Investigation of Melissococcus plutonius isolates from 3 outbreaks of European foulbrood disease in commercial beekeeping operations in western Canada

European foulbrood (EFB) disease is an economically important bacterial disease of honey bee larvae caused by enteric infection with Melissococcus plutonius. In this study, we investigated 3 clinical outbreaks of EFB disease in commercial beekeeping operations in western Canada in the summer of 2020 and characterized the Melissococcus plutonius isolates cultured from these outbreaks according to genetic multi-locus sequence type and in vitro larval pathogenicity. We isolated M. plutonius sequence type 19 from EFB outbreaks in British Columbia and Alberta, and a novel M. plutonius sequence type 36 from an EFB outbreak in Saskatchewan. In vitro larval infection with each M. plutonius isolate was associated with decreased larval survival in vitro by 58.3 to 70.8% (P < 0.001) compared to non-infected controls. Further elucidation of mechanisms of virulence of M. plutonius, paired with epidemiologic investigation, is imperative to improve EFB management strategies and mitigate risks of EFB outbreaks in western Canada.