The Discovery of Tetracapsuloides bryosalmonae Infecting Enhanced Migratory Pacific Salmon Populations in the Great Lakes.

Chinook (Oncorhynchus tshawytscha) and Coho Salmon (Oncorhynchus kisutch) get seasonally restocked in the Great Lakes watershed to support local ecosystems, aquaculture and recreational fishing industry. The myxozoan parasite Tetracapsuloides bryosalmonae (Cnidaria, Malacosporea) causes Proliferative Kidney Disease (PKD), a severe chronic lymphoid immunopathology affecting farmed and wild salmonid populations in Central Europe and Northwestern America. We opportunistically sampled migratory Pacific salmon broodstock at fish weirs in Michigan rivers during the autumns of 2020-2023, joining seasonal harvesting campaigns, collecting a total of 591 Chinook and 463 Coho Salmon. T. bryosalmonae was detected in broodstock that matured in Lakes Michigan and Huron, with no manifest clinical signs of PKD. Infection prevalence varied among stocks, locations, and years, ranging from 3 to 48% in Coho and 1 to 20% in Chinook. Sequence analysis of SSU rDNA and mitochondrial COI markers confirmed the detection of T. bryosalmonae. Histological examinations revealed both coelozoic sporogonic and histozoic extrasporogonic parasite stages in the posterior kidneys of adult Chinook and Coho. Immunohistochemistry, optimized using a new commercially available T. bryosalmonae-specific antibody, further confirmed parasite stages in the anterior and posterior kidney. Further study is needed to characterize this previously undetected, regionally reportable myxozoan parasite and to assess the impact that PKD occurrence may have on fisheries and aquaculture activities in the Great Lakes region. Therefore, tracking this parasite through adequate regulations even in asymptomatic fish hosts is essential, and may guide suitable conservation management actions to safeguard local fish populations and improve preparedness for future potential disease outbreaks.