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Diseases of Aquatic Organisms

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DAO 103:35-43 (2013)  -  DOI:

Candidatus Branchiomonas cysticola’ is a common agent of epitheliocysts in seawater-farmed Atlantic salmon Salmo salar in Norway and Ireland

Susan O. Mitchell1,2,*, Terje M. Steinum1,*, Elena R. Toenshoff3, Agnar Kvellestad4, Knut Falk1, Matthias Horn3, Duncan J. Colquhoun1,**

1Norwegian Veterinary Institute, Ullevålsveien 68, 0454 Oslo, Norway
2Vet Aqua International, Unit 7b Oranmore Business Park, Oranmore, Galway, Ireland
3Department of Microbial Ecology, University of Vienna, Althanstr. 14A, 1090 Vienna, Austria
4Department for Basic Sciences and Aquatic Medicine, Norwegian School of Veterinary Science, Ullevålsveien 72, 0033 Oslo, Norway
*These authors contributed equally to the study**Corresponding author. Email:

ABSTRACT: The prevalence and geographical distribution of the recently described endosymbiont ‘Candidatus Branchiomonas cysticola’ in Atlantic salmon Salmo salar gill epithelial cell cysts was investigated in seawater-farmed fish suffering proliferative gill inflammation (PGI). To this end, we developed a specific and sensitive real-time PCR assay for detection of the bacterium. ‘Ca. B. cysticola’ was found to be highly prevalent in Atlantic salmon gills sampled over 7 yr and from 17 geographically distant seawater locations in Norway and Ireland. ‘Ca. B. cysticola’ was found in significantly greater quantities in fish with large numbers of epitheliocysts, and fluorescence in situ hybridization confirmed its localisation within cysts. ‘Ca. Piscichlamydia salmonis’, a bacterium previously linked to epitheliocysts, was identified at relatively low levels of infection, apparently independent of epitheliocyst prevalence. These results suggest that ‘Ca. B. cysticola’ is the main cyst-forming bacterium in seawater-farmed Atlantic salmon in the studied countries. Our results also suggest a relationship between load of ‘Ca. B. cysticola’ and extent of pathological changes. Taken together with a previously described association between epitheliocyst load and severity of PGI in Norwegian salmon, the results could indicate a role for ‘Ca. B. cysticola’ in gill diseases such as PGI.

KEY WORDS: ‘Candidatus Branchiomonas cysticola’ · ‘Candidatus Piscichlamydia salmonis’ · Epitheliocystis · PGI · Proliferative gill disease · Gill pathology · Atlantic salmon

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Cite this article as: Mitchell SO, Steinum TM, Toenshoff ER, Kvellestad A, Falk K, Horn M, Colquhoun DJ (2013) ‘Candidatus Branchiomonas cysticola’ is a common agent of epitheliocysts in seawater-farmed Atlantic salmon Salmo salar in Norway and Ireland. Dis Aquat Org 103:35-43.

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