The epidemiology and laboratory investigation of ulcer disease in Atlantic salmon (salmo salar) in Atlantic Canada

The aims of this research were to gain a better understanding of the epidemiology of ulcer disease affecting Atlantic salmon (Salmo salar) in Atlantic Canada and devise management recommendations for ulcer disease outbreaks.\nIn Chapter 2, we investigated the epidemiology of ulcer disease in Atlantic Canada. The overall prevalence of ulcer disease within 29 Atlantic Canadian salmon farms, from 2014 to 2016, was 41%. The descriptive epidemiology suggests cages have a point source exposure to the causative agent, with non-uniform exposure across farms, and a low likelihood for horizontal transmission of the pathogen between cages and farms. Ulcer disease outbreaks also exhibit a seasonal pattern, with disease onset occurring from late-summer to mid-autumn. Our study did not identify any management factors that significantly reduced the severity of ulcer disease outbreaks on affected farms.\nIn Chapter 3, we conducted an experiment to 1) describe the pathology of ulcer disease in Atlantic salmon by an Atlantic Canadian Moritella viscosa isolate, and 2) determine if M. viscosa was easily transmitted horizontally in Atlantic salmon. Atlantic salmon were bath-challenged in 11°C salt water containing M. viscosa. After observing early skin lesions in these fish, we diverted the water from each bath-challenge tank to an adjacent tank of previously unexposed fish for the remainder of the experiment. Our initial bath-challenge fish expressed clinical signs of ulcer disease within four days of exposure, but we observed no clinical signs of disease in fish within the “delayed-challenge” tanks. Our results suggest ulcer disease induced with an Atlantic Canadian strain of M. viscosa, at an elevated temperature, produced pathology similar to winter ulcer disease and this organism is not highly transmissible between fish. Hypernatremia, hyperchloremia, and hypoproteinemia in fish with skin lesions were suggestive of osmoregulatory issues. The majority of fish sampled in our bath-challenge tanks did not have systemic infections until they developed visible skin ulcerations. We also found the more severe the skin lesions associated with M. viscosa, the less likely the fish had digesta present in their gastrointestinal tracts.\nIn Chapter 4, we performed an experiment to determine if extracellular products (ECP) from an Atlantic Canadian strain of M. viscosa could induce a tissue response similar to that observed with M. viscosa infections in Atlantic salmon. We injected fish subcutaneously with ECP filtrate broth. All experimental fish expressed early lesions, with five fish (out of 60) developing skin ulcers. Our results suggest the ECP of M. viscosa produced a tissue response similar to ulcer disease that was described in bath-challenges with M. viscosa; however, the tissue response in our ECP-injected fish was more acute, with melanization of the skin at the injection site. The majority of fish with skin lesions had normal serum sodium, chloride, total protein, and osmolality levels, and elevated creatine kinase levels. This is suggestive of acute skeletal muscle damage. We also found the more severe the skin lesions associated with the ECP, the less likely the fish had digesta present in their gastrointestinal tracts.

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