Factors associated with severity of naturally occurring piscirickettsiosis in netpen‐ and tank‐reared juvenile Atlantic salmon at a research aquarium in western Canada

Abstract

The opportunistic examination of factors associated with an outbreak of piscirickettsiosis (SRS) is described in Atlantic salmon Salmo salar post‐smolts held in an open netpen or in tanks supplied with raw sea water at a research aquarium in western Canada. During the outbreak, seawater temperature was significantly higher and salinity significantly lower in the netpen compared with the tanks. Mortality in the netpen began approximately 3 weeks prior to that in the tanks, and cumulative mortality in the netpen (34%) was significantly higher than in the tanks (12%). Piscirickettsia salmonis was confirmed by qPCR in tissues from moribund and dead fish and from colonies grown on enriched blood agar medium. Neither P. salmonis nor SRS were observed in salmon held concurrently in UV‐irradiated sea water. The elevated mortality was curtailed by treatment with oxytetracycline. These observations further indicate warmer, less saline and periodically hypoxic seawater are risk factors for SRS. UV irradiation of sea water is shown to be a tool for SRS management in fish‐holding facilities.

Development of a Piscirickettsia salmonis immersion challenge model to investigate the comparative susceptibility of three salmon species

“…In another study (Garcés et al., 1991), mortalities of up to 100% in coho and Atlantic salmon following ip injection with high doses (> 10 3.3 TCID 50 /fish) of an unidentified Chilean P. salmonis isolate precluded any distinction of susceptibility between these species. In western Canada, outbreaks of SRS or a similar disease have been reported in cultured Atlantic salmon, Chinook salmon and coho salmon (Brocklebank et al., 1992), and the infection has been recognized in pink salmon (Brocklebank et al., 1992; Jones et al., 2020). In contrast, there have been no reports of SRS or P. salmonis infection in sockeye salmon and our data suggest a reduced likelihood of infection or SRS in migratory sockeye salmon.…”

“…NBF‐fixed tissue samples were dehydrated in an alcohol gradient, clarified in xylene and infiltrated with paraffin wax for subsequent sectioning at 3 µm, staining and examination by light microscopy. Histological sections were stained with either haematoxylin and eosin, Giemsa or Gram stain (Jones, Long, MacWilliams, Polinski, & Garver, 2020). The presence and distribution of bacterial cells in histological sections were assessed by in situ hybridization (ISH) (Venegas, Contreras, Larenas, & Smith, 2004) with the following modifications.…”

“…For each qPCR run, the number of copies per reaction (c/rxn) was determined from a standard curve generated by amplifying a 10‐fold serially diluted double‐stranded DNA gBLOCK fragment (Integrated DNA Technologies), which included the P. salmonis primer and probe binding sites, ranging from 10 7 to 10 1 c/rxn. The limit of detection (LOD) for this assay is 5 c/rxn (Jones et al., 2020). Only samples with values at or above the LOD were considered positive.…”

“…Histological sections were stained with either haematoxylin and eosin, Giemsa or Gram stain (Jones, Long, MacWilliams, Polinski, & Garver, 2020). The presence and distribution of bacterial cells in histological sections were assessed by in situ hybridization (ISH) (Venegas, Contreras, Larenas, & Smith, 2004) with the following modifications.…”

Piscirickettsia salmonis shedding and tissue burden, and hematological responses during cohabitation infections in chum Oncorhynchus keta, pink O. gorbuscha and Atlantic salmon Salmo salar

“…Bacterial burdens in the water and tissue samples were determined using a qPCR assay designed for the P . salmonis 23S gene [ 20 , 21 ]. The limit of detection (LOD) for this assay is 5 copies/reaction (c/rxn) or 1 genome equivalent (GEq) [ 21 ].…”