Bovine leukemia virus is easily spread among cows, especially at factory farms, but only 5 percent are made sick by the disease -- which is found at nearly all large dairy farms. Photo by smereka/Shutterstock

BERKELEY, Calif., Sept. 17 (UPI) -- Researchers were surprised to find in a new study that a significant number of breast cancer cells from more than 200 women had evidence of exposure to bovine leukemia virus, or BLV.

BLV infects dairy and beef cattle's blood cells and mammary tissues, and was for a long time thought not to be able to infect humans.


Nearly all bulk milk tanks at large factory farms are infected with BLV, but only about 5 percent of cows get sick if they have the virus, according to the U.S. Department of Agriculture. The same group of researchers at the University of California Berkeley involved in the new study also found last year that BLV could be transmitted to humans.

"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 Dr. Gertrude Buehring, a professor of virology in the University of California Berkeley, in a press release. "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."

Using donated tissue from the Cooperative Human Tissue Network, the researchers compared breast tissue from 239 women who either had breast cancer or did not.

They found that 59 percent of breast cancer cells had evidence of exposure to BLV based on the presence of viral DNA in the cells. Cells from women who had not had breast cancer only had evidence of exposure 29 percent of the time.

When researchers further analyzed the data, they found the risk of developing breast cancer was 3.1 times higher if BLV was present in a woman than if it was not. Buehring said the odds ratio is higher than other well known risk factors for breast cancer, including obesity, alcohol consumption and hormones.

While Buehring said the study does not prove that BLV causes breast cancer, it would not be the first virus shown to cause cancer -- hepatitus B virus causes liver cancer and human papillomavirus can cause cervical and anal cancer.

"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."

The study is published in PLOS ONE.