Monday, 25 June 2012

Bee vs. Virus. Who decides? One of the most frequently occurring quotes going around referring to bees is "If the bee disappeared off the surface of the globe then man would only have four years of life left. No more bees, no more pollination, no more plants, no more animals, no more man," (often attributed to Albert Einstein). Regardless of whether or not this came from Einstein, the idea is still one worth thinking about. Honeybees (Apis mellifera) are often the most significant pollinators of plants, so their disappearance would be something of a concern.

Bees face many challenges. More and more people have heard of colony collapse disorder (CCD), a phenomenon whereby colonies of bees seem to suddenly die out. The most infuriating aspect of CCD is that the absolute cause is unknown; whether it’s biotic factors such as viruses, chemical factors, such as pesticides or even simply a lack of nutrition. There are many diseases of bees, including numerous viruses, but the most significant issue bees (and their beekeepers) face is Varroa destructor; a small mite which has caused immense problems wherever it’s ventured.   

A couple of papers have come out recently regarding the demise of bee colonies, both concentrating on the interactions between infestation of a colony with the Varroa mite and Deformed Wing Virus (DWV), a small RNA virus of the family Iflaviridae.

The first study was published in Science and benefits from studying the ‘before and after Varroa’ scenarios, as opposed to studies of colonies in regions already infested with the mite. The scientists compared Hawaiian islands with and without the mite, and then again once the mite had spread to other islands. Firstly they found that over the years Varroa prevalence increased. In turn, as Varroa levels increased, the infection levels of DWV in each bee also increased, whilst the diversity of the DWV strains infecting the colonies reduced. Effectively the Varroa mite had rendered a relatively resistant population susceptible to DWV and as a result DWV exploded through the population.

The Deformed Wing Virus relationship to Varroa; infestation with Varroa allows  DWV to explode.

A second study, published in PLoS Pathogens, found a similar result; as the extent of Varroa infection increased, so did the levels of DWV. Whereas the Hawaiian study observed the situation on a grand scale, the authors of the PLoS looked further into the interactions and found evidence that allowing mites to feed on bee larvae resulted in an increase in DWV levels. When they looked into the effect of the virus on the bee’s immune system, they observed that the expression of certain genes relating to bee immunity altered in response to infection by DWV. Essentially, bees infected with DWV were immunosuppressed, but as a result of DWV, not Varroa feeding on the larvae. Instead, it would appear that, by trying to repair the feeding holes made by Varroa, the bee inadvertently weakens its resistance to DWV, thus promoting DWV replication.

These studies add to the growing amounts of data linking Varroa infestation and DWV as a cause of colony collapse. They also highlight how it’s possible to view a colony as an organism itself, rather than individuals; things such as Varroa affect the colony as a whole, and, ultimately, it is the colony that collapses. 

< Nazzi F, Brown SP, Annoscia D, Del Piccolo F, Di Prisco G, Varricchio P, Della Vedova G, Cattonaro F, Caprio E, & Pennacchio F (2012). Synergistic parasite-pathogen interactions mediated by host immunity can drive the collapse of honeybee colonies. PLoS pathogens, 8 (6) PMID: 22719246