Effects of the Corticosteroid Kenalog on the Carrier State of Juvenile Channel Catfish Exposed toEdwardsiella ictaluri

Abstract

Studies were conducted on the carrier state following experimentally induced infections of Edwardsiella ictaluri in juvenile channel catfish Ictalurus punctatus. The cumulative mortality of fish exposed to the bacterium at l.0 × 108 colony-forming units/mL for 30 s was 71% after 3 weeks at a water temperature of 25°C. The bacterium could not be recovered from the posterior kidneys of 10 fish examined 1, 2, or 3 months after initial exposure, but antibodies were still evident at each sampling. However, the bacterium was isolated from 6 of 20 fish at 4 months postexposure, 14 d after intraperitoneal injection with the corticosteroid Kenalog (0.8 mg/g fish). In a second study (done 4 months after immersion exposure) and a third (done 6 months after immersion exposure), Kenalog treatments were shown to increase the susceptibility of both previously exposed and unexposed channel catfish to an immersion challenge with E. ictaluri. Previously exposed fish that received Kenalog but were not challenged suffered no mortality, but two of eight fish harbored the organism at the end of study 2 (14 d posttreatment). In contrast, reexposure of fish that received Kenalog resulted in specific mortality of two of eight fish, and the bacterium was isolated from all of the six surviving fish (study 2). In the absence of the drug, previously exposed fish did not experience any mortality with or without a second exposure (studies 2 and 3) but the bacterium was recovered from one of eight reexposed fish (study 2). These results indicate that the recovery of E. ictaluri from carriers is enhanced by treatments with corticosteroids, which also appear to increase a carrier's susceptibility to a second challenge with the bacterium. Although serum antibody titers were a good indicator of prior exposure, their levels did not correlate with the recovery of the bacterium from suspected carrier fish.