Moritella viscosa - Winter ulcers

Introduction

Moritella viscosa is a gram-negative bacterium that is the causative agent of winter ulcer disease. Winter ulcers often appear during periods of cold water and high salinity and the disease is characterized by the formation of dermal and subdermal ulcers. When the temperature increases, or salinity decreases, fish may recover from an outbreak of winter ulcers.

Challenge by immersion

VESO Aqualab has long experience in conducting challenge trials with Moritella viscosa, and we recommend models based on bath challenge of smolts in sea water which mimic a natural disease situation. Pre-smolts are photoperiod-manipulated to smoltify, transferred to sea water and acclimatized at 8-10°C before challenge. Development of ulcers and mortality is observed throughout a 15-30-day period. The mortality rate may be controlled by adjusting the water temperature.

Confirmatory diagnosis of Moritella viscosa is based on colony morphology and viscosity following growth on blood agar plates from smears from the pronephros of dead fish. Principal outcome parameters are prevalence and severity of ulceration and mortality.