First record of viral erythrocytic necrosis and Ceratomyxa shasta Noble, 1950 (Myxozoa: Myxosporea) in feral pink salmon (Oncorhynchus gorbuscha Walbaum)

FIRST REPORT IN CANADA OF CERATONOVA GASTEROSTEA (CNIDARIA: MYXOZOA) IN THREESPINE STICKLEBACK GASTEROSTEUS ACULEATUS FROM THE FRASER RIVER, BRITISH COLUMBIA

A survey of threespine stickleback (Gasterosteus aculeatus L.) and juvenile Chinook salmon (Oncorhynchus tshawytscha (Walbaum)) from the lower Fraser River, British Columbia, Canada and adjacent marine waters was conducted for the presence of myxozoan parasites of the genus Ceratonova Atkinson, Foott, and Bartholomew, 2014 (Bivalvulida, Ceratomyxidae). Quantitative polymerase chain reaction (PCR; small subunit ribosomal RNA gene) combined with conventional PCR and sequencing (ribosomal internal transcribed spacer region 1) detected Ceratonova gasterostea Atkinson, Foott, and Bartholomew, 2014 in 89 of 269 (33.1%) sticklebacks from the Fraser River and in 1 of 51 (2%) from Howe Sound. In contrast, Ceratonova shasta (Atkinson, Foott, and Bartholomew, 2014) was detected in 2 of 26 (7.7%) Chinook salmon from the Fraser River. Reciprocal infections were not detected. Light microscopic examination of the stickleback intestine revealed the presence of an intense infection with large numbers of pseudoplasmodia which interacted with enterocytes through pseudopodia-like projections. Myxospores with characteristic elongate valve cells were visible within mature pseudoplasmodia and free in the intestinal lumen. This is the first report of C. gasterostea in Canada.

Distribution and Phylogeny of Erythrocytic Necrosis Virus (ENV) in Salmon Suggests Marine Origin

Viral erythrocytic necrosis (VEN) affects over 20 species of marine and anadromous fishes in the North Atlantic and North Pacific Oceans. However, the distribution and strain variation of its viral causative agent, erythrocytic necrosis virus (ENV), has not been well characterized within Pacific salmon. Here, metatranscriptomic sequencing of Chinook salmon revealed that ENV infecting salmon was closely related to ENV from Pacific herring, with inferred amino-acid sequences from Chinook salmon being 99% identical to those reported for herring. Sequence analysis also revealed 89 protein-encoding sequences attributed to ENV, greatly expanding the amount of genetic information available for this virus. High-throughput PCR of over 19,000 fish showed that ENV is widely distributed in the NE Pacific Ocean and was detected in 12 of 16 tested species, including in 27% of herring, 38% of anchovy, 17% of pollock, and 13% of sand lance. Despite frequent detection in marine fish, ENV prevalence was significantly lower in fish from freshwater (0.03%), as assessed with a generalized linear mixed effects model (p = 5.5 × 10⁻⁸). Thus, marine fish are likely a reservoir for the virus. High genetic similarity between ENV obtained from salmon and herring also suggests that transmission between these hosts is likely.

Myxobolus arcticus and Parvicapsula minibicornis infections in sockeye salmon Oncorhynchus nerka following downstream migration in British Columbia

Factors influencing the health of sockeye salmon Oncorhynchus nerka in British Columbia, Canada, are important for fisheries management and conservation. Juvenile salmon originating from the Fraser River were screened for 3 enzootic parasites (Myxobolus arcticus, Parvicapsula minibicornis, Ceratonova shasta) and the bacterium Renibacterium salmoninarum. Fish were collected from the Strait of Georgia in 2010, 2011 and 2012 and genotyped to stock of origin. Trends in infection status were estimated by year, spawning zone and catch area. The annual prevalences of P. minibicornis (n = 1448) were 23.3, 6.5 and 8.1%, and for M. arcticus (n = 1343), annual prevalences were 40.4, 66.3 and 27.4%, respectively. Logistic regression showed that P. minibicornis was most strongly associated with salmon from the lower Fraser River spawning zone and increased with distance caught from the mouth of the Fraser River. In contrast, infection with M. arcticus was most strongly associated with salmon from the middle Fraser River spawning zone, and there was no trend related to distance from the Fraser River. Neither R. salmoninarum nor C. shasta were detected. These observations are discussed in the context of salmon life history and pathogen biology.

A survey of microparasites present in adult migrating Chinook salmon (Oncorhynchus tshawytscha) in south-western British Columbia determined by high-throughput quantitative polymerase chain reaction

Microparasites play an important role in the demography, ecology and evolution of Pacific salmonids. As salmon stocks continue to decline and the impacts of global climate change on fish populations become apparent, a greater understanding of microparasites in wild salmon populations is warranted. We used high-throughput, quantitative PCR (HT-qRT-PCR) to rapidly screen 82 adult Chinook salmon from five geographically or genetically distinct groups (mostly returning to tributaries of the Fraser River) for 45 microparasite taxa. We detected 20 microparasite species, four of which have not previously been documented in Chinook salmon, and four of which have not been previously detected in any salmonids in the Fraser River. Comparisons of microparasite load to blood plasma variables revealed some positive associations between Flavobacterium psychrophilum, Cryptobia salmositica and Ceratonova shasta and physiological indices suggestive of morbidity. We include a comparison of our findings for each microparasite taxa with previous knowledge of its distribution in British Columbia.