EXPERIMENTAL EDWARDSIELLA TARDA INFECTION IN NONABRADED CHANNEL CATFISH ICTALURUS PUNCTATUS BY IMMERSION

Authors: Wiedenmayer, Alyssa; Evans, Joyce; Klesius, Phillip

Submitted to: Fisheries Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2005
Publication Date: 8/1/2006
Citation: Wiedenmayer, A.A., Evans, J.J., Klesius, P.H. 2006. Experimental Edwardsiella tarda infection in nonabraded channel catfish Ictalurus punctatus by immersion. Fisheries Science. 72(5) 1124-1126.

Interpretive Summary: A method was developed to experimentally induce edwardsiellosis, disease resulting from infection by Edwardsiella spp., in healthy channel catfish (Ictalurus punctatus Rafinesque) with Edwardsiella tarda. E. tarda has been identified as the etiological agent responsible for chronic to acute edwardsiellosis in a variety of fish species including catfish. Channel catfish, 140 individuals of USDA strain 103 with an average weight of 24 g, were randomly separated into groups of 20 and exposed to concentrations of E. tarda at 9.3 x 105, 1.9 x 106, 6.2 x 107, and 3.0 x 108 CFU/mL in 3 L bath immersions for 30 minutes. Two control groups of 20 fish each were exposed to tryptic soy broth and water in 3 L bath immersions for 30 min. The fish were held, 20 individuals per each 57 L liter glass tank, and observed for 96 h post infection for visible signs of disease, including cutaneous lesions, gas filled abscesses, loss of pigmentation, abnormal protrusion of the eyeball, opaqueness of the eye, and abnormal swimming and behavioral patterns, and mortality due to E. tarda infection. The results indicated that lethal infections by bath immersion were obtained at 6.2 x 107and 3.0 x 108 CFU/mL. Cumulative mortality was 78% and 70%, respectively. No mortality was noted in fish exposed to 9.3 x 105 and 1.9 x 106 CFU/mL. Brain, liver, intestine, and head kidney tissue samples were culture positive for E. tarda from dead and morbid fish. Control fish had no mortalities due to E. tarda infection. The LD50, the dose of bacteria lethal to 50% of individuals within a given population, was determined to be 3.4 x 107 CFU/mL. The results demonstrate that healthy channel catfish are susceptible to edwardsiellosis following bath immersion exposure to E. tarda without prior mechanical skin injury.

Technical Abstract: A method was developed to experimentally induce edwardsiellosis in healthy channel catfish (Ictalurus punctatus Rafinesque) with Edwardsiella tarda. The development of a reliable method to produce infected catfish is essential to determine the efficacy of E. tarda vaccines and for pathogenesis studies. A previous study reported that only channel catfish with mechanical skin injury were shown to be susceptible to E. tarda infection after bath immersion exposure (Darwish et al. 2000). Channel catfish, USDA 103, (N=140) were randomly separated into groups of 20 fish and exposed to concentrations of E. tarda at 9.3 x 105, 1.9 x 107, 6.2 x 107, and 3.0 x 108 CFU/mL in 3 L bath immersions for 30 minutes (min). Control groups (N = 2) of 20 fish each were exposed to tryptic soy broth in 3 L bath immersions for 30 min. The fish were held, 20 individuals per tank, and observed for 96 h post infection for signs and mortality due to E. tarda. Lethal infections by bath immersion were obtained at 6.2 x 107and 3.0 x 108 CFU/mL. Cumulative mortality was 78% and 70%, respectively. No mortality was noted in fish exposed to 9.3 x 105 and 1.9 x 107 CFU/mL. Brain, liver, intestine, and head kidney samples were culture positive for E. tarda from dead and morbid fish. Control fish sham infected had no mortalities. The LC50 concentration was 3.4 x 107 CFU/mL. The results demonstrate that healthy channel catfish are susceptible to edwardsiellosis following bath immersion exposure to E. tarda without prior mechanical skin injury. This method should allow researchers to induce edwardsiellosis in healthy channel catfish to test the efficacy of vaccines and for pathogenesis studies.