Pathogen levels, lysozyme, and cortisol response in channel catfish with susceptibility differences to Edwardsiella ictaluti

Differences in bacterial loads, clearance rates, nonspecific immune response. and cortisol response were measured for three family groups of channel catfish Ictalurus punctatus susceptible to enteric septicemia of catfish (ESC) and three ESC-resistant families. Resistant families had lower mortality during challenge (P < 0.0001). Bacterial loads on day 5 postexposure were higher (P < 0.01) in susceptible families than resistant families for blood, kidney, and spleen. Blood, kidney, and spleen bacterial levels were highly correlated. Pathogen clearance was evident by day 12 in fish from the susceptible families and measurable only in kidney and spleen from resistant families on day 20. Lysozyme activity increased on day 1 (P = 0.0074) in resistant fish and on day 2 in susceptible fish (P < 0.0001). An acute stress response was evident for both resistant (21.4 +/- 1.7 ng/mL [mean SD]) and susceptible families (30.0 +/- 1.6 ng/mL). Plasma cortisol levels were elevated in susceptible fish throughout challenge, except for day 12, but in resistant fish they recovered to near basal levels immediately after an acute response on day 2. There was no correlation between cortisol and lysozyme levels. Differences in lysozyme activity and pathogen levels suggest that the nonspecific immune response was effective during the early stages of infection in resistant fish.