The Two Prevalent Genotypes of an Emerging Infectious Disease, Deformed Wing Virus, Cause Equally Low Pupal Mortality and Equally High Wing Deformities in Host Honey Bees

Abstract

Deformed wing virus (DWV) is an emerging infectious disease of the honey bee (Apis mellifera) that is considered a major cause of elevated losses of honey bee colonies. DWV comprises two widespread genotypes: the originally described genotype A, and genotype B. In adult honey bees, DWV-B has been shown to be more virulent than DWV-A. However, their comparative effects on earlier host developmental stages are unknown. Here, we experimentally inoculated honey bee pupae and tested for the relative impact of DWV-A versus DWV-B on mortality and wing deformities in eclosing adults. DWV-A and DWV-B caused similar, and only slightly elevated, pupal mortality (mean 18% greater mortality than control). Both genotypes caused similarly high wing deformities in eclosing adults (mean 60% greater wing deformities than control). Viral titer was high in all of the experimentally inoculated eclosing adults, and was independent of wing deformities, suggesting that the phenotype ‘deformed wings’ is not directly related to viral titer or viral genotype. These viral traits favor the emergence of both genotypes of DWV by not limiting the reproduction of its vector, the ectoparasitic Varroa destructor mite, in infected pupae, and thereby facilitating the spread of DWV in honey bees infested by the mite.

Outcomes of honeybee pupae inoculated with deformed wing virus genotypes A and B

“…The only characteristic clearly distinguishing the DWV variants was the higher viral load of DWV-B than DWV-A in the heads of inoculated bees ( Figure 6). This supports previous conclusions that DWV-B is better able to replicate in honeybees at higher levels through the mite transmission route (Tehel et al 2019). Nevertheless, our results suggested that there is no correlation between the efficiency of viral replication (Figure 7a) and the capability of DWV-A to efficiently trigger wing deformities in inoculated pupae (Figure 8b).…”

“…They reported that an injection of 10 4 DWV equivalent genome induces deformed wings in about 50% of emerging bees. We found that this dose led to higher rates (67 to 74%) and was comparable to the 60% elevation in wing deformities reported by Tehel et al (Tehel et al 2019). This difference may be due to biological reasons such as the DWV strain used or to differences in the honeybee's tolerance to the infection.…”

“…In this study, we performed experimental inoculation of honeybee pupae with viral suspensions prepared from naturally infected adult honeybee heads in order to evaluate the virulence of DWV-A and DWV-B according to three criteria: rate of mortality, rate of wing deformities, and viral load in the head of inoculated honeybees. Our results are also discussed in the light of two recent studies on DWV inoculation of pupae (Remnant et al 2019;Tehel et al 2019). Notably, our results support the previous conclusion that DWV variants induce no significant increase in pupae death compared to control pupae.…”

“…Our observation supports a previous study reporting bees with deformed wings in colonies from V. destructor -free areas (Forsgren et al 2012), even though the suggested source of stress (subnormal temperatures or genetic predispositions) differed from a needle injection. Tehel et al who experimented with DWV inoculation of pupae, also found control bees with wing deformities despite no significant increase in DWV loads (Tehel et al 2019). Moreover, we found that DWV-A and DWV-B were circulating in our winter apiary colonies despite no mites being detected (Table II).…”

“…The results reported in this table were calculated taking into account the dilution factors that were used to adjust the main DWV variant at 10 4 copies/2 μl nd virus not detected in the viral suspension used to prepare the inoculum honeybee pupae when they were inoculated with bee extract containing high levels of SBV and BQCV, whereas pupae inoculated with high levels of DWV resulted in near 100% survival (Remnant et al 2019). Tehel et al found that both DWV-A and DWV-B caused slightly elevated pupal mortality (the mean mortality being 18% higher than for the controls) (Tehel et al 2019). Both studies support previous findings suggesting that the V. destructor mite could affect negatively the prevalence of highly virulent viruses (Martin et al 2012;Mondet et al 2014).…”