Deformed wing virus prevalence and load in honeybees in South Africa

Abstract

Deformed wing virus (DWV) is an emerging honeybee pathogen that has appeared across the globe in the past 40 years. When transmitted by the parasitic varroa mite, it has been associated with the collapse of millions of colonies throughout the Northern Hemisphere. However, despite the presence of the mite in the Southern Hemisphere, infested colonies survive. This study investigated the prevalence of DWV genotypes A, B and C along with their viral loads in South Africa and compared the findings with recent data from Brazil, the UK and the USA. We found that DWV-B was the most prevalent genotype throughout South Africa, although the total DWV viral load was significantly lower (2.8E+07) than found in the Northern Hemisphere (2.8E+07 vs. 2.7E+10, p > 0.00001) and not significantly different to that found in Brazil (5E+06, p = 0.13). The differences in viral load can be explained by the mite resistance in Brazil and South Africa, since mite-infested cells containing high viral loads are removed by the bees, thus lowering the colony's viral burden. This behaviour is much less developed in the vast majority of honeybees in the Northern Hemisphere.

Ten Years of Deformed Wing Virus (DWV) in Hawaiian Honey Bees (Apis mellifera), the Dominant DWV-A Variant Is Potentially Being Replaced by Variants with a DWV-B Coding Sequence

“…Ultimately, this study has shown that since 2010 when DWV-B was not detected, the viral load and prevalence of DWV-B have increased to the point at which DWV-B now dominates colonies found on Big Island and co-dominates with DWV-A on Oahu. Thus far, this increase in DWV-B fits with what has been observed in numerous other regions ( Figure 2 ) [ 16 , 20 , 30 ]. We know that DWV-B replicates to greater titers than DWV-A when injected into pupae [ 47 , 48 ] whilst being equally [ 48 ] or less virulent [ 49 ].…”

“…as other studies using the same methodology have found a reduced DWV burden in resistant, not treated, managed populations in South Africa and Brazil [30]. [9], 2012 [29], 2012 * [17], 2015/16 [26] and 2019 (this study).…”

“…The managed untreated colonies had similar DWV-A levels to treated colonies, which were also significantly greater than levels in feral colonies. This is unexpected as other studies using the same methodology have found a reduced DWV burden in resistant, not treated, managed populations in South Africa and Brazil [ 30 ].…”

“…Our results indicate a large proportion of RdRp sequences now contain those that match the DWV-B variant. This suggests that the Hawaiian Islands of Oahu and Big Island are transitioning from DWV-A to DWV-B dominance, mirroring that observed in the UK, USA, Europe, South Africa (Figure 2) [16,20,[28][29][30][31][32]. However, to confirm this would require future studies analyzing the full genome sequence of past and present samples from each island.…”

“…[ 37 ] only detected DWV-B within 3 of their 27 honey bee samples from Brazil. Whereas, in South Africa, DWV-B appeared to dominate from the mite’s introduction in 1997 or shortly afterward [ 30 ]. The median viral genome equivalent of DWV-A is similar on Oahu and Big Island, but the median viral genome equivalent of DWV-B on Oahu is half the value on Big Island ( Table 1 ).…”