Could This Be the Reason for the Decline?
A viral pathogen that typically infects plants has
been found in honey bees and could help explain their decline. Researchers
working in the U.S. and Beijing, China report their findings in mBio,
the online open-access journal of the American Society for Microbiology.
The routine screening of bees for frequent and rare
viruses "resulted in the serendipitous detection of Tobacco Ringspot
Virus, or TRSV, and prompted an investigation into whether this plant-infecting
virus could also cause systemic infection in the bees," says Yan Ping Chen
from the U.S. Department of Agriculture's Agricultural Research Service (ARS)
laboratory in Beltsville, Maryland, an author on the study.
"The results of our study provide the first
evidence that honeybees exposed to virus-contaminated pollen can also be
infected and that the infection becomes widespread in their bodies," says
lead author Ji Lian Li, at the Chinese Academy of Agricultural Science in
"We already know that honey bees, Apis
melllifera, can transmit TRSV when they move from flower to flower, likely
spreading the virus from one plant to another," Chen adds.
Notably, about 5% of known plant viruses are
pollen-transmitted and thus potential sources of host-jumping viruses. RNA
viruses tend to be particularly dangerous because they lack the 3'-5'
proofreading function which edits out errors in replicated genomes. As a result,
viruses such as TRSV generate a flood of variant copies with differing
One consequence of such high replication rates are
populations of RNA viruses thought to exist as "quasispecies," clouds
of genetically related variants that appear to work together to determine the
pathology of their hosts. These sources of genetic diversity, coupled with
large population sizes, further facilitate the adaption of RNA viruses to new
selective conditions such as those imposed by novel hosts. "Thus, RNA
viruses are a likely source of emerging and reemerging infectious
diseases," explain these researchers.
Toxic viral cocktails appear to have a strong link
with honey bee Colony Collapse Disorder (CCD), a mysterious malady that
abruptly wiped out entire hives across the United States and was first reported
in 2006. Israel Acute Paralysis Virus (IAPV), Acute Bee Paralysis Virus (ABPV),
Chronic Paralysis Virus (CPV), Kashmir Bee Virus (KBV), Deformed Wing Bee Virus
(DWV), Black Queen Cell Virus (BQCV) and Sacbrood Virus (SBV) are other known
causes of honeybee viral disease.
When these researchers investigated bee colonies
classified as "strong" or "weak," TRSV and other viruses
were more common in the weak colonies than they were in the strong ones. Bee
populations with high levels of multiple viral infections began failing in late
fall and perished before February, these researchers report. In contrast, those
in colonies with fewer viral assaults survived the entire cold winter months.
TRSV was also detected inside the bodies of Varroa
mites, a "vampire" parasite that transmits viruses between bees while
feeding on their blood. However, unlike honeybees, the mite-associated TRSV was
restricted to their gastric cecum indicating that the mites likely facilitate
the horizontal spread of TRSV within the hive without becoming diseased
themselves. The fact that infected queens lay infected eggs convinced these
scientists that TRSV could also be transmitted vertically from the queen mother
to her offspring.
"The increasing prevalence of TRSV in
conjunction with other bee viruses is associated with a gradual decline of host
populations and supports the view that viral infections have a significant
negative impact on colony survival," these researchers conclude. Thus,
they call for increased surveillance of potential host-jumping events as an
integrated part of insect pollinator management programs.
Labels: Bee Decline, Pathogenic Plant Virus Jumps to Honey Bees. Viral Pathogen California Ag News