Comparative pathogenesis of piscirickettsiosis in Atlantic salmon (Salmo salar L.) post‐smolt experimentally challenged with LF‐89‐like and EM‐90‐like Piscirickettsia salmonis isolates

Abstract

Piscirickettsiosis (SRS) is the most prevalent bacterial disease in Chilean salmon aquaculture and is responsible for high economic losses. The aim of this study was to comparatively characterize the pathogenesis of SRS in post-smolt Atlantic salmon during the early and late stages of infection with Piscirickettsia salmonis LF-89-like (PS-LF-89) and EM-90-like (PS-EM-90) using a cohabitation challenge. The pathogenesis of cohabitant fish infected with the two isolates was relatively different due to cohabitant fish infected with PS-EM-90 showing higher cumulative mortality and shorter time until death compared with PS-LF-89 fish. PS-LF-89 caused an SRS infection characterized by kidney and liver lesions, whereas PS-EM-90 caused systemic and haemorrhagic disease characterized by kidney, liver, heart, brain, skeletal muscle and intestine lesions. Decreased serum concentration of total proteins and albumin as well as increased serum ALT, AST and creatinine levels in fish infected with both isolates confirmed that changes in liver and kidney function occurred during infection. Tissue damage, expressed as an SRS histoscore, showed a strong positive correlation with the bacterial load expressed as abundance of P. salmonis 16S rRNA transcripts in the livers and kidneys of fish affected with either isolate, but the correlation was significantly higher in fish infected with PS-EM-90. The results contribute to improving the understanding of the bacteria-host interaction.

Comparison of gene expression in post‐smolt Atlantic salmon challenged by LF‐89‐like and EM‐90‐like Piscirickettsia salmonis isolates reveals differences in the immune response associated with pathogenicity

“…To evaluate the expression of immune response genes, we used the same head kidney samples previously obtained from post‐smolt Atlantic salmon infected via cohabitation with LF‐89‐ like and EM‐90‐ like P. salmonis reported by Rozas‐Serri et al. (). Briefly, the cohabitation challenge was performed using an infectious dose of 10 5.6 u.f.c./0.1 ml of the laboratory isolate PS‐LF‐89 and the field isolate PS‐EM‐90.…”

“…The relative quantification of mRNA transcribed from the 16S rRNA gene of PS ‐ LF‐89 and PS‐EM‐90 was performed using RT‐qPCR in a StepOnePlus™ Real‐Time PCR system (Applied Biosystems, Life Technologies, Waltham, MA, USA) with the Brilliant III Ultrafast RT‐qPCR Master Mix kit (Agilent Technologies, Santa Clara, CA, USA) as described by Rozas‐Serri et al. (). Briefly, amplification was performed in a final volume of 15 μl containing 300 nM primers, 400 nM probe, 300 nM 6‐carboxyl‐X‐rhodamine (ROX) (50 nM) and 100 ng of total RNA.…”

“…Piscirickettsia salmonis isolates show high levels of similarity, although the EM‐90 isolate is situated separately (Mauel et al., ). The LF‐89‐ like and EM‐90‐ like field isolates exhibit genomic differences that appear to determine different degrees of virulence (Bohle et al., ) and different SRS pathogenesis in post‐smolt Atlantic salmon infected via cohabitation (Rozas‐Serri et al., ). In Chile, the strategy for controlling SRS focuses primarily on antimicrobial therapy and vaccination (Jakob et al., ; Rozas & Enriquez, ).…”

“…However, comparative studies of the innate and adaptive immune responses of Atlantic salmon infected with LF‐89 or EM‐90 P. salmonis via cohabitation have not been described. Our research group recently described a cohabitation challenge model for the P. salmonis isolates LF‐89 and EM‐90 in Atlantic salmon (Rozas‐Serri et al., ). Consequently, the aim of this study was to describe and comparatively quantify the innate and adaptive immune responses of post‐smolt Atlantic salmon infected via cohabitation with LF‐89‐ like and EM‐90‐ like P. salmonis by analysing the expression of genes involved in immunity.…”

The mutagenesis of a type IV secretion system locus of Piscirickettsia salmonis leads to the attenuation of the pathogen in Atlantic salmon, Salmo salar

“…Interestingly, the EM-90-like strain, which tolerates a warmer and less nutrient-rich environment, was responsible for the majority of piscirickettsiosis cases recorded in Chile in 2015, infecting exclusively Salmo salar (Saavedra et al, 2017). However, and despite their relevance in the current epidemiological situation of piscirickettsiosis, data concerning the experimental behaviour of EM-90-like isolates have only recently been presented (Rozas-Serri et al, 2017). …”