenuated Cryptobia salmositica per fish. They were challenged with 100000 virulent C. salmositica per fish 4 wk post-vaccination. Both vaccinated AA-deficient and AA-supplemented fish were protected while unvaccinated controls had high parasitaemias and cryptobiosis (e.g. anaemia, abdominal distension with ascites) after being challenged with the pathogen. AA-deficiency did not significantly affect titres of complement fixing antibodies (CFAb) in vaccinated and vaccinated/challenged fish. However, detectable CFAb was delayed 1 wk in vaccinated and vaccinated/challenged fish fed the AA-deficient diet. Also the parasitaemias in infected and vaccinated/challenged AA-deficient fish were consistently lower than those in AA-supplemented fish. This indicates that AA may have directly and indirectly promoted more rapid multiplication of the virulent parasite.

KEY WORDS: Cryptobia salmositica . Ascorbic acid . Vaccination . Cryptobiosis . Protection

Published in DAO Vol. 24, No. 1 (1996) on January 4
ISSN: 0177-5103. Copyright © Inter-Research, Oldendorf/Luhe, 1996