Piscirickettsia salmonis, the aetiological agent of piscirickettsiosis, is a Gram-negative, facultative intracellular bacterium that infects marine fish (Fryer, Lannan, Giovannoni, & Wood, 1992). Outbreaks of salmonid piscirickettsiosis, or salmonid rickettsial septicaemia (SRS), occur in farmed salmonids in Canada, Chile, Ireland, Norway and Scotland and tend to follow stressful environmental events such as algal blooms and elevated temperatures (Branson & Nieto Diaz-Munoz, 1991; Cusack, Groman, & Jones, 2002; Rozas & Enríquez, 2014). SRS causes significant economic losses in the Chilean salmon farming industry (Rozas & Enríquez, 2014) whereas outbreaks in farmed salmonids elsewhere tend to be of lower severity. Susceptible salmonid species include pink salmon (Oncorhynchus gorbuscha), rainbow trout (O. mykiss), coho salmon (O. kisutch), chinook salmon (O. tshawytscha) and Atlantic salmon (Salmo salar). In western Canada, P. salmonis has been reported in wild and cultured Pacific salmon including Chinook and pink salmon as well as in cultured Atlantic salmon (Brocklebank, Evelyn,

Background Infection with the myxozoan parasite Parvicapsula pseudobranchicola causes disease in wild and farmed salmonids in Norway. In the northeast Pacific Ocean, the parasite has been reported in Pacific salmon Oncorhynchus spp. without evidence of disease. The objectives of the present study were to confirm the identity of P. pseudobranchicola in the Pacific, document its host and geographic ranges, and describe associated pathological changes. Methods Ocean-entry year wild pink salmon Oncorhynchus gorbuscha, chum salmon O. keta, Chinook salmon O. tshawytscha, coho salmon O. kisutch and sockeye salmon O. nerka were collected in summer and autumn surveys near Vancouver Island (VI) and from a winter survey in the Gulf of Alaska. Samples were also obtained from farmed Atlantic salmon Salmo salar and Chinook salmon near VI. Samples were analysed by qPCR and histology using conventional staining or in situ hybridisation. Parasite sequence was obtained from small subunit ribosomal RNA gene (SSU rDNA). Results Identical 1525 base-pair SSU rDNA sequences from infected pink salmon, chum salmon and Chinook salmon shared 99.93% identity with a P. pseudobranchicola sequence from Norwegian Atlantic salmon. In autumn surveys, the prevalence was greatest in chum salmon (91.8%) and pink salmon (85.9%) and less so in Chinook salmon (68.8%) and sockeye salmon (8.3%). In farmed salmon, the prevalence was zero in Atlantic salmon (n = 967) and 41% in Chinook salmon (n = 118). Infections were preferentially sited in pseudobranch and visualised by in situ hybridisation. Heavy parasite burdens in all species of Pacific salmon were inconsistently associated with focal granulomatous pseudobranchitis. Conclusions In the northeast Pacific, widespread occurrence of P. pseudobranchicola in Pacific salmon together with its absence or sporadic occurrence in farmed Atlantic salmon differs from its epidemiology in Norway, despite similar pathological development in the pseudobranch. Consequences of the infections to the health of wild Pacific salmon, identity of the invertebrate host and the distribution and abundance of infective actinospores are unknown and remain high priorities for research. Graphical Abstract