The epidemiology of skin ulcers in saltwater reared Atlantic salmon (Salmo salar) in Atlantic Canada

Abstract

Skin ulcers have been increasing in occurrence on Atlantic salmon (Salmo salar) farms in Atlantic Canada during summer and autumn months when water temperatures are elevated above 10 °C. The clinical presentation in Atlantic Canada resembles lesions associated with winter ulcer disease in Europe caused by Moritella viscosa, but this bacterium is not always identified from skin lesions in North American cases. The objectives of our study were to: 1) describe the onset, duration, magnitude, and temporal and spatial distributions of Atlantic Canadian skin ulcer cases at the cage and farm levels; 2) develop hypotheses regarding potential sources and transmission routes for infection based on the patterns of disease occurrence; and 3) identify potential risk factors associated with total percent mortality during skin ulcer outbreaks. We summarized weekly cage-level data from 29 salmon farms in Atlantic Canada, from April 2014 to January 2016. Cage and farm-level prevalence, onset, duration, and total percent mortality associated with skin ulcer outbreaks were determined. The association of potential risk factors for skin ulcers with total percent mortality during skin ulcer outbreaks was assessed using a mixed-effects linear regression model with random farm effects. The overall farm-level prevalence of skin ulcers on Atlantic Canadian farms included in our study was 41%. The descriptive epidemiology of skin ulcers suggests cages are likely exposed to the causative agent(s) only at certain times of the year and exposure is not uniform across farms based on the patterns of disease occurrence. The onset of skin ulcer outbreaks in our study occurred from late-summer to mid-autumn. The pattern of disease is also suggestive of point source exposures to the causative agent across farms. In our final regression model, year of outbreak onset, and the interaction between the diagnosis of skin ulcers at the cage level and antibiotic treatment during the outbreaks were significantly associated with the log of total percent mortality during the outbreaks (P < .001). Our results did not identify any management factors that could reduce the severity of skin ulcer outbreaks on affected farms.