DWV-A lethal to honey bees (Apis mellifera) : a colony level survey of DWV variants (A, B & C) in England, Wales and 32 states across the US

The strong association between Varroa destructor and Deformed wing virus (DWV), and high overwintering colony losses (OCL) of honey bees is well established. Three DWV master variants: DWV-A, B and C, have been described. However, their various roles in colony mortality remains an open question. The aim of this study is to investigate the seasonal prevalence, viral load, and changing distribution of the three DWV master variants within honey bee colonies from England, Wales and 32 states across the US. Here we report that in 2016, DWV-B was the most prevalent (100%, n=249) and dominant (95%) strain in England and Wales compared to the US (56% n=217 and 23%, respectively) where DWV-A was the most prevalent (83%, n=217) and dominant (63%) strain. DWV-C was detected in low viral loads (<1E+07 genome equivalents per bee) and at lower prevalence compared to DWV A & B (58% in England and Wales, n=203, and 14% across the US, n=124). DWV-B prevalence and dominance in England and Wales coincided with low OCL (6%). Whilst, a 60% loss was reported by participating US beekeepers, where DWV-A prevalence (89%, n=18) and load were significantly (p = 0.002) higher (1E+08 – 1E+11) when compared to the colonies that survived (49% [n=27], 1E+06 – 1E+10). DWV-B had low prevalence (56%, n = 18) in the colonies that died with viral loads of <1E+10. DWV-B was routinely detected in high viral loads (<1E+10) in surviving colonies from all sample locations (100% prevalence), providing evidence that DWV-A appears to exhibit increased virulence over DWV-B at the colony and population level