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

Abstract

Chinook salmon Oncorhynchus tshawytscha are highly susceptible to Renibacterium salmoninarum, the causative agent of bacterial kidney disease (BKD). Previously we demonstrated that introduced Chinook salmon from Lake Michigan, Wisconsin (WI), USA, have higher survival following R. salmoninarum challenge relative to the progenitor stock from Green River, Washington, USA. In the present study, we investigated the pathological and immunological responses that are associated with differential survival in the 2 Chinook salmon stocks following intra-peritoneal R. salmoninarum challenge of 2 different cohort years (2003 and 2005). Histological evaluation revealed delayed appearance of severe granulomatous lesions in the kidney and lower overall prevalence of membranous glomerulopathy in the higher surviving WI stock. The higher survival WI stock had a lower bacterial load at 28 d post-infection, as measured by reverse-transcriptase quantitative polymerase chain reaction (RT-qPCR). However, at all other time points, bacterial load levels were similar despite higher mortality in the more susceptible Green River stock, suggesting the possibility that the stocks may differ in their tolerance to infection by the bacterium. Interferon-γ, inducible nitric oxide synthase (iNOS), Mx-1, and transferrin gene expression were up-regulated in both stocks following challenge. A trend of higher iNOS gene expression at later time points (≥28 d post-infection) was observed in the lower surviving Green River stock, suggesting the possibility that higher iNOS expression may contribute to greater pathology in that stock.

Bench‐top validation testing of selected immunological and molecular Renibacterium salmoninarum diagnostic assays by comparison with quantitative bacteriological culture

“…This possibility is supported by the work of Metzger et al . (), in which BKD lesions were identified by histopathology in the pancreatic interstitium, spleen capsule and liver of some juvenile Chinook salmon 14 days after intraperitoneal injection with R. salmoninarum ; the injected fish were from the same Wisconsin stock as those used in the current study. The ability of both ELISA and FAT to detect non‐viable R. salmoninarum may have also contributed to higher diagnostic sensitivity in comparison with culture.…”

“…; Metzger et al . ) have been developed to detect the expression of R. salmoninarum messenger RNA (mRNA), which has a half‐life usually measured in minutes (e.g., von Gabain et al . ; Belasco et al .…”

“…; Metzger et al . ). Spawning fall Chinook salmon to provide SPF progeny for this research were obtained from Strawberry Creek, Wisconsin in October 2004.…”

“…; Metzger et al . ), which indicated that the infection would likely be progressing at the time of sampling, but that most fish would probably have low to moderate infection levels.…”

Experimental challenges with Renibacterium salmoninarum in Arctic charr Salvelinus alpinus

“…Infection trials, applying various techniques, have been carried out in economically important salmonid species (Fryer & Lannan 1993), especially rainbow trout Oncorhynchus mykiss (Pascho et al 1997, Senson & Stevenson 1999, Jansson et al 2008 and Chinook salmon Oncorhynchus tshawytscha (Murray et al 1992, McKibben & Pascho 1999, Alcorn et al 2005, Metzger et al 2010, Elliott et al 2015. Other species, including Arctic charr, have received less attention: we were able to locate only 2 papers, where Arctic charr, along with other salmonids, were infected with R. salmoninarum by intraperitoneal (i.p.)…”