Co-infection with two iflaviruses (deformed wing virus and sacbrood virus) affects viral and immune dynamics and synergistically increases honey bee mortality

The honey bee microbiome includes a wide variety of viruses. While most of them usually remain commensal, some can become pathogenic in specific contexts. Of these, one is that of deformed wing virus (DWV) and another, sacbrood virus (SBV). Although co-infection is the norm rather than the exception, most of the time these viruses have been studied independently. When investigated as co-infections, past studies have focused on their effects on the honey bee brood. In this study, we co-inoculated adult honey bees at emergence with DWV by injection and SBV orally (acting as the viral transmission by Varroa destructor and by trophallaxis or food, respectively), either simultaneously or sequentially. Using optical counters, we were able to track the survival and behaviour of these honey bees within colonies. Through regular in-hive sampling, we monitored the evolution of their viral loads as well as the expression of eight immune genes involved in honey bee anti-viral immunity. Here, we show that co-inoculations of DWV and SBV synergistically increase the virulence of DWV and conditionally promote the replication of both viruses. Finally, our results show that immune responses in adult honey bees depend on DWV genotypes and whether replication originates from a superinfecting virus or a virus already present in bees. Author SummaryHoney bees are highly social pollinators that live in crowded colonies. Their population density and the high frequency of interactions between individuals favours disease transmission and makes colonies susceptible to pathogen outbreaks. Many viruses commonly infect honey bees, however, they are often studied as single infections. As an effort to better understand interactions between honey bees and multiple viral populations, we co-inoculated young bees with two common honey bee viruses (deformed wing virus and sacbrood virus), released them in colonies and monitored their health and behaviour. Our findings show evidence of synergies between both viruses, as we show that a virus seemingly harmless for adult bees (sacbrood virus) may actually increase the virulence of another virus (deformed wing virus). These results highlight the importance of monitoring and studying multiple pathogens at once for a better understanding of the threat they represent to colony health and survival.