Characterization of Melissococcus plutonius persistence in adult worker honeybees

European foulbrood (EFB) is a reemerging bacterial disease of honey bees caused by Melissococcus plutonius. In western Canada, increased incidence of EFB could be potentiated by genetic determinants of pathogenicity, antimicrobial resistance, or impaired regime and dosage of antibiotics. M. plutonius infects honeybee larvae, often resulting in larval starvation and death, particularly when associated with colony stress. Infected larvae that survive, become asymptomatic carriers of M. plutonius and may facilitate intra and inter colony transmission of bacteria. However, the persistence of M. plutonius in the digestive tract of adult workers has not been determined. A pilot study of overwintering adult worker honeybees infected with low (107 CFU), medium (109 CFU), and high (1011 CFU) doses of M. plutonius found that the infected bees remained carriers of M. plutonius for one week based on quantitative PCR (qPCR) and standard plate counts. Studies are currently underway to replicate this experiment using newly emerged summer worker bees which will be infected with the same doses of M. plutonius as above and maintained in laboratory cages and/or paint-marked and subsequently introduced into 4 experimental field colonies. The bacterial load of honeybees from infected and control groups will be monitored once every 5 days for six weeks. M. plutonius contamination of hive matrices and non-experimental bees within these field colonies will also be sampled to evaluate transmission M. plutonius within a colony. Results of this study are anticipated to inform and optimize integrated pest management and antimicrobial control strategies for EFB.