MLVA genotyping of Moritella viscosa reveals serial emergence of novel, host-specific clonal complexes in Norwegian salmon farming

• Published in: Journal of Fish Diseases
• Main Authors: Sørgaard, Martin, Sveinsson, Karoline Overn, Patel, Sonal, Nilsen, Hanne Katrine, Olsen, Anne Berit, Vaagnes, Øyvind, Colquhoun, Duncan John, Gulla, Snorre
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2023
• Subjects: Atlantic salmon; Northeast Atlantic; Salmo salar
• Online Access: https://hdl.handle.net/11250/3083300; https://doi.org/10.1111/jfd.13766

A Multi-Locus Variable number of tandem repeat Analysis (MLVA) genotyping scheme was developed for the epidemiological study of Moritella viscosa, which causes ‘winter ulcer’ predominantly in sea-reared Atlantic salmon (Salmo salar L.). The assay involves multiplex PCR amplification of six Variable Number of Tandem Repeat (VNTR) loci, followed by capillary electrophoresis and data interpretation. A collection of 747 spatiotemporally diverse M. viscosa isolates from nine fish species was analysed, the majority from farmed Norwegian salmon. MLVA distributed 76% of the isolates across three major clonal complexes (CC1, CC2 and CC3), with the remaining forming minor clusters and singletons. While 90% of the salmon isolates belong to either CC1, CC2 or CC3, only 20% of the isolates recovered from other fish species do so, indicating a considerable degree of host specificity. We further highlight a series of ‘clonal shifts’ amongst Norwegian salmon isolates over the 35-year sampling period, with CC1 showing exclusive predominance prior to the emergence of CC2, which was later supplanted by CC3, before the recent re-emergence of CC1. Apparently, these shifts have rapidly swept the entire Norwegian coastline and conceivably, as suggested by typing of a small number of non-Norwegian isolates, the Northeast Atlantic region as a whole. publishedVersion