Prevalence and analysis of Renibacterium salmoninarum infection among juvenile Chinook salmon Oncorhynchus tshawytscha in North Puget Sound

Abstract

Renibacterium salmoninarum causes bacterial kidney disease (BKD), a chronic and sometimes fatal disease of salmon and trout that could lower fitness in populations with high prevalences of infection. Prevalence of R. salmoninarum infection among juvenile Chinook salmon Oncorhynchus tshawytscha inhabiting neritic marine habitats in North Puget Sound, Washington, USA, was assessed in 2002 and 2003. Fish were collected by monthly surface trawl at 32 sites within 4 bays, and kidney infections were detected by a quantitative fluorescent antibody technique (qFAT). The sensitivity of the qFAT was within an order of magnitude of the quantitative real-time PCR (qPCR) sensitivity. Prevalence of infection was classified by fish origin (marked/hatchery vs. unmarked/likely natural spawn), month of capture, capture location and stock origin. The highest percentages of infected fish (63.5 to 63.8%) and the greatest infection severity were observed for fish collected in Bellingham Bay. The lowest percentages were found in Skagit Bay (11.4 to 13.5%); however, there was no difference in prevalence between marked and unmarked fish among the capture locations. The optimal logistic regression model of infection probabilities identified the capture location of Bellingham Bay as the strongest effect, and analysis of coded wire tagged (CWT) fish revealed that prevalence of infection was associated with the capture location and not with the originating stock. These results suggest that infections can occur during the early marine life stages of Chinook salmon that may be due to common reservoirs of infection or horizontal transmission among fish stocks.

Pathological and immunological responses associated with differential survival of Chinook salmon following Renibacterium salmoninarum challenge

“…Expression of the Renibacterium salmoninarum major soluble antigen (msa) (Chien et al 1992, Chase et al 2006 or abc transporter permease (Rhodes et al 2006) genes was measured by reverse-transcriptase quantitative PCR (RT-qPCR) as a proxy of bacterial load. Since these assays measure RNA rather than DNA, RT-qPCR may be better able to discriminate live from dead bacteria.…”

“…The msa RTqPCR was performed as previously described (Chase et al 2006) with the following modifications: (1) the probe was labeled with a 5' 6-carboxyfluorescein (FAM) and 3' non-fluorescent quencher (NFQ) and a minor grove binding (MGB) linker (probe purchased from Applied Biosystems) and (2) the target template was cDNA rather than genomic DNA. The abc assay was performed as described (Rhodes et al 2006) but again targeted cDNA rather than genomic DNA. Bacterial load was assayed in the same experimental cDNA samples as the host immune genes described below.…”

“…R. salmoninarum is widespread in natural and captive populations of Chinook salmon Oncorhynchus tshawytscha in the Pacific Northwest region of North America (Elliott et al 1997, Rhodes et al 2006. Chinook salmon have been shown to be particularly susceptible to BKD relative to other salmonid species (Fryer & Lannan 1993), and there is limited understanding of what constitutes a protective immune response to R. salmoninarum in Chinook salmon.…”

Interpreting dual ELISA and qPCR data for bacterial kidney disease of salmonids

“…Categories for ELISA values were set using the thresholds described in Pascho & Mulcahy (1987; see our Table 1). DNA extractions and qPCR were performed as previously described (Rhodes et al 2006), except the probe was a 5'-FAM-BHQ1-3' labeled oligonucleotide. This primer-probe set is directed against a stable, single-copy intergenic region of the Renibacterium salmoninarum genome, and it can be used to quantify the number of bacterial cells present.…”

“…Bacterial kidney disease (BKD) of salmonids is a persistent, often debilitating condition frequently observed among stocks of Chinook salmon, coho salmon, and sockeye salmon of the US Pacific Northwest and Alaska (Meyers et al 2003, Arkoosh et al 2004, Rhodes et al 2006. The enzyme-linked immunosorbent assay (ELISA) has been the diagnostic method of choice in regional hatcheries and captive broodstock programs for nearly 2 decades because of several desirable features.…”