Aquaculture Canada and WAS North America 2022

August 15 - 18, 2022

St Johns, Newfoundland, Canada

EMERGING VARIANTS OF Moritella viscosa


Julia Tandberg*, Anette Furevik, Thu Nguyen, Helen Behzaad, and Siv Tunheim


PHARMAQ part of Zoetis

Harbitzalléen 2A, P.O.Box 267 Skøyen

N-0213 Oslo, Norway


Moritella viscosa has been described as the dominating agent of winter-ulcer in Atlantic salmon (Salmo salar), lumpfish (Cyclopteridae) and rainbow trout (Oncorhynchus mykiss). Outbreaks of ulcerative disease in farmed fish occurs across the North Atlantic region and is an impeding factor for sustainable growth within the industry. Several commercially available multivalent vaccines contain inactivated bacteria to control skin diseases. Although challenge models confirm specific protection against disease development, they also show differences in vaccine efficacy. Grove et. al described in 2010 two major genetic clades within M. viscosa  (classic and variant) and an increased occurrence of winter-ulcers has, furthermore, been registered in Norway and UK over the last years.

The present study describes the collection and characterization of M. visocsa isolates from 2016-2020 in the North Atlantic region with focus on isolates harvested from Norwegian outbreaks of winter-ulcer. The isolates collected has undergone both genetic and serological analysis using gyrB and OMP sequencing in addition to ELISA, demonstrating several sub-groups of both classic and variant M. viscosa. Infection trials using Norwegian variant and classic isolates of the bacterium, both in experimental vaccines and for challenge, demonstrate that the two genetic clades provide limited cross protection. The results of the present study indicate both genetically, serological and clinical differences between classic and variant isolates of M. viscosa, demonstrating the need for further vaccine development against winter-ulcers.