A new study by University of California (UC) Berkeley researchers establishes for the first time a link between infection with the bovine leukemia virus and human breast cancer. In the study, published on September 2, 2015 in the open-access journal PLOS ONE, researchers analyzed breast tissue from 239 women for the presence of BLV, comparing samples from women who had breast cancer with women who had no history of the disease. 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 Dr. 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.” BLV 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 5 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 because 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.
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