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In this study, 318 bacterial strains were isolated from the gastrointestinal (GI) tracts of 29 rainbow trout, Oncorhynchus mykiss (Walbaum). These bacteria were screened in vitro for their ability to inhibit growth of Flavobacterium psychrophilum, the causative agent of coldwater disease. Bacteria observed to inhibit F. psychrophilum growth were further screened against rainbow trout bile, as an indicator of their ability to survive in the GI tract. This screening resulted in narrowing the pool to 24 bacterial isolates. Those 24 isolates were then tested for pathogenicity in rainbow trout by intraperitoneal injection. Following a 28-day challenge, eight isolates were shown to cause direct mortality and were eliminated from further study. As a result, 16 bacterial isolates were identified as probiotic candidates with the potential to control or reduce disease caused by F. psychrophilum.

Nonlethal sampling techniques have previously been evaluated for detection of a variety of viral salmonid pathogens. However, many of these studies have used molecular assays in lieu of widely accepted cell culture techniques to evaluate the sampled tissues. Samples were collected from female steelhead Oncorhynchus mykiss broodstock using three potential nonlethal sampling methods (mucus/skin scrape, pectoral fin clip, and gill tissue biopsy) and evaluated for the presence of infectious hematopoietic necrosis virus (IHNV) via cell culture techniques. The results were compared with those from samples collected using a standard lethal sampling method (pooled anterior kidney and spleen tissues) applied to the same fish. Of the three nonlethal sampling techniques that were evaluated, fin clipping was the easiest and least invasive method. Furthermore, fin tissue was as sensitive as or more sensitive than kidney/spleen tissue for detecting IHNV in this population of fish. However, with the exception of gill tissue, the nonlethal samples did not appear to be appropriate surrogates for lethally collected tissues with regard to identifying an active infection in a particular fish. Nevertheless, nonlethal sampling coupled with cell culture appears to be suitable for helping to define the IHNV status of a steelhead population. Received July 27, 2016; accepted December 11, 2016.

Myxobolus cerebralis (Mc) is a myxozoan parasite causing whirling disease in hatchery- and natural-origin salmonids. To minimize spread of this parasite and the incidence of its associated disease, fish health professionals routinely screen fish for Mc before stocking or moving the fish to Mc-free waters. Sample collection for Mc traditionally entails lethal sampling of cranial tissue followed by pepsin-trypsin digest (PTD) and screening of the sample for mature myxobolid myxospores (PTD method); however, nonlethal sampling methods would be advantageous in some circumstances, such as when dealing with rare or otherwise valuable fish. Accordingly, we compared Mc detections in cranial cartilage by using the PTD method with PCR assays of fin biopsies collected from juvenile Chinook Salmon Oncorhynchus tshawytscha and adult steelhead O. mykiss. Cranial samples were also analyzed using PCR methods for comparative purposes. Results indicated that Mc could be detected by PCR in fin clips, but the results generated by this approach differed significantly from those associated with PTD- and/or PCR-based analysis of cranial cartilage samples. Polymerase chain reaction-based analysis-of individual head samples and head digest pools in both species as well as fins in steelhead-yielded more positive detections than PTD analysis alone. The PCR-based analysis of head and fin tissues yielded different Mc detection rates in both species, but the nature of the detection disparity varied depending on the species and/or life stage of the fish. We conclude that for lethal cranial samples, neither PTD nor PCR should be used alone, but using these techniques in concert may provide the most complete and accurate estimation of Mc presence in a group of salmonids. If imperiled or highly valuable fish are in question, nonlethal fin samples may be used to generate some information regarding Mc status, with the understanding that parasite DNA detections do not necessarily signify mature infections or disease.

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