Pathology associated with the rosette agent, a systemic protist infecting salmonid fishes

K. D. Arkush, S. Frasca, R. P. Hedrick

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41 Scopus citations


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 evident. In the nodular form of the disease, multifocal granulomas were identified in the kidney, liver, and spleen. Parasites ranged 2–6 mm in diameter in both disease presentations and were strongly periodic acid–Schiff (PAS) positive, argyrophilic, basophilic following Giemsa staining, and acid-fast negative. Transmission electron microscopy revealed that the parasite was surrounded by a trilaminar cell wall and had a ribosome-laden cytoplasm with scattered segments of rough endoplasmic reticulum, vesicular mitochondria, and a single nucleus. Variable numbers of electron-dense granules and lipid droplets were present in the cytoplasm, and solitary concentric bodies were identified in some of the organisms. The agent was isolated from kidney tissue of a naturally infected chinook salmon and was propagated in the chinook salmon embryo cell line (CHSE-214). Parasites from these cultures were injected at a dose of 1.6 3 107 organisms per fish into chinook salmon, coho salmon O. kisutch, rainbow trout O. mykiss, brown trout Salmo trutta, and brook trout Salvelinus fontinalis. At 3 and 6 months postinfection, chinook salmon and coho salmon were most heavily infected, followed by rainbow trout and brown trout. Few parasites were detected in brook trout. Evidence from natural outbreaks and experimental infections of chinook salmon and coho salmon suggests that the rosette agent is a significant pathogen of at least two salmon species. Trout appear to be more resistant, but their potential role as carriers of the pathogen remains unknown.

Original languageEnglish (US)
Pages (from-to)1-11
Number of pages11
JournalJournal of Aquatic Animal Health
Issue number1
StatePublished - Jan 1 1998

ASJC Scopus subject areas

  • Aquatic Science


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