Factors associated with severity of naturally occurring piscirickettsiosis in netpen- and tank-reared juvenile Atlantic salmon at a research aquarium in western Canada.

The opportunistic examination of factors associated with an outbreak of piscirickettsiosis (SRS) is described in Atlantic salmon Salmo salar post-smolts held in an open netpen or in tanks supplied with raw sea water at a research aquarium in western Canada. During the outbreak, seawater temperature was significantly higher and salinity significantly lower in the netpen compared with the tanks. Mortality in the netpen began approximately 3 weeks prior to that in the tanks, and cumulative mortality in the netpen (34%) was significantly higher than in the tanks (12%). Piscirickettsia salmonis was confirmed by qPCR in tissues from moribund and dead fish and from colonies grown on enriched blood agar medium. Neither P. salmonis nor SRS were observed in salmon held concurrently in UV-irradiated sea water. The elevated mortality was curtailed by treatment with oxytetracycline. These observations further indicate warmer, less saline and periodically hypoxic seawater are risk factors for SRS. UV irradiation of sea water is shown to be a tool for SRS management in fish-holding facilities.

Caligus rogercresseyi infestation is associated with Piscirickettsia salmonis-attributed mortalities in farmed salmonids in Chile.

Caligidosis and Piscirickettsiosis are currently the most important sanitary challenges for the Chilean salmon industry. Caligidosis is caused by the ectoparasite, Caligus rogercresseyi and Piscirickettsiosis is caused by the intracellular bacterium, Piscirickettsia salmonis. Both diseases are highly prevalent and widely distributed in farming areas in Chile. The co-occurrence of the two diseases is frequently reported on salmon farms. However, there is little epidemiological evidence as to whether these two diseases are associated and generate interactive effects. This study was undertaken to evaluate the potential effects of C. rogercresseyi infestation on P. salmonis-attributed mortalities in farmed salmonids in Chile. Using a linear regression model, the potential association between the mean abundance of adult C. rogercresseyi in a period of 10 weeks and Piscirickettsiosis cumulative mortalities observed in the following 10 weeks was evaluated, while controlling for important confounders. These two 10-week windows were set around the time-point at which Piscirickettsiosis weekly mortality exceeded 0.1% for the first time in a production cycle. We found that the mean abundance of adult C. rogercresseyi was significantly associated with the Piscirickettsiosis cumulative mortality, suggesting the two diseases have a synergistic relationship. This relationship was of the same intensity in Atlantic salmon and rainbow trout. Our findings highlight the importance of taking effective control measures for C. rogercresseyi as a part of the strategies in place to reduce P. salmonis-attributed mortalities on salmon farms in Chile.