Plant virus can infect honeybees
I saw this news alert on Microbe World, which pointed to a primary research article in the American Society of Microbiology’s journal mBio. Honeybee colonies worldwide have undergone a phenomenon referred to as “colony collapse,” where the worker bees from a colony disappear. Pesticides, various viruses and bacteria, mites, and habitat loss have all been proposed as explanations however no clear front runner among these hypotheses has emerged. Honeybees are major pollinators of crops, with estimates that bees are responsible for pollinating up to $200 billion of crops each year. The number of beehives in the US has declined by 50% over the past 30 years, highlighting the seriousness of the problem.
The current study is a collaboration between researchers at the Chinese Institute of Agricultural Sciences, the US Dept. of Agriculture, the University of North Carolina, and Emory University in Atlanta, GA entitled “Systemic Spread and Propagation of a Plant-Pathogenic Virus in European Honeybees.” Viral infections do not easily jump between species. Indeed, although epidemiologists are extremely worried and closely track influenza viruses in mammal and avian populations, the so-called “bird flu” in itself does not readily infect humans. The concern about bird flu outbreaks is over a random mutation that allows a bird flu isolate to be easily spread between humans. Threats posed by viruses that infect a species extremely unrelated to humans are essentially non-existent, and so far there have only been rudimentary reports that this happens. This report from 2010 represents the first time that a plant virus had been implicated in human disease, however this study has not been followed up so far. The current study goes further, and clearly shows by a variety of methods that a plant virus can effectively replicate in insect cells, and infection of colonies with this virus is correlated with colony collapse disorder.
Most plant viruses are spread by herbivorous insect vectors that spread the virus from an infected plant to a healthy plant, and there is evidence that plant viruses can manipulate the behavior of insects that feed on infected plants. Tobacco Ringspot Virus (TRSV) infects a number of plants of economic importance, and causes malformation and stunted growth. A number of biting insect vectors have been implicated in the spread of TRSV, however additional spread via infected seeds is also critical in the long term spread of the virus. Honeybees have been found to play a role in this process by transferring infected pollen from plant to plant, however it had not been described previously that the bees could actually be infected with the virus by the pollen that they carried.
The researchers used bees from colonies kept at the USDA labs in Maryland, USA. Samples of workers from these colonies were analyzed for presence of the virus by several methods. Viruses were purified from whole bee homogenates, and visualized directly by electron microscopy–the picture above demonstrates the presence of virus particles that showed the same morphology as TRSV particles from plant sources. Several molecular analyses were also used, including polymerase chain reaction of isolated bee tissues using specific primer sets, and in situ hybridization of microscope slides of whole body sections of worker bees. Both of these approaches showed the presence of virus in a variety of tissues, indicating that the virus was actually replicating within the tissues of bees as opposed to being carried on the outside of the insect on pollen particles.
They further demonstrated the presence of TRSV in bees by analyzing the presence of virus in Varrao mites, which are obligate parasites of the honeybee and have been an important part of the problems facing the beekeeping industry. Indeed, it appears that the mites are able to transmit the virus between bees and are critical in the spread of the virus throughout a hive. The main block preventing easy infection of bees is the step of transfer and binding of virus to insect cells, and biting mites circumvent this process by introducing the virus directly into the body of the insect. The researchers found a strong correlation with the presence of TRSV in bee colonies plus other viral diseases of honeybees in colonies deemed “weak” or in danger of collapse. In contrast, “strong” colonies showed very low levels of TRSV and other seasonal viral infections of bees. They conclude by suggesting that the data argues that survival of honeybee colonies is dependent on infection with TRSV along with other specific viruses of bees.