Direct Evidence for Infection of Varroa destructor Mites with the Bee-Pathogenic Deformed Wing Virus Variant B, but Not Variant A, via Fluorescence In Situ Hybridization Analysis

Deformed wing virus (DWV) is a bee-pathogenic, single- and positive-stranded RNA virus that has been involved in severe honey bee colony losses worldwide. DWV, when transmitted horizontally or vertically from bee to bee, causes mainly covert infections not associated with any visible symptoms or damage. Overt infections occur after vectorial transmission of DWV to the developing bee pupae through the ectoparasitic mite Varroa destructor. Symptoms of overt infections are pupal death, bees emerging with deformed wings and shortened abdomens, or cognitive impairment due to brain infection. So far, three variants of DWV, DWV-A, DWV-B, and DWV-C, have been described. While it is widely accepted that V. destructor acts as vector of DWV, the question of whether the mite only functions as a mechanical vector or whether DWV can infect the mite, thus using it as a biological vector, is hotly debated because in the literature data can be found that support both hypotheses. In order to settle this scientific dispute, we analyzed putatively DWV-infected mites with a newly established protocol for fluorescence in situ hybridization of mites and demonstrated DWV-specific signals inside mite cells. We provide compelling and direct evidence that DWV-B infects the intestinal epithelium and the salivary glands of V. destructor. In contrast, no evidence for DWV-A infecting mite cells was found. Our data are key to understanding the pathobiology of DWV, the mite’s role as a biological DWV vector, and the quasispecies dynamics of this RNA virus when switching between insect and arachnid host species. IMPORTANCE Deformed wing virus (DWV) is a bee-pathogenic, originally rather benign, single- and positive-stranded RNA virus. Only the vectorial transmission of this virus to honey bees by the ectoparasitic mite Varroa destructor leads to fatal or symptomatic infections of individuals, usually followed by collapse of the entire colony. Studies on whether the mite only acts as a mechanical virus vector or whether DWV can infect the mite and thus use it as a biological vector have led to disparate results. In our study using fluorescence in situ hybridization, we provide compelling and direct evidence that at least the DWV-B variant infects the gut epithelium and the salivary glands of V. destructor. Hence, the host range of DWV includes both bees (Insecta) and mites (Arachnida). Our data contribute to a better understanding of the triangular relationship between honey bees, V. destructor, and DWV and the evolution of virulence in this viral bee pathogen.