OAHN Aquatics Network Project: Baseline survey for Lactococcus garvieae group in Ontario salmonid facilities

Project Lead: Drs. Alexandra Reid and Claire Jardine

Collaborators: Marcia Chaisson, AHL molecular lab

Executive Summary

Piscine lactococcosis is a trio of related bacteria causing acute septicemia and meningoencephalitis in salmonids, considered an emerging disease in North America since an outbreak in 2020, as well as a zoonotic disease.  Testing identified Lactococcus petauri as the source of the explosive north American outbreak and vaccination started in Ontario using an autogenous bacterin in fish. In California, L. petauri was also detected in soil and asymptomatic wild fish species. Because of this, we decided to sample farmed or captive salmonids and the environment at different fish farms through the province, and all Ministry of Natural Resources (MNR) culture stations and substations. We also decided to validate and compare detections on Lactococcus selective media against conventional fish media. 11/14 detections were only on the Lactococcus selective media. While we detected a low number of positives the first year, the second year we not only expanded to include additional sites we detected a small outbreak of Lactococcus petauri, although producers did not report explosive mortality. We also detected L. garvieae, which was not known to be in Ontario. No MNR cultured fish were ever positive, but two environmental samples in two different culture stations were positive once each during the period of high disease activity in farmed fish. The second year, for personnel reasons, MNR fish were sampled at the Animal Health Laboratory and environmental samples were discontinued. Genetically, almost all strains detected were unique and tended not to be found serially, which suggests that treatment or other factors are removing the pathogen and we did not find evidence of persistence in the environment. The only related strains were detected in two farms with shared management practices. This suggests that biosecurity, including vaccination, is a strong tool to prevent piscine lactococcosis in Ontario. We strongly recommend culturing fish samples during the warmer water temperatures onto Lactococcus specific media to ensure piscine lactococcosis can be detected.