Beyond Bee Books: Clever stuff with DWV

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Dave Blacks blog prefix
While it has become relatively straightforward to measure Deformed Wing Virus (DWV) levels in honeybee adults and off-spring with modern analysis methods it has not been so easy to study the possible transmission routes, and to differentiate one possible source from another. It had been shown many years ago that DWV can be transmitted when the queen lays eggs, and that drone semen can contain the virus, and more recently that varroa mites inject the viral particles when they feed.

A clever study from the USDA-ARS at Beltsville has used DWV labelled with a gene with a green fluorescent protein (GRP) to demonstrate an additional way of passing the infection on, in this case during the detection and removal of infected pupae and larvae, a behaviour we encourage by selecting for hygienic or VSH strains of bee. This was not unexpected, but without this experimental technique it wasn’t clear whether bees were being infected in this way, or in some other way. The use of GRP and laboratory isolation has made this infection route pretty certain.

Some consequences follow. It has already been shown (with radioactively labelled sugar syrup) that food from a single bee can be shared with hundreds and even thousands of other bees in a single day. To be blunt, the ‘removal’ of infected off-spring is a euphemism for cannibalism - bees waste nothing and pupae and larvae are a valuable investment in protein that has to be recovered. Food sharing behaviour (trophallaxsis) ensures that while only a few bees are involved in ‘clean-up duties’ the viral particles are rapidly spread to the other adults. To use our post-CoVid vocabulary, these larvae become super-spreaders. Bees have some tolerance for orally acquired viruses, but will succumb to very high levels. This reservoir of infection persists even after mites have been removed, what we recognise as Parasitic Mite Syndrome (PMS).

So here is the good news. The ‘label’ the researchers attached to the mite’s genome was not an essential protein and was quite likely to be jettisoned when the virus replicates in the host tissue. By looking at the ratio between GRP-labelled and not labelled DWV they had a kind of clock, because the ratio grew as the virus replicated and shed the marker. The original infection was established with DWV-GRP, but subsequent infections tended towards DWV, and the virus circulating in the population post-cannibalism was largely a replicate. This continued spread and replication only occurs as long as the bees are not killed by the virus they acquire. Over time, it looks like natural selection operates to ensure the virus strains that will persist are of a less virulent kind.

Francisco Posada‑Florez, Zachary S. Lamas, David J. Hawthorne, Yanping Chen, Jay D. Evans & Eugene V. Ryabov. Pupal cannibalism by worker honeybees contributes to the spread of deformed wing virus. Scientific Reports (2021) 11:8989