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A Virulent Strain of Deformed Wing Virus (DWV) of Honeybees (Apis mellifera) Prevails after Varroa destructor-Mediated, or In Vitro, Transmission
The globally distributed ectoparasite Varroa destructor is a vector for viral pathogens of the Western honeybee (Apis mellifera), in particular the Iflavirus Deformed Wing Virus (DWV). In the absence of Varroa low levels DWV occur, generally causing asymptomatic infections. Conversely, Varroa-infested colonies show markedly elevated virus levels, increased overwintering colony losses, with impairment of pupal development and symptomatic workers. To determine whether changes in the virus population were due Varroa amplifying and introducing virulent virus strains and/or suppressing the host immune responses, we exposed Varroa-naïve larvae to oral and Varroa-transmitted DWV. We monitored virus levels and diversity in developing pupae and associated Varroa, the resulting RNAi response and transcriptome changes in the host. Exposed pupae were stratified by Varroa association (presence/absence) and virus levels (low/high) into three groups. Varroa-free pupae all exhibited low levels of a highly diverse DWV population, with those exposed per os (group NV) exhibiting changes in the population composition. Varroa-associated pupae exhibited either low levels of a diverse DWV population (group VL) or high levels of a near-clonal virulent variant of DWV (group VH). These groups and unexposed controls (C) could be also discriminated by principal component analysis of the transcriptome changes observed, which included several genes involved in development and the immune response. All Varroa tested contained a diverse replicating DWV population implying the virulent variant present in group VH, and predominating in RNA-seq analysis of temporally and geographically separate Varroa-infested colonies, was selected upon transmission from Varroa, a conclusion supported by direct injection of pupae in vitro with mixed virus populations. Identification of a virulent variant of DWV, the role of Varroa in its transmission and the resulting host transcriptome changes furthers our understanding of this important viral pathogen of honeybees.

http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1004230
 

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This confirms the natural experiment reported by Martin in June 2012 Science.
Martin studied the invasion of Varroa into Hawaii in 2009. Oahu had long exposure, the Hilo side of the Big Island 2 year exposure, and Maui and Kaiau were Varroa free.

Martin found that DWV strain changed to a single highly virrulent one that was easily transmitted by the Varroa, rather than a low level and diverse background.






Cite::Global Honey Bee Viral Landscape
Altered by a Parasitic Mite

Stephen J. Martin,1* Andrea C. Highfield,2 Laura Brettell,1 Ethel M. Villalobos,3
Giles E. Budge,4 Michelle Powell,4 Scott Nikaido,3 Declan C. Schroeder2*

Emerging diseases are among the greatest threats to honey bees. Unfortunately, where and when
an emerging disease will appear are almost impossible to predict. The arrival of the parasitic
Varroa mite into the Hawaiian honey bee population allowed us to investigate changes in the
prevalence, load, and strain diversity of honey bee viruses. The mite increased the prevalence
of a single viral species, deformed wing virus (DWV), from ~10 to 100% within honey bee
populations, which was accompanied by a millionfold increase in viral titer and a massive reduction
in DWV diversity, leading to the predominance of a single DWV strain. Therefore, the global
spread of Varroa has selected DWV variants that have emerged to allow it to become one of the
most widely distributed and contagious insect viruses on the planet.

@article{martin2012global,
title={Global honey bee viral landscape altered by a parasitic mite},
author={Martin, Stephen J and Highfield, Andrea C and Brettell, Laura and Villalobos, Ethel M and Budge, Giles E and Powell, Michelle and Nikaido, Scott and Schroeder, Declan C},
journal={Science},
volume={336},
number={6086},
pages={1304--1306},
year={2012},
publisher={American Association for the Advancement of Science}
}
 

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Old news to me...I have been saying for years that it seems to me that DWV is one one of the main culprits killing my bees. Scientists are only figuring it put now?
 

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OD, don't be so dismissive of scientists. I would say that Martin knows very well that DWV is highly virulent with Varroa, what it did in the Hawaii paper was very elegant: he showed that DWV changed overnight from a relatively benign background issue to a highly virulent strain when Varroa appeared by selection and amplification of a particular strain. This is accelerated evolution in action, observed in real time. He provides a mechanism by which the Varroa becomes a co-conspirator to the viral killer, and one that is out of control of the Varroa genome itself.

The importance: the coevolution of benign Varroa (as hypothesized by Seeley, and promoted by "TF" die-hards) is made very difficult, if the killing effect is out of the hands of the mite.
 

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OD, don't be so dismissive of scientists.
I am sure their studies are very informative and by far more than I can offer....but....these reports just seems five or more years past due. I was reporting massive DWV dieoffs five years ago.

http://youtu.be/49G00nN6wSg
 
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