Horizontal transmission of deformed wing virus: pathological consequences in adult bees (Apis mellifera) depend on the transmission route

Recent reports on a steady decline of honeybee colonies in several parts of the world caused great concern. There is a consensus that pathogens are among the key players in this alarming demise of the most important commercial pollinator. One of the pathogens heavily implicated in colony losses is deformed wing virus (DWV). Overt DWV infections manifested as deformed-wing syndrome started to become a threat to honeybees only in the wake of the ectoparasitic mite Varroa destructor, which horizontally transmits DWV. However, a direct causal link between the virus and the symptom 'wing deformity' has not been established yet. To evaluate the impact of different horizontal transmission routes, and especially the role of the mite in the development of overt DWV infections, we performed laboratory infection assays with pupae and adult bees. We could demonstrate that pupae injected with DWV dose-dependently developed overt infections characterized by deformed wings in adult bees, suggesting that DWV, if transmitted to pupae by the mite, is the causative agent of the deformed-wing syndrome. The OID50 (overt infection dosage) was approximately 2500 genome equivalents. Injecting more than 1x10(7) DWV genome equivalents into adult bees also resulted in overt infections while the same viral dosage fed to adult bees only resulted in covert infections. Therefore, both infection of adult bees through DWV-transmitting phoretic mites and infection of nurse bees through their cannibalizing DWV-infected pupae might represent possible horizontal transmission routes of DWV.