Asymptomatic Infection of <i>Renibacterium salmoninarum</i> in Hatchery-reared Juvenile Chum Salmon <i>Oncorhynchus keta</i> Resulted in Mass Mortalities after Long-term Rearing

Abstract

A previous study detected the major soluble antigen (msa) gene and its corresponding mRNA from Renibacterium salmoninarum (R.s.) in the kidney tissue and surface mucus of asymptomatic chum salmon fry and juveniles by nested PCR (nPCR), quantitative realtime PCR (qPCR) and a reverse transcription nested PCR assay (RT-nPCR) (Suzuki et al., 2017a). The chum salmon had been transferred from a hatchery in Hokkaido at the fry-stage (0.5 g) and reared at a wetlaboratory in the Salmon and Freshwater Fisheries Research Institute in a fresh water environment for 10 months (Suzuki et al., 2017a). In January 2017, the same group of asymptomatic chum salmon juveniles and another group of asymptomatic chum salmon juveniles, which were transferred at the fertilized-egg stage from another hatchery in Hokkaido to the wet laboratory and reared until juvenile, began to die. Most of the dead fish showed the clinical signs of bacterial kidney disease (BKD). The purpose of the present study was to examine the outbreaks of BKD in chum salmon juveniles subclinically infected with R.s. Materials and Methods Fish Two groups of asymptomatic chum salmon (hereafter referred to as Group-A and Group-B) were examined. Group-A fish were the progeny of returning chum salmon female adults in 2015 at Site A (Suzuki et al., 2017a, 2017b) and transferred from Site A at the fry-stage weighing 0.5 g in March 25, 2016, and reared until juveniles at the wet laboratory in the Salmon and Freshwater Fisheries Research Institute (Suzuki et al., 2017a). BKD had not previously been reported at this institute. The fish were confirmed by nPCR, qPCR and RT-nPCR for the presence of the msa gene and msa mRNA to indicate subclinical infection with R.s. seven times (Suzuki et al., 2017a). Group-B fish were transferred from another hatchery in Hokkaido at the fertilized-egg stage in October 8, 2015, to the wet laboratory and reared until juvenile. Group-B fish were confirmed by nPCR and qPCR to be subclinically infected with R.s. twice.

Specific pathogens and microbial abundance within liver and kidney tissues of wild marine fish from the Eastern Mediterranean Sea

“…The internal organs, in contrast to gills and skin, do not directly contact the outer environment, and thus may represent the organ’s specific symbiotic microbiome and provide important information. Already 30 years ago, a wide range of bacteria in kidney and liver of fish (Mudarris and Austin, ; Toranzo et al , ; Starliper and Tesk, ) was reported and in recent years more studies have focused on the presence of bacteria in apparently healthy fish internal organs (Pujalte et al , ; Suzuki, ). One explanation for the presence of bacteria in what should be considered a ‘sterile organ’ is the breakdown of immunological defence mechanisms as a result of stress, which enhances the presence of bacteria in immune system cells and in the blood’s circulation (Tort, ).…”