In ovo methods for utilizing the modified live Edwardsiella ictaluri vaccine against enteric septicemia in channel catfish

Abstract

Eyed channel catfish ( Ictalurus punctatus) eggs were vaccinated by immersion exposure (10 min) with either the modified live Edwardsiella ictaluri isolate RE-33 grown in brain heart infusion broth (trial 1) or the lyophilized AQUAVAC-ESC™ vaccine (trial 2). AQUAVAC-ESC™ is the modified live E. ictaluri isolate (RE-33) that was licensed, produced and marketed as a vaccine against enteric septicemia of catfish by Intervet. Trial 1 consisted of vaccinated (RE-33 E. ictaluri at 1×10 5 CFU/ml) and non-vaccinated (controls) treatments with the eyed eggs hatching into fry 4 days following treatment. No mortality was recorded in these fish for 33 days post vaccination. In trial 2, three treatments were used: single vaccinated, vaccinated and fry booster vaccinated 7 days following initial immunization; and non-vaccinated (controls). The vaccination in trial 2 was carried out according to the manufacturer's recommendation for use on channel catfish fry (i.e. AQUAVAC-ESC™ label). In trial 2, the eyed eggs hatched 24 h post vaccination and no control fish or single-vaccinated fish died following vaccination. Three fry died in the booster vaccinated treatment. No rifampicin-resistant E. ictaluri was isolated from these dead fish. Thirty-three (trial 1) and sixty days (trial 2) following vaccination, fish were challenged with E. ictaluri (isolate AL-93-75) at 1×10 7 CFU/ml. In trial 1, the relative percentage survival (RPS) of vaccinates was 87.9; however, data did not fit Amend's criteria of 60% mortality in the non-vaccinated treatment and no significant difference was seen between mortality of the vaccinates or control treatments in trial 1 [Dev. Biol. Stand. 49 (1981) 447]. In trial 2, mean percentage mortality in the non-vaccinated treatment (controls) (64.2±5.8) was significantly higher ( p=0.003) than mean percentage mortality in the single vaccinated treatment and booster vaccinated treatment (25.8±5.1 and 46.7±0.8, respectively). Relative percentage survival was 59.7 in the single vaccinated treatment and 27.3 in the booster vaccinates. Safety and efficacy of the modified live E. ictaluri vaccine (AQUAVAC-ESC™) was demonstrated in eyed channel catfish eggs following single vaccination. Booster vaccination did not enhance vaccine efficacy; however, timing of the booster may have been too soon following initial vaccination of eyed eggs.