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The role of the varroa mite in the spread of deformed wing virus.

In the June 8, 2012 issue of the journal, Science, researchers from the UK and Dr. Ethel Villalobos and Scott Nikaido of UH Honeybee Project present information regarding the role of the Varroa in the spread of the Deformed Wing Virus (DWV) in Hawaii.

Deformed Wing Virus (DWV) is a common honeybee virus that can be transmitted by the Varroa mite and has been linked to the occurrence of Colony Collapse Disorder in the continental US and other parts of the world. DWV causes severe deformities, shortened life span, and even death to infected bees. Infection by DWV can be expressed in two ways by individual bees. In some instances, the bees will show an "overt infection", which is characterized by crumpled, non-functional wings, as well as distended, discolored abdomens, and a shortened life span. DWV can also be present in bees without showing visible signs, this is called a "covert infection". Covert infections are more common when the virus is transmitted from parent to offspring without involving Varroa. These normal looking bees, are carriers of the virus, but do not have deformed wings. However, the life span of bees with a covert infection is also reduced.

Honeybees are more likely to express an overt infection when the viral transmission involves Varroa and the viral load in the infected honeybee is high. This study revealed that the arrival of Varroa in Hawaii had a profound effect on the number of colonies infected with DWV. The prevalence of DWV in colonies in Varroa-free areas was 6% to 13%, but in colonies from areas where Varroa is established, the prevalence of DWV rose dramatically to 75% to 100%. This increase in the prevalence of DWV was also accompanied by a million-fold increase in the amount of viruses which infected each individual bee.

Our work has provided proof that there is a strong selection in favor of certain DWV strains which can replicate inside the Varroa mite's body and that these "Varroa adaptable strains" will displace other variants of the virus. The high number of viral particles found per bee in Varroa infested colonies also suggest that the mite's introduction affects the amount of viral material to which each bee is exposed. These Varroa induced changes in the viral landscape increase the likelihood that individual bees may express obvious physical signs of this viral disease and may have contributed to the death of millions of honeybee colonies, wherever Varroa and DWV co-occur.