Detection Of Lactococcus spp. in environmental samples and wild fish from Four Lakes in Southern California

Authors: ABRAHAM, TAYLOR - University Of California, Davis; YAZDI, ZEINAB - University Of California, Davis; LITTMAN, ERIC - University Of California, Davis; SHAHIN, KHALID - University Of California, Davis; HECKMAN, TAYLOR - University Of California, Davis; QUIJANO CARDÉ, EVA MARIS - University Of California, Davis; NGUYEN, DIEM - University Of California, Davis; HUA, RUIXUE - University Of California, Davis; SOTO, ESTEBAN - University Of California, Davis; ADKISON, MARK - California Department Of Fish & Game; VEEK, TRESA - California Department Of Fish & Game; MUKKATIRA, KAVERY - California Department Of Fish & Game; RICHEY, CHRISTINE - California Department Of Fish & Game; KWAK, KEVIN - California Department Of Fish & Game; MOHAMMED, HAITHAM - Assiut University; ORTEGA, CESAR - Universidad Autonoma Del Estado De Hidalgo; AVENDAÑO-HERRERA, RUBEN - Universidad Andres Bello (UNAB); HYATT, MICHAEL - Wildlife Conservation Society; KELEHER, WILLIAM - Kennebec River Biosciences; Welch, Timothy - Tim

Submitted to: Journal of Aquatic Animal Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2023
Publication Date: 9/25/2023
Citation: Abraham, T., Yazdi, Z., Littman, E., Shahin, K., Heckman, T.I., Quijano Cardé, E., Nguyen, D., Hua, R., Soto, E., Adkison, M., Veek, T., Mukkatira, K., Richey, C., Kwak, K., Mohammed, H., Ortega, C., Avendaño-Herrera, R., Hyatt, M.W., Keleher, W., Welch, T.J. 2023. Detection Of Lactococcus spp. in environmental samples and wild fish from Four Lakes in Southern California. Journal of Aquatic Animal Health. 35(3):129-198. https://doi.org/10.1002/aah.10188.
DOI: https://doi.org/10.1002/aah.10188

Interpretive Summary: Piscine lactococcosis is an emerging bacterial disease of fish that is caused by Lactococcus garvieae L. petauri, and L. formosensis. In 2020, outbreaks of Lactococcosis, caused by L. petauri, were detected in farmed rainbow trout at 3 California state fish hatcheries in southern California. In this study, we conducted surveillance for the etiological agents of piscine lactococcosis in the ecosystems near one of the California outbreak sites in order to better understand the distribution of these pathogens in the neighboring aquatic environments and hosts during this outbreak. L. garvieae was isolated from Largemouth bass from one of the four lakes near the outbreak site, however, these strains were not infectious when tested in laboratory challenges. DNA indicative of Lactococcus spp. was also detected in fish, water, or sediment samples in all 4 of the lakes sampled using a recently developed quantitative PCR assay. However, it was not possible to identify the Lactococcus species present in these samples. These studies failed to identify an environmental reservoir of pathogenic strains of Lactococcus at the California outbreak site. However, since the sampling was carried out at a time of year when the lakes were too warm to support stocked trout, we cannot discount the possibility that trout are a seasonal reservoir of this pathogen. This epidemiologic survey provides a better understanding of the environmental reservoirs of Lactococcus spp. which will aid in the development of control measures that help prevent future outbreaks.

Technical Abstract: Piscine lactococcosis is an emerging disease of salmonids in North America. While historically attributed solely to the bacteria L. garvieae, recent investigations have revealed that lactococcosis is also caused by the closely related species L. petauri and L. formosensis. These three species are indistinguishable by conventional diagnostic methods, leading to the misidentification of L. petauri or L. formosensis as L. garvieae. To gain an understanding of the presence of these bacteria in California, 359 fish and 161 environmental samples were collected from four lakes near two affected fish farms during an outbreak of piscine lactococcosis in California in 2020. Lactococcus garvieae was isolated from the brains of two largemouth bass (Micropterus salmoides) in one of the lakes by standard microbiological methods. Additionally, Lactococcus spp. were detected in 14 fish (8 bluegills and 6 largemouth bass) from 3 out of the 4 lakes using a recently developed quantitative PCR (qPCR) assay. Of the collected environmental samples, all 4 lakes tested positive for Lactococcus spp. in the soil samples, while 2 of the 4 lakes tested positive in the water samples through qPCR. The second objective of the study was to compare the virulence of the Lactococcus spp. in rainbow trout (Oncorhynchus mykiss) and largemouth bass. Treatment groups were injected with representative isolates of L. petauri (n=17), L. garvieae (n=2), or L. formosensis (n=4), and monitored for 14 days post-challenge (dpc). Challenged largemouth bass did not show any signs of infection post-injection throughout the challenge period. Rainbow trout infected with L. petauri showed clinical signs within 3 dpc and presented a significantly higher cumulative mortality (62.2%; p<0.0001) at 14 dpc when compared to L. garvieae (0%) and L. formosensis (10%) infected treatments. The current study demonstrates the potential use of the developed qPCR assay in detection of the agents of piscine lactococcosis in the environment, and its potential use as a tool for piscine lactococcosis surveillance in natural and commercial settings.