Pathology Associated with the Rosette Agent, A Systemic Protist Infecting Salmonid Fishes

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Abstract
Mortality and morbidity were observed among 1–5-year-old captive broodstock of Sacramento River winter-run chinook salmon Oncorhynchus tshawytscha that had been reared in seawater and were infected with the systemic protist termed the “rosette agent.” Two types of lesions were found in naturally occurring infections. The first was disseminated and was characterized by systemic dispersion of parasites accompanied by minimal host inflammatory cell response, whereas the second was limited and nodular with parasites restricted to granulomas in the kidney, spleen, and liver. In the disseminated form of the disease, the parasite was detected within hematopoietic, epithelial, and mesenchymal cell types. Aggregates of the organism and associated cellular debris were found in the kidney, liver, spleen, heart, gill, brain, ovary, testis, and hindgut. Renal tubular necrosis, membranous glomerulonephritis, necrotizing interstitial nephritis, multifocal hepatocellular necrosis, and necrotizing vasculitis were...