Identification of Streptococcus phocae strains associated with mortality of Atlantic salmon (Salmo salar) farmed at low temperature in Chile

This work describes the identification of the causal agent of disease outbreaks that occurred in Atlantic salmon (700-750g) cage-cultured at water temperature of 5±0.5°C in a farm located around the city of Puerto Natales in the Antarctic region of Chile, reaching a cumulative mortality lower than 1% of the affected population. Infections most often were characterized by exophthalmia with accumulation of purulent and haemorrhagic fluid around eyes and ventral petechial haemorrhages. Abundant pure growth cultures from kidney samples and mixed cultures from external lesions were obtained. Using biochemical, sequencing techniques six different isolates were studied and two bacterial species were identified, corresponding to Streptococcus phocae and Aliivibrio sp. Challenge tests by intraperitoneal injection showed that a representative S. phocae isolate 151 is pathogenic for Atlantic salmon. No mortalities or morphological alterations were observed in fish injected with Aliivibrio sp. (isolate 222). Infections of Atlantic salmon with S. phocae have been restricted to temperatures higher than 15^SC; however our data represent the first report of S. phocae associated with mortality of salmonids farmed in cold water (5°C). Additional studies are necessary to evaluate the risk for salmonid cultured below 10^°C, particularly because the prevalence of infections by S. phocae could be underrated due to their possible confusion with other bacterial infections.