Carrier State of Channel Catfish Infected with Edwardsiella ictaluri

Channel catfish Ictalurus punctatus that were naturally exposed to Edwardsiella ictaluri were acquired from a catfish farmer and transported to the laboratory. The 6-month-old channel catfish (mean length, 14.9 cm) had a normal appearance without symptoms of enteric septicemia of catfish (ESC. For 5 d, one group of channel catfish was given feed containing the antibiotic Romet-30® (8 kg/tonne) at a daily rate of 1% (about 20 g/fish), and the other group received feed without medication. About one-third of the channel catfish in the untreated group developed clinical signs of ESC and died within 3 d after transportation to the laboratory; survivors were killed after 11 d because of their poor condition. The group of channel catfish treated with Romet-30 had no signs of ESC or deaths caused by ESC. The presence of E. ictaluri in the trunk kidney oftreated channel catfish was demonstrated by a microscopic immunofluorescence method and selective enrichment culture at 90 and 270 d. The percentage of treated channel catfish positive for the presence of E. ictaluri increased from about 40 to 70% between 90 and 270 d. The percentage of treated channel catfish positive for antibodies against E. ictaluri increased from 44 to 82% between 30 and 270 d. Challenge infection of these fish with E. ictaluri showed that most were protected from ESC. The results show that healthy channel catfish exposed to E. ictaluri and subsequently treated with Romet-30 can be carriers of E. ictaluri and may serve as a reservoir of infection for fish susceptible to ESC.