A Summer Mortality Outbreak of Lactococcosis by Lactococcus garvieae in a Raceway System Affecting Farmed Rainbow Trout (Oncorhynchus mykiss) and Brook Trout (Salvelinus fontinalis)

Simple SummaryLactococcus garvieae is the etiological agent of lactococcosis, a bacterial disease affecting many species of fish and causing major economic losses in aquaculture. In this study we described, for the first time, the isolation of L. garvieae in brook trout farmed in northwestern Italy by performing a molecular and epidemiological characterization. Results confirmed water as vehicle of infection, favoring the transmission of the pathogen between rainbow trout farmed in the upstream compartments of a raceways system and the brook trout located in downstream tanks. Abstract Lactococcosis is a fish disease of major concern in Mediterranean countries caused by Lactococcus garvieae. The most susceptible species is the rainbow trout (Oncorhynchus mykiss), suffering acute disease associated with elevated mortalities compared to other fish species. References reported that other salmonids are also susceptible to the disease, but no mortality outbreak has been described to date. The aim of this study was to present a mortality outbreak that occurred in brook trout (Salvelinus fontinalis) farmed in northwestern Italy during the summer of 2018. Fish exhibited clinical signs, such as exophthalmos, diffused hemorrhages localized in the ocular zone, hemorrhagic enteritis, and enlarged spleen. L. garvieae was isolated in all fish. Molecular and epidemiological characterization of the isolates, through Pulsed Field Gel Electrophoresis (PFGE), confirmed the initial hypothesis of water as vehicle of infection favoring transmission between rainbow trout farmed in upstream compartments and brook trout located in downstream tanks. Moreover, several environmental conditions affected and promoted the outbreak, among them the high-water temperature, which probably induced a physiological stress in brook trout, being way above the optimal temperature for this species, increasing the susceptibility to infection.