Comparative Study of the Sensitivity of Bacterial Culture of Aeromonas salmonicida from Individual and Five-fish pool Samples from Naturally Infected Lahanton Cutthroat Trout

Introduction The question of bacterial culture sensitivity for individual versus pooled samples was discussed at the 2001 Fish Health Biologist Meeting in Tucson, AZ, as procedures for the USFWS Handbook were being developed. The USFWS Handbook strives to conform with methods specified by the Office International des Epizooties (OIE 2000) and the American Fisheries Society (AFS) Blue Book (Thoesen, 1996), however these two methodologies differed. The OIE method specifies up to 5-fish pools of target tissue, while the Bluebook recommended the use of inoculating loops to sample individual fish for bacteriological testing. A comparative study of the two methodologies most commonly used for detection of bacteria was suggested as a means to eva luate the sample method sensitivity and determine if either method would preclude a positive test result during a fish health inspection. The Ca-Nv Fish Health Center volunteered to conduct a comparative study to determine if bacterial culture sensitivity was negatively affected by pooling kidney tissues during sample collection from adult broodstock known to be infected with Aeromonas salmonicida. It is important to utilize naturally infected fish because they more closely represent the range of host and environmental variables present in a fish population; this range of variables has as a major impact on the accuracy of the test-result interpretation (OIE, 2000). The study also addressed the effects of delayed inoculation of kidney tissue onto growth media to determine if assay sensitivity was altered when samples were collected in the “field” versus inoculated at a later time in the laboratory.