Co-inoculation with deformed wing virus and sacbrood virus affects viral and immune dynamics and synergistically increases honey bee mortality.

Honey bees are commonly infected with viruses, including deformed wing virus (DWV-A and DWV-B) and sacbrood virus (SBV), which cause morphological symptoms and death in developing bees and primarily asymptomatic infections in adult bees. Co-infections occur regularly in colonies, but they have rarely been studied, especially in adult bees. In this study, we co-inoculated young adult honey bees with DWV by injection (simulating vectored transmission by Varroa) and feeding them with SBV (simulating oral transmission) before reintroducing them in colonies. Through the use of optical counters and regular sampling, we tracked their survival and behaviour, and quantified the dynamics of viral loads in treated bees 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 SBV. We also show that SBV may play a role in the replication of DWV in specific contexts. Finally, our results show that immune responses in adult honey bees depend on virus genotype (i.e., DWV), their relative abundance and the pre-existing natural infections before virus injection). Together, these results confirm the existence of deleterious interactions between deformed wing virus and sacbrood virus, impacting honey bee health and colony dynamics.