Abstract
Lactococcus garvieae is a Gram-positive bacterium and one of the most important emergent pathogens of wild and cultured salmonids in the USA.1-3 Infection by L. garvieae is particularly prevalent in salmonids at water temperatures above 15°C.1 Since 2017, outbreaks of lactococcosis have occurred in at least three states causing significant economic losses to conservation programs and private aquaculture operations. A lack of efficacious vaccines, mounting antimicrobial resistance, and limited treatment options have led to mortalities approaching 90% in some epizootics. In CA, significant outbreaks occurred on farms in the Eastern Sierras and Southern CA with mortality reaching 75%. Medicated feed failed to control the outbreaks, and in some facilities, depopulation resulted in euthanasia of over 3 million fish.3 Isolates recovered from different outbreaks in CA in 2020 formed a unique genotype, suggesting the emergence of a virulent local bacterial strain. To gain a better understanding of the pathogenesis of lactococcosis in rainbow trout (Oncorhynchus mykiss), and to identify the experimental route of infection that closely mimics natural disease, fingerlings were challenged with L. garvieae utilizing different delivery methods and conditions. In the first trial, trout maintained at 13 or 18°C were challenged by intracoelomic (IC) injection with 102, 104 and 106 colony-forming units (CFUs) and monitored for 21 days. The fish challenged at 18°C exhibited a similar clinical presentation to those affected in state hatcheries and aquaculture farms, including abnormal orientation and/or buoyancy, emaciation, coelomic distension, exophthalmos, cutaneous erythema, and ulcerated skin. Internal lesions included hemorrhagic coelomic effusion, coelomic fat erythema and petechial hemorrhage in multiple organs. Trout exposed to L. garvieae at 13°C experienced mortalities of 7%, 7% and 0%, and based on posterior kidney culture, a bacterial persistence of 0%, 20% and 0%, respectively, when exposed to 102, 104 and 106 CFU/fish. In contrast, fish exposed to the same numbers of bacteria experienced mortalities of 60%, 84% and 91%, and bacterial persistence of 50%, 67% and 60% in kidneys of survivors, respectively, when challenged at 18°C, confirming the important role of temperature in the pathogenesis of lactococcosis in trout. In the second trial, the susceptibility of white sturgeon (Acipenser transmontanus), koi (Cyprinus carpio), Nile tilapia (Oreochromis niloticus), Chinook salmon (Oncorhynchus tshawytscha), and trout to 104 L. garvieae CFUs was compared using IC challenges. Twenty-one days post-challenge (DPC), a cumulative percent mortality of 96% and 56% was detected in trout and salmon, respectively. No mortality occurred in other fish. Additionally, 17% of surviving salmon remained persistently infected. While no bacteria were recovered from challenged sturgeon, koi and tilapia 21 DPC, cohabitation challenges lasting 21 days using surviving fish resulted in 6% mortality of naïve trout housed with tilapia, demonstrating that non-salmonids can become sub-clinical shedders of this pathogen. The data obtained in this study confirms the virulent nature of the emerging L. garvieae isolates recovered in CA in 2020; and provides useful information regarding controlled experimental infections in salmonids that can be used in the development of prophylactic and therapeutic protocols to combat this important pathogen.
Acknowledgements
The authors wish to thank the Hubbs-SeaWorld Research Institute and the William R. Pritchard Veterinary Medical Teaching Hospital at the University of California-Davis for their support of the Aquatic Animal Health Fellowship. The authors acknowledge the support by the California Department of Fish and Wildlife, and the team of the Center for Aquatic Biology and Aquaculture at UC Davis for support with housing and care of fish used in this project.
*Presenting author
+Student presenter
Literature Cited
1. Vendrell D, Balcázar JL, Ruiz-Zarzuela I, de Blas I, Gironés O, Múzquiz JL. Lactococcus garvieae in fish: a review. Comp Immunol Microbiol Infect Dis. 2006; 29(4): 177–198.
2. Meyburgh CM, Bragg RR, Boucher CE. Lactococcus garvieae: an emerging bacterial pathogen of fish. Dis Aquat Org. 2017; 123: 67–79.
3. California Department of Fish and Wildlife. Bacterial Outbreak Forces Euthanization of Fish at Three Southern California Hatcheries. Online at: https://cdfgnews.wordpress.com/2020/07/20/bacterial-outbreak-forces-euthanization-of-fish-at-three-southern-california-hatcheries/. Accessed Nov 20, 2020.