Deformed wing virus (DWV) is a honeybee viral pathogen either persisting as an inapparent infection or resulting in wing deformity. The occurrence of deformity is associated with the transmission of DWV through Varroa destructor during pupal stages. Such infections with DWV add to the pathology of V. destructor and play a major role in colony collapse in the course of varroosis. Using a recently developed RT-PCR protocol for the detection of DWV, individual bees and mites originating from hives differing in Varroa infestation levels and the occurrence of crippled bees were analysed. It was found that 100 % of both crippled and asymptomatic bees were positive for DWV. However, a significant difference in the spatial distribution of DWV between asymptomatic and crippled bees could be demonstrated: when analysing head, thorax and abdomen of crippled bees, all body parts were always strongly positive for viral sequences. In contrast, for asymptomatic bees viral sequences could be detected in RNA extracted from the thorax and/or abdomen but never in RNA extracted from the head. DWV replication was demonstrated in almost all DWV-positive body parts of infected bees. Analysing individual mites for the presence of DWV revealed that the percentage of DWV-positive mites differed between mite populations. In addition, it was demonstrated that DWV was able to replicate in some but not all mites. Interestingly, virus replication in mites was correlated with wing deformity. DWV was also detected in the larval food, implicating that in addition to transmission by V. destructor DWV is also transmitted by feeding.
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Deformed wing virus (DWV) is a viral pathogen of the European honeybee (Apis mellifera), associated with clinical symptoms and colony collapse when transmitted by the ectoparasitic mite Varroa destructor. In the absence of V. destructor, DWV infection does not result in visible symptoms, suggesting that mite-independent transmission results in covert infections. True covert infections are a known infection strategy for insect viruses, resulting in long-term persistence of the virus in the population. They are characterized by the absence of disease symptoms in the presence of the virus and by vertical transmission of the virus. To demonstrate vertical transmission and, hence, true covert infections for DWV, a detailed study was performed on the vertical-transmission routes of DWV. In total, 192 unfertilized eggs originating from eight virgin queens, and the same number of fertilized eggs from the same queens after artificial insemination with DWV-negative (three queens) or DWV-positive (five queens) semen, were analysed individually. The F0 queens and drones and F1 drones and workers were also analysed for viral RNA. By in situ hybridization, viral sequences were detected in the ovary of an F0 queen that had laid DWV-positive unfertilized eggs and was inseminated with DWV-positive semen. In conclusion, vertical transmission of DWV from queens and drones to drone and worker offspring through unfertilized and fertilized eggs, respectively, was demonstrated. Viral sequences in fertilized eggs can originate from the queen, as well as from drones via DWV-positive semen.
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