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

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.

Keywords: Lactococcus garvieae; aquaculture; brook trout; fish diseases; outbreak.

Figure 1

Clinical signs of lactococcosis observed in brook trout: (a) exophthalmos and haemorrhages in the periorbital and intraocular area; (b) ruptured globe; (c1) enlarged spleen (black arrow), hemorrhagic enteritis (white arrow), and (c2) diffused haemorrhages on liver.

Figure 2

Pulsed-field gel electrophoresis patterns of ApaI (A) and SmaI (B) digests of genomic DNA of Lactococcus garvieae clinical isolates. Lane 1 and 18, Salmonella serotype Branderup strain H9812; lanes 2–6 and lane 13, brook trout isolates from samples S-8, S-9, S-10, S-11 S-12, and S-7; lanes 7–12 and lanes 14–17, rainbow trout isolates of samples T-1, T-2, T-3, T-4, T-5, T-6, T-7, T-13, T-14, and T-15.