A new study by University of
California, Berkeley, researchers establishes for the first time a link between
infection with the bovine leukemia virus and human breast cancer.
The virus is widespread. In
some cases, all of the milk from large herds carries the virus.
In the study, published this
month in the journal PLOS ONE
and available online, researchers analyzed breast tissue from 239 women,
comparing samples from women who had breast cancer with women who had no
history of the disease for the presence of bovine leukemia virus (BLV).
MedicaL News reports that they
found that 59 percent of breast cancer samples had evidence of exposure to BLV,
as determined by the presence of viral DNA.
By contrast, 29 percent of
the tissue samples from women who never had breast cancer showed exposure to
BLV.
"The association
between BLV infection and breast cancer was surprising to many previous
reviewers of the study, but it's important to note that our results do not
prove that the virus causes cancer," said study lead author Gertrude
Buehring, a professor of virology in the Division of Infectious Diseases and
Vaccinology at UC Berkeley's School of Public Health.
"However, this is the
most important first step. We still need to confirm that the infection with the
virus happened before, not after, breast cancer developed, and if so, how,"
she said.
Bovine leukemia virus
infects dairy and beef cattle's blood cells and mammary tissue.
The retrovirus is easily
transmitted among cattle primarily through infected blood and milk, but it only
causes disease in fewer than five percent of infected animals.
A 2007 U.S. Department of
Agriculture survey of bulk milk tanks found that 100 percent of dairy
operations with large herds of 500 or more cows tested positive for BLV
antibodies.
This may not be surprising
since milk from one infected cow is mixed in with others.
Even dairy operations with
small herds of fewer than 100 cows tested positive for BLV 83 percent of the
time.
What had been unclear until
recently is whether the virus could be found in humans, something that was
confirmed in a study led by Buehring and published last year in Emerging
Infectious Diseases. That paper overturned a long-held belief that the
virus could not be transmitted to humans.
"Studies done in the
1970s failed to detect evidence of human infection with BLV," said
Buehring.
"The tests we have now
are more sensitive, but it was still hard to overturn the established dogma
that BLV was not transmissible to humans.
“As a result, there has been
little incentive for the cattle industry to set up procedures to contain the
spread of the virus," he said.
The new paper takes the
earlier findings a step further by showing a higher likelihood of the presence
of BLV in breast cancer tissue. When the data was analyzed statistically, the
odds of having breast cancer if BLV were present was 3.1 times greater than if
BLV was absent.
"This odds ratio is
higher than any of the frequently publicized risk factors for breast cancer,
such as obesity, alcohol consumption and use of post-menopausal hormones,"
said Buehring.
There is precedence for
viral origins of cancer. Hepatitis B virus is known to cause liver cancer, and
the human papillomavirus can lead to cervical and anal cancers. Notably,
vaccines have been developed for both those viruses and are routinely used to
prevent the cancers associated with them.
"If BLV were proven to
be a cause of breast cancer, it could change the way we currently look at
breast cancer control," said Buehring. "It could shift the emphasis
to prevention of breast cancer, rather than trying to cure or control it after
it has already occurred."
Buehring emphasized that
this study does not identify how the virus infected the breast tissue samples
in their study. The virus could have come through the consumption of
unpasteurized milk or undercooked meat, or it could have been transmitted by
other humans.
