Nancy and Pete B. Thanks for the kind feedback. The feeling is mutual- I
enjoy the informed discussion on emerging scholarship and appreciate
everyone who contributes.
Along those lines, what do you all make of the global rise of DWV-B? While
there is too much to unpack in one post, it seems plain that DWV-B is
rapidly replacing DWV-A as the dominant variant in apiaries around the
world.
Two papers that I’m aware of that have tried to broach this topic are by the
Norton et al team:
In Accumulation and Competition Amongst Deformed Wing Virus Genotypes in
Naïve Australian Honeybees Provides Insight Into the Increasing Global
Prevalence of Genotype B
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full> they
observe in part:
An immediate effect of V. destructor appears to be a reduction in the
genetic diversity of DWV in honeybees both in the field (Martin et al., 2012
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full#B44> )
and in experiments using injection of DWV to mimic vector transmission
(Ryabov et al., 2014
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full#B67> ).
Over time, the distribution of DWV genotypes changes so that one DWV
genotype prevails within honeybee populations. DWV-B has become the most
common variant in the United Kingdom (UK) and Europe (McMahon et al., 2016
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full#B46> ;
Kevill et al., 2019
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full#B32> ;
Manley et al., 2019
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full#B39> ).
In North-America DWV-A remains the most common genotype (Ryabov et al., 2017
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full#B65> ;
Kevill et al., 2019
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full#B32> ).
However, Ryabov et al. (2017)
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full#B65>
found that DWV-B prevalence in the United States increased from 3% in 2010
to 65% in 2016. Similarly, Kevill et al. (2019)
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full#B32>
found that DWV-B was prevalent in 56% of tested colonies in 2016, and the
dominant genotype in 23% of those colonies. Kevill et al. (2019)
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full#B32>
predicted that DWV-B prevalence will continue to increase and supersede
DWV-A with time, as observed in England and Wales. Such change in relative
prevalence suggests that the different DWV genotypes compete within their
host. The increased prevalence of DWV-B may potentially be explained by
differences in replication rate within the host, difference in virulence and
associated host mortality, or a combination of both.
They then introduce the logic behind testing varroa-naïve bees for the
purpose of attempting to evaluate the comparative virulence of DWV-B versus
DWV-A (and presage what may be in-store for Australian beekeepers):
Clearly while the global association between V. destructor and DWV seems
irrefutable, determining whether virulence differences exist between DWV
genotypes remains a challenge... Australian honeybees are naïve to both V.
destructor and DWV and are therefore an ideal model to determine the
dynamics between different DWV genotypes. We infected white-eyed pupae to
reflect the life stage at which V. destructor first vectors DWV to honeybees
(Bailey and Ball, 1991
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full#B2> ).
We infected pupae by injecting either a single DWV genotype or two genotypes
(co-infection). Co-infection allowed us to determine the extent to which
different DWV genotypes compete within the same host. We further determined
if there is a relationship between viral load and host damage (mortality).
From this study (and validation of previous studies) they conclude in part
that:
... DWV-B reaches higher viral loads than DWV-A after injection. Similarly
to our study, in English and Welsh colonies (Kevill et al., 2019
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full#B32> )
and in the United States (Ryabov et al., 2017
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full#B65> ;
Kevill et al., 2019
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full#B32> ),
mean DWV-B loads were approximately sevenfold higher than DWV-A when
colonies contain both genotypes. Yet there are exceptions. In some
co-infected colonies in the United States that died over winter, Kevill et
al. (2019)
<https://www.frontiersin.org/articles/10.3389/fmicb.2020.00620/full#B32>
found significantly higher DWV-A loads relative to DWV-B.
Our data provide some explanation for the continued global increase in
prevalence of DWV-B over DWV-A. Low mortality in pupae and the ability of
DWV-B to accumulate to higher loads relative to DWV-A, even during
co-infection, are likely to be contributing factors to the increasing
prevalence of DWV-B.
In a follow-up paper entitled, Adaptation to vector- based transmission in a
honeybee virus
<https://www.researchgate.net/publication/350826765_Adaptation_to_vector-bas
ed_transmission_in_a_honey_bee_virus> they provide a lot of data and
research on this phenomenon but to this dumb country boy the most
interesting conclusion from the study is the following:
Our findings suggest that the global increase in DWV- B prevalence is not
driven by selective pressure by the vector. Rather, DWV- B is able to
persist in colonies at higher viral loads relative to DWV- A in the presence
and absence of V. destructor. The interplay between V. destructor and DWV
genotypes within honeybee colonies may have broad consequences upon viral
diversity in sympatric taxa as a result of spillover.
On a personal level, I asked a USDA study researcher why the results from my
apiary had no evidence of DWV-A but relatively high titers of DWV-B. She
replied quite succinctly that: ... we have seen a decline of DWV-A as DWV-B
rises.
What are we to make of all this?
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