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A commercially prepared vaccine against Edwardsiella ictaluri was used to vaccinate 12-day-old channel catfish fry by immersion, or by immersion plus an oral booster 2 months later. One month after the fish were fed the booster vaecine, they were challenged by waterborne exposure to 21 x 10" eells ml~' of E. ictaluri. Immersion only vaccinated fish suffered 6-7% mortality and immersion plus oral-boosted fish had a 3-3% mortality. Mortality among non-vaccinated controls was 96-7% and was significantly (/*<0-01) above the vaccinated mortality. The relative per cent survival for the immersion-only fish was 93-1, while it was 96-6 for the immersion plus orai-boosted fish. Agglutinating antibody titres of the vaccinated fish were signifieantly {F<005) higher than the control fish. When the ponds were drained 6 months after stocking, 42 7% of non-vaecinated, 56-3% of immersion-only and 70-8% of immersion plus oral-boosted fish were harvested. Survival of immersion plus orally-boosted fish was significantly (P<0-05) higher than the controls of immersion-only fish. Duplieate populations of immersion plus oral-booster-vaccinated fish grew 34% and 56% faster, respectively, on an average daily gain than the control fish, while immersiononly fish in one pond grew 20% less per day and fish in the second pond grew 48% faster.

Channel catf~sh Ictalurus punctatus that s u m v e d a natural infect~on of Edwardslella lctalun demonstrated a strong relahonsh~p between E ictaluri aggluhnahon a n t~b o d y tlter and degree of protechon upon challenge Intrapentoneal injection of 2 0 X lo7 cells fish-' killed 100 % of f~s h with ant~body titers of 0 and 128 (low) 77 8 % of fish with bters from 256 to 512 (medium) and 57 7 % of the fish with hters of >l024 (high) A second challenge tnal usmg 5 1 x 105 cells f~s h -' produced 72 2 % mortahty in fish with no detectable antibody titers 51 3 % In low a n t~b o d y titer flsh 25 0 % in m e d~u m antibody tlter fish and 6 5 % in h~g h a n t~b o d y titer fish These results demonstrate that channel catfish have protective antibodies after they are exposed to E lctalun but that a titer of over 256 1s necessary to ensure detectable protect~on against an ~njected challenge If fish are challenged with large numbers of pathogen this protechve immunity can be overcome Administration of E ictalun cell extract impregnated feed every 5 or 10 d to fish vaccinated by inlection resulted in the fish malntainmg theu anbbody titer, whereas the antibody titers of fish receiving no oral vaccine continued to dechne

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