Isolation and characterization of Lactococcus garvieae from rainbow trout, Onchorhyncus mykiss, from California, USA.

Authors: SHAHIN, KHALID - University Of California, Davis; VEEK, TRESA - California Department Of Fish & Game; HECKMAN, TAYLOR - University Of California, Davis; LITTMAN, ERIC - University Of California, Davis; MUKKATIRA, KAVERAMMA - California Department Of Fish & Game; ADKINSON, MARK - California Department Of Fish & Game; Welch, Timothy; IMAI, DENISE - University Of California, Davis; PASTENKOS, GABRIELLE - University Of California, Davis; CAMUS, ALVIN - University Of Georgia; SOTO, ESTEBAN - University Of California, Davis

Submitted to: Transboundary and Emerging Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2021
Publication Date: 7/30/2021
Citation: Shahin, K., Veek, T., Heckman, T.I., Littman, E., Mukkatira, K., Adkinson, M., Welch, T.J., Imai, D.M., Pastenkos, G., Camus, A., Soto, E. 2021. Isolation and characterization of Lactococcus garvieae from rainbow trout, Onchorhyncus mykiss, from California, USA. Transboundary and Emerging Diseases. https://doi.org/10.1111/tbed.14250.
DOI: https://doi.org/10.1111/tbed.14250

Interpretive Summary: Lactococcus garvieae is a commercially important pathogen in aquaculture as the causative agent of lactococcosis, a devastating disease that can cause high levels of loss in cultured rainbow trout. In the summer of 2020 outbreaks of lactococcosis were discovered at four rainbow trout aquaculture facilities in California. Experimental infection studies verified the virulence of the CA outbreak strains and formally confirmed causation of the outbreaks. Molecular analysis provided evidence that the bacterial strains found at the four outbreak sites were identical suggesting that the outbreaks may have originated from a common source. In addition, the CA outbreak strains were similar to other L. garvieae isolates associated with disease outbreaks in trout. Further work characterizing the ability of these strains to form biofilms and their sensitivity to disinfectants was performed to support disinfection efforts. Future research is needed to better understand the emergence of this pathogen and to develop prophylactic and therapeutic protocols for its control.

Technical Abstract: Lactococcus garvieae is an emergent bacterial pathogen of salmonid fish in North America that causes acute infections particularly at water temperatures above 15'C. During 2020, L. garvieae was detected in rainbow trout, Onchorhyncus mykiss, cultured in southern California and the Eastern Sierras. Infected fish exhibited high mortalities and non-specific clinical signs of lethargy, erratic swimming, dark skin pigmentation, and exophthalmia. Macroscopic changes included external and internal hemorrhages, mainly in the eyes, liver, coelomic fat, intestine and brain. Histological examination revealed splenitis, branchitis, panophthalmitis, hepatitis, enteritis, and coelomitis with variable degrees of tissue damage among evaluated fish. Pure colonies of L. garvieae were isolated from infected trout and specific PCR primers for L. garvieae confirmed the preliminary diagnosis. Multi-locus sequence analysis showed that the strains recovered from diseased trout represent a novel genetic group. Isolates were able to form biofilms within 24h that increased their resistance to disinfection by hydrogen peroxide. Laboratory challenge methods for inducing lactococcosis in steelhead trout, O. mykiss, were evaluated by intracoelomic injection with serial dilutions of L. garvieae. The LD50 21 days post challenge was ~ 20 colony-forming units/fish. Experimentally infected trout presented similar clinical signs, gross changes, and microscopic lesions as those with natural disease fulfilling Koch’s postulates and demonstrating the high virulence of the recovered strains.