Lactococcosis in farmed rainbow trout

Heindrich Snyman, Véronique LePage, Calvin Kellendonk, Patricia Bell-Rogers, Qiumei You, Lisa, Ledger, Jason Eidt, Nathan Bennoit, Hugh Cai

Animal Health Laboratory (Snyman, Kellendonk, Bell-Rogers, You, Ledger, Eidt, Bennoit, Cai), LePage Aquatic Veterinary Services (LePage) 

AHL Newsletter, 2020;24(4):15.

Over the late summer, a commercial grow-out rainbow trout aquaculture operation had been experiencing slow but sustained daily mortalities.  Both regular and triploid trout strains were used, and fish were routinely raised in separate open water cages.  Average water temperatures over the period ranged from 17 to 20°C.  Mortalities progressively increased, peaking when water temperatures exceeded 20°C and resulting in an overall cumulative mortality rate ranging between ~ 30 to 85%, depending on age group and type (regular vs. triploid trout).  During the peak of mortality, on-farm postmortems were performed and a set of representative tissue samples and whole dead fish were submitted to the Animal Health Laboratory for further analysis.  Furunculosis, a low-grade bacterial infection caused by Aeromonas salmonicida, was initially suspected.  Tissues were collected for histopathology and bacterial culture, and furunculosis was ruled out through negative A. salmonicida PCR testing of pooled organ samples.

Mortalities tended to sink in the raceways rather than float, making it difficult to retrieve fresh dead fish, and along with the accompanying warm water temperatures resulted in advanced autolysis.  Nonetheless, histologically there was mild mixed histiocytic and lymphocytic inflammation within the periocular adipose tissue, angles of the cornea, and along the choroidal layer and vascular rete of the eyes (peri- and endophthalmitis).  One eye was partially collapsed with scattered fibrin thrombi within the ocular vascular rete.  There were scattered dense colonies of gram-positive coccoid bacteria within these thrombi, as well as within the foci of intraocular and periocular inflammation (Fig. 1).  Aerobic culture of spleens, kidneys, and gill surface swabs all yielded heavy pure cultures of Lactococcus garvieae, consistent with the observed gram-positive cocci.  Ongoing on-farm monitoring and sampling of mortalities consistently retrieved heavy pure cultures of L. garvieae in subsequent submissions.

Lactococcus garvieae causes a hyperacute hemorrhagic septicemic syndrome in fish called lactococcosis. The disease typically occurs in production systems where water temperatures exceed 15°C.  Uni- or bilateral exophthalmia is commonly observed (Fig. 2), along with ascites and widespread visceral organ congestion and hemorrhage.  Numerous freshwater and marine species of commercial interest are affected, but it is a particularly important emerging disease in fresh water rainbow trout aquaculture where it can lead to significant economic losses.  Over the past summer, there have been large outbreaks of lactococcosis in a number of freshwater hatcheries in Southern California, resulting in the destruction of very large numbers of fish (> 3 million) in an attempt to contain the disease.

This is the first documented outbreak of lactococcosis in Ontario farmed fish.  Rising ambient environmental temperatures could represent a potential risk factor for this disease in Ontario rainbow trout aquaculture.  Clinically, it was opted to treat some groups and not others in a strategic way depending on age, severity of disease, and impending harvest dates.  Treated fish were given oxytetracycline.  Mortalities subsided in all groups once temperatures decreased back to <18°C.

                

Figure 1. Periocular adipose tissue with lymphohistiocytic inflammation and abundant gram-positive cocci. (Gram stain) 

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Figure 2.  Rainbow trout exhibiting prominent bilateral exophthalmos which was particularly common during peak mortalities when water temperature exceeded 20°C.  Courtesy of Véronique LePage.

References
1. C.M. Meyburgh, R.R. Bragg, C.E. Boucher. Lactococcus garvieae: an emerging bacterial pathogen of fish. Dis Aquat Org 217;123:68-79.

2. California Department of Fish and Wildlife (CDFW)-FAQ for Lactococcus garvieae outbreak in Southern California fish hatcheries July 20, 2020  https://nrm.dfg.ca.gov/FileHandler.ashx?DocumentID=180707&inline