Is a virus from tobacco killing all of the honey bees?

Answer: Probably not, but it’s interesting that it is found in bees at all

Honey bees are hosts to more known parasites and pathogens than any other social insect, with 72 documented so far. Many of the most economically important ones, including varroa mites, have resulted from introductions of foreign exotic pests. In biology terminology, this is known as a “host shift”, where a parasite from one species jumps ship and starts to parasitize another species. In the case of varroa, the mites shifted hosts from the Eastern honey bee (Apis cerana) to our Western honey bee (Apis mellifera). Because our bees had not evolved any natural defenses against varroa, the mites are highly virulent; that is, our bees succumb very quickly to them (at least compared to their original hosts).

Well, it looks as if history is repeating itself, as another parasite seems to have shifted hosts. This time, however, it is very, very unusual. Host shifts usually occur between closely related species, so the jump from one host to another isn’t a very long one. This is why our pets don’t get sick whenever we get the flu, or we don’t catch some weird bacteria that plagues salamanders. So it is quite odd for honey bees to get infected by a virus that usually afflicts, of all things, a wide variety of plants.

A recent report out of the USDA Honey Bee Research Lab in Beltsville, MD, verifies that the tobacco ringspot virus (TRSV) has not only been detected in honey bees, it replicates in them. Finding the virus on the surface of bees or in their pollen loads might be expected, since this is a pollen-bourne virus thrives in many plant species and is found in ~5% of their pollen. But even if bees happen to pick up the virus incidentally, bees and plants are so far apart on the evolutionary tree that the virus would find it incredibly difficult to use the bees as alternative hosts. Nonetheless, the evidence that infected bees increase their viral loads over time suggests that TRSV is indeed doing just that.

TRSV-FigureThe details of the study, however, reveal several key findings that are likely not being covered in typical media reports. First, they found the virus within the bodies of Varroa mites. As such, like most other RNA viruses that infect bees, these pesky parasites could very well be serving as a vector of the disease that help its spread. Second, the researchers dissected many different body parts of the bees and found virus in most (but not all) organs. This means that, while the virus may be replicating within the adult bees, they aren’t specialized or targeting particular functions of the bee. This is actually good news, since it means it that the virus isn’t specialized enough to effect particular bodily functions and therefore isn’t likely to be symptomatic. Further, while the virus was only just now described in bees, we do not know for sure that this leap was a recent one; there is a history of disease agents being present in bees for some time before they are recognized by scientists or beekeepers.  Finally, and perhaps most importantly, TRSV has a far lower infection rate (~10-20% for unhealthy colonies, 0% in healthy colonies) compared to other known viruses such as deformed wing virus (DWV; ~60-70% in unhealthy colonies, 20-30% in healthy colonies).

In the end, it is far too early to make any strong statements about how problematic this newly detected virus might be for bee health. Anything new dealing with bee disease is usually ascribed to being the link to Colony Collapse Disorder, but the evidence just isn’t sufficient to make such an overgeneralization. However, it does remind us that our beleaguered honey bees are hosts to many things that make them sick—both new and old—and we need to remain vigilant for all of them.

Reference

Li, J. L., R. S. Cornman, J. D. Evans, J. S. Pettis, Y. Zhao, C. Murphy, W. J. Peng, J. Wu, M. Hamilton, H. F. Bonscristiani, L. Zhou, J. Hammond, and Y. P. Chen. (2014). Systemic spread and propagation of a plant-pathogenic virus in European honeybees, Apis mellifera. mBio, 5: e00898-13 [LINK]

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