ARS | Publication request: Detection of Edwardsiella ictaluri in frozen catfish: Epidemiological application

Submitted to: World Aquaculture Society Meeting
Publication Type: Abstract Only
Publication Acceptance Date: November 30, 2009
Publication Date: March 1, 2010
Citation: Bebak, J.A., Shoemaker, C.A., Arias, C., Klesius, P.H. 2010. Detection of Edwardsiella ictaluri in frozen catfish: Epidemiological application [abstract]. World Aquaculture Society. p. 91.

Technical Abstract: Sampling large numbers of pond-cultured fish species for population studies will most likely occur in a hot ambient environment without immediate access to laboratory facilities. One solution to maintain sample integrity may be to place fish on dry ice and then transfer them to storage at -80ºC for processing at a later date. The three main objectives of this laboratory study were, 1) to determine whether a channel catfish (Ictalurus punctatus Rafinesque) fingerling classified as positive for Edwardsiella ictaluri (Enterobacteriaceae, Gram-negative rod) infection according to bacterial culture before freezing was also classified as positive after freezing at -80ºC for 58-60 days, 2) to determine how direct culture from the kidney, culture of homogenate and standard PCR agree with bacterial culture (colony forming units (CFU)/g kidney tissue) in terms of classifying fish as positive or negative, and 3) to estimate diagnostic sensitivity and diagnostic specificity for direct culture, culture of homogenate and standard PCR. Channel catfish (Ictalurus punctatus) fingerlings were injected I.C. (intracoelomic) with various concentrations of Edwardsiella ictaluri, sampled at six to nine days post-injection, frozen at -80ºC, and then sampled 59-63 days post-freezing. Bacterial culture and PCR assay of homogenized kidney tissue were compared. As bacterial concentration (CFU E. ictaluri/g kidney tissue) decreased, the ability of each assay to detect positive fish also decreased, especially when there were <10,000 CFU/g tissue. Culture of homogenized kidney tissue proved to be the most reliable at correctly classifying catfish, whether they were subclinically or clinically infected. The results indicate that, whether fish are assayed while fresh or previously frozen, sample size estimates should consider diagnostic sensitivity and specificity of the assays, especially if subclinically infected fish are included. Values for diagnostic sensitivity and diagnostic specificity for the various assays, for this study population, will be presented.