Is a virus responsible for the disappearing bees?

This article is reposted from the old Wordpress incarnation of Not Exactly Rocket Science. There's been more work on CCD since, but I'm reposting this mainly because of some interesting follow-up research that will I will post about tomorrow.

In 2006, American and European beekeepers started noticing a strange and worrying trend - their bees were disappearing. Their hives, usually abuzz with activity, were emptying. There were no traces of the workers or their corpses either in or around the ghost hives, which still contained larvae and plentiful stores of food. It seemed that entire colonies of bees had apparently chosen not to be.

The cause of the aptly named 'Colony Collapse Disorder', or CCD, has been hotly debated over the last year. Fingers were pointed at a myriad of suspects including vampiric mites, pesticides, electromagnetic radiation, GM crops, climate change and poor beekeeping practices. And as usual, some people denied that there was a problem at all.

But a large team of US scientists led by Diana Cox-Foster and Ian Lipkin have used modern genomics to reveal a new villain in this entomological whodunnit - a virus called Israeli Acute Paralysis Virus or IAPV. By and large, the team found that where there was IAPV, there was CCD. The virus and the affliction were so stongly connected that Cox-Foster and Lipkin estimated that a hive infected with IAPV had a 96% chance of suffering from CCD. Once infected, the chances of a colony collapsing shot up by 65 times.

The team apprehended IAPV by taking a full inventory of the bees' microscopic hive-mates. Using particularly fast technology, they sequenced the entire set of genetic material found in several colonies - bees, microbes and all. They even looked at samples of royal jelly from China used to feed larvae, and apparently healthy imported bees from Australia, to try and track down the possible sources of an infection.

Thankfully, the full genome of the honeybee had recently been published. By subtracting that from their initial data, Cox-Foster and Lipkin were left with the sequences of the bees' bedfellows, which they identified by matching these sequences against publicly available genetic libraries.

A US beekeeper tending a hive - most have lost signficiant proportions of their bees of late.The search turned up a host of bacteria, fungi, viruses and more. Then began a steady process of elimination. Many of the bacteria and fungi in the pool were well known species that happily co-exist alongside the bees, mostly inside their digestive systems. And of the remaining fungi and viruses with the potential to cause disease, almost all were found both in colonies affected by CCD and those untouched by it.

The one exception was IAPV. In 51 samples of workers from a variety of hives, Cox-Foster and Lipkin found the molecular fingerprints of IAPV in 83% of colonies affected by CCD and just 5% of the unaffected colonies.

The idea that CCD is caused by a virus makes sense in the light of previous findings. Beekeepers were starting to realise that they could transmit CCD by reusing equipment that had previously been used to tend to infected colonies. And tellingly, irradiating this equipment stopped the spread, presumably by killing whatever infectious agent was lurking on it.

The researchers believe that the virus, first discovered in Israel three years ago, was introduced into the United States within bees imported from Australia. This practice began in 2004, which coincides with the first reports of CCD.

Cox-Foster and Lipkin do not believe that IAPV is acting alone. It most likely has accomplices. Infected bees in Israel showed certain symptoms, such as shivering wings, that the Australian or CCD-afflicted bees do not. They also died outside their hive while CCD-bees vanish without a trace. The researchers believe that the virus may be interacting with other aspects of the bees' environment that have been previously put forward as causes of CCD.

A honeybee carrying a Varroa mite. But the real culprit behind CCD is the microscopic virus IAPV.The blood-sucking Varroa mite (right), which is absent in Australia, could be making the bees more susceptible to infection by weakening their immune systems. The chemicals used to control these mites, and the pesticides sprayed on bee-pollinated plants, could be having a similar effect.

The researchers caution that they haven't proved that IAPV causes CCD and indeed, later studies would establish that IAPV is far from the only, or even the most important, factor behind CCD. I'll be discussing the latest discovery in this tale tomorrow.

Reference: Cox-Foster, Conlan, Holmes, Palacios, Evans, Moran, Quan, Briese, Hornig, Geiser, Martinson, van Engelsdorn, Kalkstein, Drysdale, Hui, Zhai, Cui, Hutchison, Simons, Egholm and Lipkin. 2007. A metagenomic survey of microbes in honey bee colony collapse disorder. Science doi:10.1126/science.1146498

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By Manuka Honey (not verified) on 07 Sep 2009 #permalink