EFFECTS OF DIETARY ASCORBIC-ACID DEFICIENCY ON CRYPTOBIA-SALMOSITICA INFECTION AND ON VACCINATION AGAINST CRYPTOBIOSIS IN ONCORHYNCHUS-MYKISS

Eight groups of Oncorhynchus mykiss were fed diets either lacking or s upplemented with ascorbic acid (AA, 500 mg kg(-1) diet) for 10 wk prio r to vaccination against cryptobiosis. The concentrations of AA in liv ers and kidneys of the fish correlated with those of the exogenous die tary AA. Fish were vaccinated intraperitoneally with 100 000 attenuate d Cryptobia salmositica per fish. They were challenged with 100 000 vi rulent C. salmositica per fish 4 wk post-vaccination. Both vaccinated AA-deficient and AA-supplemented fish were protected while unvaccinate d controls had high parasitaemias and cryptobiosis (e.g. anaemia, abdo minal distension with ascites) after being challenged with the pathoge n. AA-deficiency did not significantly affect titres of complement fix ing antibodies (CFAb) in vaccinated and vaccinated/challenged fish. Ho wever, detectable CFAb was delayed 1 wk in vaccinated and vaccinated/c hallenge fish fed the AA-deficient diet. Also the parasitaemias in inf ected and vaccinated/challenged AA-deficient fish were consistently lo wer than those in AA-supplemented fish. This indicates that AA may hav e directly and indirectly promoted more rapid multiplication of the vi rulent parasite.