Researchers from the University of California at Berkeley and University of California at Davis have uncovered an association between the presence of bovine leukemia virus in cattle and breast cancer in humans.
A team led by UC Berkeley virology professor Gertrude Buehring performed a study, published in the journal PLOS ONE, determining that contact with the cattle-borne bovine leukemia virus could be a cause of breast cancer.
Breast cancer is the second-most common form of cancer among women in the United States, according to the National Cancer Institute.
The Cooperative Human Tissue Network provided the researchers with breast tissue samples from 239 donors.
The study found that 59 percent of the cancerous samples used had been exposed to the virus, while only 29 percent of the tissue samples from women without breast cancer exhibited BLV antibodies.
According to Gladys Block, a UC Berkeley professor emerita of nutritional epidemiology who co-authored the study, the odds of having breast cancer were roughly three times greater in subjects in which BLV was present.
''We calculate that 37 percent of all breast cancers are attributable to this cause,'' Block said. ''That's a fantastically large attributable risk.''
For more than 30 years, BLV, a virus that causes leukemia and lymphoma in cattle, was not believed to infect humans. But new technology introduced in the 1990s such as DNA sequencing allowed for more sensitive techniques to detect viruses and enabled Buehring to publish a study in 2003 that challenged that long-held belief.
''(Our study) overturned a 30-year dogma that humans did not get infected with this virus and it was not transmissible from cattle,'' Buehring said.
Last year, Buehring and her team published a study confirming the presence of bovine leukemia virus in the mammary epithelial cells of human breast tissue, where breast cancer originates.
Though the team's most recent study does not conclude that BLV causes cancer, the study found that there is a strong correlation between the two.
''We're only saying that there is an association between the virus and breast cancer,'' said Hanne Jensen, a UC Davis professor of pathology and laboratory medicine and a co-author of the paper. ''We still need to do many more studies.''
The researchers are still unsure of how BLV is transmitted to humans. According to Buehring, that is the most important question that needs to be answered. In the meantime, she said she hopes her research will inspire others to develop a vaccine.
''If this virus did play a role and turned out to be responsible for a large portion of those cases, then we could think about preventative measures,'' Buehring said. ''This would change the whole picture of breast cancer.''