Susceptibility of channel catfish to infection with edwardsiella ictaluri: Effect of exposure method

Three methods of infecting channel catfish Ictalurus punctatus with Edwardsiella ictaluri were compared. Groups of fish were exposed to the bacterium by intraperitoneal (IP) injection, bath immersion with stress, or bath immersion without stress. We determined infection rate by culturing tissues for the presence of E. ictaluri over a 3-week period. Fish in the IP-injected and bath-stressed groups had higher rates of infection than the bath-unstressed fish. Three weeks after exposure, the cumulative percentage of fish that cultured positive for E. ictaluri was 46.6% in the IP-exposed group, 52. 7% in the bath-stressed group, and only 15.7% in the bath-unstressed group. Tissues from the IP-injected and the bath-stressed fish also cultured positive at an earlier time postexposure. Edwardsiella ictaluri were isolated from liver, head kidney, brain, spleen, and trunk kidney at some time during the 3-week sampling period. Anti-E. ictaluri antibody levels determined by agglutination and enzyme-linked immunosorbent assay indicated significantly different antibody titers depending on method of exposure to E. ictaluri. The bath-stressed group showed measurable antibody levels earlier and maintained higher titers than the bath-unstressed group throughout the experiment. This Study confirms that stressed fish are more susceptible to infection with E. ictaluri than unstressed fish.